JP7572068B2 - RFID tag, passive RFID tag light sensor, and method for determining light irradiance - Patents.com - Google Patents

Description

The present invention relates to an RFID tag, a passive RFID tag light sensor, and a method for determining the irradiance of light.

RFID (Radio Frequency Identification) technology uses electromagnetic fields or radio waves to send and receive data contactlessly, and uses that data for authentication. It can simultaneously read multiple RFID tags from a distance, and can determine which RFID tag the received data came from by using the unique identification information written into the RFID tag's IC.

The ability to read multiple tags at once is an advantage of RFID technology, but when trying to read information from just one specific tag among multiple tags, there is a problem in that it becomes difficult to distinguish between them. Therefore, in order to solve this problem, systems have been proposed that combine the identification function of RFID tags with a photodetector (see, for example, Patent Documents 1 to 4).

In these systems, for example, among multiple RFID tags equipped with photodetectors, some are capable of obtaining information only on RFID tags located in a predetermined location by irradiating light only on the photodetector of the RFID tag located in the predetermined location and transmitting light detection information indicating whether the photodetector detected light or not to the RFID reader/writer (see, for example, Patent Documents 1 and 4).

In such systems, a threshold or reference value is set to determine whether or not the photodetector detects light, and information about the RFID tag located at a given location is obtained by comparing the set value with the signal from the photodetector (see, for example, Patent Documents 2 and 3).

In these systems, the position information of a photodetector that detects light above a threshold is used to identify a specific tag among multiple RFID tags and receive information from only that tag.

In these systems, for example, a power generation device or an RF wave rectifier is provided, and the photodetector is driven by DC power from these devices (see, for example, Patent Documents 2 and 4).

In these systems, the RFID tag that receives the signal from the photodetector is not limited to a specific one, but commercially available RFID tags are used (see, for example, Patent Document 1).

US Patent Application Publication No. 2008/0297323 European Patent Application Publication No. 2333701 JP 2007-264799 A Patent No. 6531655

In recent years, the application of RFID technology to the medical and nursing care fields has been considered, and diapers have been developed that use the change in characteristics of RFID tags due to wetting as a urination detection sensor. Wearable passive RFID tag sensors that do not require batteries or external power sources are expected to become more widespread in the medical and nursing care fields in the future as devices that place a low burden on the wearer. If passive RFID tag sensors could be used not only to detect urination but also to test for contaminants in urine and excrement, it is believed that this would lead to a reduction in the burden on medical and nursing care sites. For example, when considering detecting changes in components such as hematuria and occult blood in urine from changes in transmittance, what is needed is a passive LED tag as a light source and a passive RFID tag optical sensor as a light receiving element. Passive LED tags have already been realized, but such passive RFID tag optical sensors do not yet exist.

Such passive RFID tag sensors require characteristics different from those of conventional systems that combine RFID tags and photodetectors, such as those described in Patent Documents 1 to 4. In conventional systems that combine RFID tags and photodetectors, a threshold or reference value is set, and the signal from the photodetector is compared with the set value to determine whether the photodetector has detected light. This results in a binary determination of whether the photodetector has detected light or not.

On the other hand, when considering observing changes in components such as hematuria and occult blood in urine from changes in transmittance, it is necessary to quantitatively detect changes in the irradiance of light incident on the photodetector that accompany changes in transmittance. In other words, there was a problem in that a passive RFID tag optical sensor with the function of an irradiance meter was required that could quantitatively measure the irradiance of incident light, rather than making a binary judgment as to whether the photodetector detected light or not.

The present invention has been made in consideration of these problems, and aims to provide an RFID tag with a function as an irradiance meter that can quantitatively measure the irradiance of incident light, a passive RFID tag optical sensor, and a method for determining the irradiance of light.

The inventors conducted the following studies and arrived at the present invention. That is, in a conventional system combining an RFID tag and a photodetector, a power generation device made of a semiconductor or the like that generates a photoelectric effect and an RF wave rectifier are provided, and the photodetector is driven by DC power from these and is also provided with an electronic circuit for controlling the photodetector and sending its signal to a communication unit. In contrast, since the present invention is intended for wearable and disposable applications, a simple and low-cost structure that does not have the above-mentioned auxiliary components is preferable, and further, a flexible and thin tag is preferable. For this reason, a structure in which the photodetector is electrically connected directly to an impedance matching unit or antenna unit that has an IC chip, which is a component of the RFID tag, and a DC power source for driving the photodetector and an electronic circuit for control are not required is preferable.

The impedance matching section is used to achieve impedance matching between the IC chip and the antenna section. If impedance matching is not achieved between the IC chip and the antenna section, signal reflection will occur, resulting in communication failure of the RFID tag and a decrease in communication distance. The impedance matching section is made of a ring-shaped conductor pattern with an opening formed on a dielectric substrate, and impedance matching with the IC chip is achieved by controlling the imaginary part of the complex impedance (reactance) depending on the size and shape of the opening.

When a photodetector whose resistance value changes when exposed to light is connected to an impedance matching section having such characteristics, the impedance of the impedance matching section changes with light exposure, and the impedance matching state between the IC chip and the antenna section changes.

Changes in the impedance matching state between the IC chip and the antenna section change the proportion of signals that are reflected and not effectively used, which changes the information in the signal sent from the RFID tag's IC chip to the RFID reader/writer.

In addition to the identification information, the information sent to the RFID reader/writer includes the reader RSSI (Received Signal Strength Indicator), which is a value related to the power the RFID reader/writer is receiving from the RFID tag. In addition, with certain types of IC chips, it is possible to know the sensor code S, which is information related to the impedance value changed by the auto-tuning mechanism of the IC chip itself, and changes in the on-chip RSSI, which is information related to the power the IC chip is receiving.

Of these pieces of information, RSSI (dBm) changes depending on the distance between the RFID tag and the antenna of the RFID reader/writer, obstacles between them, and even the orientation of the RFID tag and the antenna of the RFID reader/writer. Therefore, although it can be used for light detection, there are restrictions when considering its use as an irradiance meter to quantitatively measure the irradiance of incident light, such as the need to keep the installation conditions of the RFID tag, RFID reader/writer, and antenna constant.

In contrast, the sensor code S is determined by the shape, size, and material of the impedance matching part, and is suitable as information for an illuminometer that quantitatively measures the illuminance of incident light.

The photodetector may be a photoconductive detector or a photovoltaic detector, and photoresistors, phototransistors, photodiodes, etc. may be used. When connecting to an impedance matching section to measure impedance changes due to light irradiation, photoresistors and phototransistors, whose resistance value changes due to light irradiation, are preferred.

Based on the above findings, the RFID tag according to the first aspect of the present invention is characterized by having an insulating layer, an impedance matching section formed on the insulating layer and including a conductor pattern having an annular opening, and an IC chip electrically connected to the conductor pattern in the vicinity of the opening, an antenna conductor section extending in a first direction and a second direction from the impedance matching section, and a photodetector electrically connected to the end of the conductor pattern having the annular opening in the first direction and the end of the conductor pattern having the annular opening in the second direction, and forming a parallel circuit with the IC chip.

The passive RFID tag optical sensor according to the first aspect of the present invention has an RFID tag according to the first aspect of the present invention, and is configured to detect the impedance change of the impedance matching section caused by light incident on the optical detector as a change in the sensor code S transmitted from the IC chip to an RFID reader/writer via wireless communication. Alternatively, the passive RFID tag optical sensor according to the first aspect of the present invention may be configured to detect the change in irradiance of light incident on the optical detector as a change in the reader RSSI transmitted from the IC chip to an RFID reader/writer via wireless communication.

The RFID tag according to the second aspect of the present invention is characterized by having an insulating layer, an impedance matching section including a first conductor pattern provided on the insulating layer and having a first annular opening, an IC chip electrically connected to the first conductor pattern near the first opening, a second conductor pattern provided adjacent to the first conductor pattern in the impedance matching section and having a second annular opening, an antenna conductor extending from the impedance matching section in a first direction and a second direction, and a photodetector electrically connected to the second conductor pattern.

The passive RFID tag optical sensor according to the second aspect of the present invention has an RFID tag according to the second aspect of the present invention, and is configured to detect the impedance change of the impedance matching section caused by light incidence on the optical detector as a change in the sensor code S transmitted from the IC chip to the RFID reader/writer via wireless communication. Alternatively, the passive RFID tag optical sensor according to the second aspect of the present invention may be configured to detect the light incidence on the optical detector as a change in the reader RSSI transmitted from the IC chip to the RFID reader/writer via wireless communication.

The passive RFID tag optical sensor according to the second aspect of the present invention may be configured to be capable of controlling the size and range of change of the sensor code S transmitted from the IC chip to the RFID reader/writer by wireless communication depending on the ratio between the area of the first opening and the area of the second opening, or the size of each area.

The RFID tags according to the first and second aspects of the present invention preferably use a frequency in the UHF band. Also, the passive RFID tag optical sensor according to the first and second aspects of the present invention preferably has the optical detector be a phototransistor.

The first method for determining the irradiance of light is a method for determining the irradiance of light using a passive RFID tag optical sensor that detects changes in the sensor code S, among the passive RFID tag optical sensors according to the first and second aspects of the present invention, characterized in that a calibration curve showing the relationship between the irradiance of light incident on the optical detector and the change in the value of the sensor code S caused by light of the irradiance is created in advance, and the irradiance of the light incident on the optical detector is determined from the measured value of the sensor code S using the calibration curve.

The second method for determining the irradiance of light is a method for determining the irradiance of light using a passive RFID tag optical sensor that detects changes in the reader RSSI, among the passive RFID tag optical sensors according to the first and second aspects of the present invention, characterized in that a calibration curve showing the relationship between the irradiance of light incident on the optical detector and the change in the value of the reader RSSI caused by the light of the irradiance is created in advance, and the irradiance of the light incident on the optical detector is determined from the measured value of the reader RSSI using the calibration curve.

The present invention provides an RFID tag with a function as an irradiance meter that can quantitatively measure the irradiance of incident light, a passive RFID tag optical sensor, and a method for determining the irradiance of light.

1A and 1B are a plan view and a cross-sectional view of an RFID tag (shape A) according to a first embodiment of the present invention. 1A is a plan view of an RFID tag (shape B) according to embodiments 1 and 3 of the present invention, FIG. 1B is an enlarged plan view of the vicinity of an impedance matching section, FIG. 1C is a cross-sectional view thereof, and FIG. 1D is a plan view of the antenna section conductor pattern. FIG. 1A is a plan view and a front view of (a) a CdS photoresistor, which is a light detector, of an RFID tag according to embodiment 1 of the present invention; (b) a phototransistor, which is a light detector, of an RFID tag according to embodiment 3 of the present invention; and (c) a small, thin phototransistor of an RFID tag according to embodiment 3 of the present invention. 1 is a perspective view showing a method of measuring an RFID tag according to an embodiment of the present invention by an RFID reader/writer under light irradiation. 1 is a graph showing the relationship between the sensor code S and the irradiance at different communication distances for the RFID tag (shape A, L1=10 cm, L2=5 cm, photodetector: photoresistor CdS cell) according to the first embodiment of the present invention. 1 is a graph showing the relationship between reader RSSI and irradiance at different communication distances for an RFID tag according to embodiment 1 of the present invention (shape A, L1=10 cm, L2=5 cm, photodetector: photoresistor CdS cell). 11 is a graph showing the relationship between the antenna length L1 of the RFID tag (shape A, photodetector: photoresistor CdS cell) according to the first embodiment of the present invention and (a) the sensor code S and (b) the reader RSSI. 13 is a graph showing the change in (a) sensor code S and (b) reader RSSI when the LED light (wavelength: 390 nm, irradiance: 24.0 mW/cm ² ) of an RFID tag (shape A, L1 = 5 cm, L2 = 5 cm, photodetector: photoresistor CdS cell) according to embodiment 2 of the present invention is turned on and off. 13 is a graph showing changes in (a) sensor code S and (b) reader RSSI with on/off of LED light (wavelength: 550 nm, irradiance: 6.0 mW/cm ² ) of an RFID tag (shape A, L1=5 cm, L2=5 cm, photodetector: photoresistor CdS cell) according to a second embodiment of the present invention. 13 is a graph showing changes in (a) sensor code S and (b) reader RSSI with on/off of LED light (wavelength: 620 nm, irradiance: 2.5 mW/cm ² ) of an RFID tag (shape A, L1=5 cm, L2=5 cm, photodetector: photoresistor CdS cell) according to a second embodiment of the present invention. 13 is a graph showing the relationship between (a) sensor code S and irradiance, and (b) reader RSSI and irradiance, for an RFID tag according to embodiment 3 of the present invention (shape A, L1 = 10 cm, L2 = 5 cm, photodetector shown in FIG. 3(b), communication distance CL = 15 cm). 13 is a graph showing the relationship between (a) sensor code S and irradiance, and (b) reader RSSI and irradiance, for an RFID tag according to embodiment 3 of the present invention (shape C, L1 = 7.5 cm, L2 = 7.5 cm, photodetector shown in FIG. 3(c), communication distance CL = 25 cm).

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In the following embodiments, the same or common parts are denoted by the same reference numerals, and the description thereof will not be repeated. For ease of understanding, the scale of each part in the drawings may differ from the actual scale. Directions such as parallel, right angle, orthogonal, horizontal, vertical, up/down, left/right, and terms such as same and equal are allowed to have deviations that do not impair the functions and effects of the embodiments. The X-axis direction, the Y-axis direction, and the Z-axis direction respectively represent directions parallel to the X-axis, the Y-axis, and the Z-axis. The X-axis direction, the Y-axis direction, and the Z-axis direction are mutually orthogonal.

The passive RFID tag optical sensor according to the embodiment of the present invention includes an antenna section consisting of an impedance matching section, an IC chip, and a radiating conductor, which are components of a UHF RFID tag, as well as a photodetector electrically connected to an impedance junction.

The photodetector, electrically connected to the impedance matching section, changes the impedance of the impedance matching section due to changes in resistance caused by light irradiation, and obtains information about the irradiance of the light incident on the photodetector from information wirelessly transmitted from the IC chip to the RFID reader/writer.

The information can be the sensor code S and the reader RSSI, and the irradiance of the light incident on the photodetector can be determined by creating a calibration curve in advance that relates to the relationship between this information and the irradiance of the incident light.

Next, we will explain each element of the RFID tag optical sensor in detail.

[First embodiment]
1 shows a plan view and a cross-sectional view of a UHF band RFID tag (shape A) 101. The RFID tag 101 includes an insulating substrate 10, an impedance matching portion 30 including an impedance matching portion insulating substrate 11, an impedance matching portion conductor pattern 31 having an opening 50, and an IC chip 20, an antenna portion conductor pattern 32, and a photodetector 21.

The insulating substrate 10 and the impedance matching section insulating substrate 11 are plate- or film-shaped members with a conductor layer formed on their surfaces, and are processed into the shapes of the impedance matching section conductor pattern 31 and the antenna section conductor pattern 32, respectively, which have an opening 50 of the impedance matching section 30, when viewed in a plan view.

In FIG. 1, the shape of the opening 50 of the impedance matching section 30 is an approximate quadrilateral having a pair of long sides parallel to the X-axis direction and a pair of short sides parallel to the Y-axis direction. However, the shape of the opening 50 is not limited to the illustrated form, and may be, for example, an approximate quadrilateral having a pair of short sides parallel to the X-axis direction and a pair of long sides parallel to the Y-axis direction. The approximate quadrilateral shape of the opening 50 may include a complete quadrilateral. Here, "approximately" means that the corners or sides are rounded. The quadrilateral may include a rectangle, a rhombus, a parallelogram, and a square. The shape of the opening 50 may be a polygon other than a quadrilateral, a circle, or an ellipse.

In FIG. 1, the shape of the antenna portion conductor pattern 32 is a substantially quadrilateral in plan view. However, the shape of the antenna portion conductor pattern 32 is not limited to the illustrated form, and may be, for example, a meander line shape having multiple bends. Furthermore, the number of bends in the meander line shape, the width of the meander line, and the widths in the X-axis and Y-axis directions are not limited. The antenna portion conductor pattern 32 forms the antenna conductor portion.

The material of the insulating substrate 10 and the impedance matching section insulating substrate 11 is not particularly limited, but examples thereof include resin substrates such as polyethylene terephthalate (PET), polyurethane (PU), polyimide (PI), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), fluorinated resin copolymer, triacetyl cellulose (TAC), polyethylene naphthalate (PEN), syndiotactic polystyrene, polyphenylene sulfide, polycarbonate, polyarylate, polysulfone, polyester sulfone, polyetherimide, cyclic polyolefin, brominated phenoxy resin, norbornene resin, cycloolefin polymer, cycloolefin copolymer, polyacetal, and composite substrates such as paper substrates, paper phenol substrates, paper epoxy substrates, glass composite substrates, and glass epoxy substrates. From the viewpoint of flexibility, resin substrates and paper substrates are preferred. From the standpoint of flexibility and strength, the thickness of the insulating substrate 10 is preferably 4 to 1000 μm, and more preferably 8 to 150 μm.

The material of the impedance matching part conductor pattern 31 and the antenna part conductor pattern 32 having the opening 50 of the impedance matching part 30 is not particularly limited, but is formed from a conductive conductor. For example, it may be a metal such as aluminum, copper, gold, platinum, silver, nickel, chromium, zinc, lead, tungsten, iron, etc. The material of the impedance matching part conductor pattern 31 and the antenna part conductor pattern 32 may be a metal oxide such as tin oxide or ITO (indium tin oxide), a conductive film using metal nanowires such as gold, silver or copper, a conductive resin mixture in which metal powder or a conductive carbon material is mixed with resin, or a conductive resin film, etc. The thickness of the impedance matching part conductor pattern 31 and the antenna part conductor pattern 32 is preferably 0.01 to 1000 μm, more preferably 1 to 100 μm, from the viewpoint of flexibility and strength.

The IC chip 20 is a semiconductor component electrically connected to the impedance matching section conductor pattern 31 near the opening 50 of the impedance matching section 30. The IC chip 20 is configured to transmit and receive predetermined information via the antenna section conductor pattern 32 between the IC chip 20 and a communication device such as an RFID reader/writer that is present outside the RFID tag 101.

The IC chip 20 may be arranged on the same layer as the impedance matching portion conductor pattern 31 having the opening 50 of the impedance matching portion 30, or may be arranged on a different layer from the impedance matching portion conductor pattern 31 having the opening 50. The IC chip 20 may be electrically connected to the impedance matching portion conductor pattern 31 having the opening 50 via a lead wire, or may be mounted on an electrode provided on the impedance matching portion conductor pattern 31 having the opening 50. The IC chip 20 may be arranged so as to straddle a slit or cutout formed in the impedance matching portion conductor pattern 31 having the opening 50.

As shown in FIG. 1, in the RFID tag (shape A) 101, the photodetector 21 is electrically connected to the impedance matching section conductor pattern 31 via the photodetector lead pin 40 shown in FIG. 3(a). The photodetector 21 is supported on both sides by the photodetector lead pin 40, giving it a suspended structure.

Figure 2 shows a plan view and a cross-sectional view of an RFID tag 102 (shape B) equipped with the photodetector 21 shown in Figure 3 (c). In the RFID tag 102 of shape B, the photodetector 21 is connected to an opening conductor pattern 33 having a second annular opening 51, which is provided adjacent to an impedance matching section conductor pattern 31 having an opening 50 of an impedance matching section 30. The photodetector 21 is positioned so as to straddle the notch of the opening conductor pattern 33 having the second annular opening 51.

In Figures 1 and 2, the impedance matching section conductor pattern 31 and the antenna section conductor pattern 32 are shown in a contacting arrangement, but the impedance matching section conductor pattern 31 and the antenna section conductor pattern 32 may be connected to each other and made of the same type of conductor, or may be made of different conductors, connected with a conductive paste, or fixed using an adhesive, pressure sensitive adhesive, or adhesive tape, etc.

The RFID tag 101 shown in FIG. 1 and the RFID tag 102 shown in FIG. 2 may be provided with an insulating layer (not shown) that covers the impedance matching section 30 and the antenna section conductor pattern 32, etc.

Figure 4 is a conceptual diagram showing a method of measuring an RFID tag using an RFID reader/writer under light irradiation. As shown in Figure 4, the measuring device 70 has an RFID reader/writer 71 and an antenna 72 of the RFID reader/writer 71. The communication distance CL is the distance between the surface of the antenna 72 and the IC chip of the RFID tag 73 mounted on the base plate 74.

In the measurement system shown in Fig. 4, the irradiance of light from a light source 75 arranged on the opposite side of the RFID reader/writer 71 was changed by using one or more neutral density filters 76 with various transmittances, and the light was irradiated to the photodetector of the RFID tag 73. The irradiance (mW/ ^cm2 ) at this time was measured by a power meter, and the RFID reader/writer 71 wirelessly received information (sensor code S and reader RSSI) from the IC chip 20 when the photodetector of the RFID tag 73 was irradiated with the irradiance (mW/ ^cm2 ), and created a calibration curve relating to the relationship between the sensor code S and the irradiance (mW/ ^cm2 ) and the relationship between the reader RSSI (dBm) and the irradiance (mW/ ^cm2 ). A pseudo-sunlight source consisting of a xenon lamp was used as the light source 75. When irradiating the RFID tag 73 with light, the front and back surfaces of the impedance matching section were covered with black vinyl tape so that light was irradiated only to the photodetector.

Fig. 5 is a graph showing the relationship between the sensor code S and the irradiance (mW/cm2) of light incident on the photodetector when an RFID tag 101 (shape A, L1 = 10 cm, ^L2 = 5 cm, photodetector: photoresistor CdS cell) is used as the RFID tag 73. The data in Fig. 5a is plotted when the communication distance CL is 37 cm (height of tag installation position: center of antenna 72 of RFID reader/writer 71), and the data in Fig. 5b is plotted when the communication distance CL is 11 cm (height of tag installation position: 11 cm below the center of antenna 72 of RFID reader/writer 71), and the relationship between the sensor code S and the irradiance (mW/ ^cm2 ) is consistent despite the difference in the installation position of the RFID tag 73.

By creating in advance a calibration curve showing the relationship between the sensor code S and irradiance (mW/ ^cm2 ) as shown in Figure 5, the irradiance (mW/ ^cm2 ) of the light incident on the photodetector can be calculated from the measurement value of the sensor code S.

The calibration curve shown by the solid curve in Figure 5 is obtained by the following formula (1). Since the relationship between irradiance and sensor code S is nonlinear, the calibration curve was obtained by curve approximation to a double exponential waveform using the following formula.

In this formula (1), x is the irradiance (mW/cm ² ), y is the sensor code S or reader RSSI, which is information from the RFID tag light sensor, and y ₀ , x ₀ , A ₁ , A ₂ , τ ₁ , and τ ₂ are parameters in the curve fit to a double exponential waveform.

6 is a graph showing the dependency of the reader RSSI on irradiance (mW/ ^cm2 ) measured simultaneously with the sensor code S. The calibration curve obtained from formula (1) is shown by a solid curve. The relationship between the reader RSSI and irradiance (mW/ ^cm2 ) differs depending on the installation position of the RFID tag, but this is because the power of the RF waves received by the RFID tag 73 differs depending on the distance and positional relationship between the RFID tag 73 and the antenna 72 of the RFID reader/writer 71, which causes a difference in the reader RSSI, which is the information transmitted from the IC chip of the RFID tag 73 to the RFID reader/writer 71.

Even when the reader RSSI is used as information, if the distance and positional relationship between the RFID tag 73 and the antenna 72 of the RFID reader/writer 71 are constant, the irradiance (mW/ ^cm2 ) of the light incident on the photodetector can be obtained from a calibration curve created by measuring in advance the relationship between the reader RSSI and irradiance (mW/ ^cm2 ), but this is often difficult as a measurement condition. In such cases, it is preferable to observe the irradiance (mW/ ^cm2 ) using the sensor code S, which is information related to the impedance of the impedance matching part of the RFID tag 73.

In the IC chip used in the RFID tag 73 up to now, only the reader RSSI could be obtained as sensing information. The reader RSSI changes depending on the distance between the antenna 72 of the RFID reader/writer 71, obstacles between them, and even the orientation of the RFID tag 73 and the orientation of the antenna 72 of the RFID reader/writer 71. Although it was possible to determine the presence or absence of light in a binary manner, it was difficult to quantitatively measure the irradiance of the incident light. In order to quantitatively measure the irradiance of the incident light, it is an effective method to utilize the change in impedance value specific to the impedance matching part of the RFID tag 73. In the first embodiment of the present invention, the IC chip 20 for UHF band RFID tags, which is capable of measuring the sensor code S, which is information related to the impedance value of the impedance matching part of the RFID tag 73, is used to realize irradiance measurement by a passive RFID tag optical sensor.

Figure 7 is a graph showing the sensor code S (Figure 7(a)) and reader RSSI (Figure 7(b)) when the antenna length L1 is changed in a UHF band RFID tag (shape A) 101. The sensor code S, which is information related to the impedance of the impedance matching part of the RFID tag 101, remains constant, whereas the reader RSSI, which is information related to the strength of the signal received by the RFID reader/writer 71 from the RFID chip, shows a significant decrease as the antenna length L1 becomes shorter. This result also shows that measuring the irradiance (mW/ ^cm2 ) using the sensor code S is a preferable method.

[Embodiment 2]
Next, a second embodiment of the present invention will be described. In the description of the second embodiment, the description of the same configuration, action, and effect as the first embodiment will be omitted or simplified by invoking the description of the first embodiment. In the second embodiment, a highly monochromatic LED light source of various wavelengths is used as the light source, and the response characteristics of an RFID tag optical sensor are shown.

8 shows the change in (a) sensor code S and (b) reader RSSI (communication distance CL = 10 cm) with turning on and off LED light irradiation (wavelength: 390 nm, irradiance: 24.0 mW/ ^cm2 ) to an RFID tag 101 (shape A, L1 = 5 cm, L2 = 5 cm, photodetector: photoresistor CdS cell). The change in sensor code S and reader RSSI with turning on and off the blue LED light was observed with good reproducibility.

As shown in FIG. 8( a), although the irradiance of the blue LED light is high at 24.0 mW/ ^cm2 , the change in the sensor code S is smaller than that predicted from the calibration curve in FIG. 5 when the pseudo sunlight is used as the light source. This is because the spectral sensitivity characteristics at 390 nm of the CdS cell used as the photodetector are low, at approximately 20% of the maximum value around 550 nm.

9 shows the change in (a) sensor code S and (b) reader RSSI (communication distance CL = 10 cm) with turning on and off LED light irradiation (wavelength: 550 nm, irradiance: 6.0 mW/ ^cm2 ) to an RFID tag 101 (shape A, L1 = 5 cm, L2 = 5 cm, photodetector: photoresistor CdS cell). The change in sensor code S and reader RSSI with turning on and off the green LED light was observed with good reproducibility.

As shown in FIG. 9( a), even though the irradiance of the green LED light is low at 6.0 mW/ ^cm2 , the change in the sensor code S is larger than that predicted from the calibration curve in FIG. 5 when the simulated sunlight is used as the light source. This is because the spectral sensitivity characteristics of the CdS cell used as the photodetector have a maximum value near 550 nm.

10 shows the change in (a) sensor code S and (b) reader RSSI (communication distance CL = 10 cm) with turning on and off LED light irradiation (wavelength: 620 nm, irradiance: 2.5 mW/ ^cm2 ) to an RFID tag 101 (shape A, L1 = 5 cm, L2 = 5 cm, photodetector: photoresistor CdS cell). The change in sensor code S and reader RSSI with turning on and off the red LED light was observed with good reproducibility.

Thus, the RFID tag 101 (shape A) showed highly sensitive response characteristics over a wide wavelength range from blue to red, even when using an LED light source with a simple structure.

[Embodiment 3]
Next, a third embodiment of the present invention will be described. In the description of the third embodiment, the description of the same configuration, action, and effect as those of the first and second embodiments will be omitted or simplified by incorporating the description of the first and second embodiments. In the third embodiment, the characteristics of an RFID tag optical sensor having a silicon semiconductor phototransistor as the optical detector 21 shown in Figs. 3(b) and (c) will be described.

As shown in FIG. 3(b), the photodetector 21, which is a phototransistor, consists of a sensor head, two photodetector emitter lead pins 40E and a photodetector collector lead pin 40C, and its shape is almost the same as the CdS cell of the photodetector 21 shown in FIG. 3(a). As shown in FIGS. 1(a) and (b), the photodetector 21 is supported on both sides by the photodetector emitter lead pins 40E and the photodetector collector lead pins 40C, giving it a suspended structure. In contrast, the photodetector 21, which is a phototransistor, shown in FIG. 3(c) is in a small, thin package that is different from the photodetector 21 shown in FIGS. 3(a) and (b).

Figure 11 shows the relationship between the sensor code S and reader RSSI of RFID tag 101 (shape A, L1 = 10 cm, L2 = 5 cm, photodetector shown in Figure 3 (b), communication distance CL = 15 cm) and irradiance. The calibration curve obtained from formula (1) is shown by a solid curve. A pseudo-sun light source consisting of a xenon lamp was used as the light source. Compared to the reader RSSI, the change in sensor code S in response to changes in irradiance was highly sensitive, but compared to the case where a CdS cell was used as the photodetector, the range of change in sensor code S was smaller.

FIG. 12 shows the relationship between the sensor code S and reader RSSI of the RFID tag 102 shown in FIG. 2 (shape B, L1=7.5 cm, L2=7.5 cm, photodetector shown in FIG. 3(c), communication distance CL=25 cm) and irradiance. The calibration curve obtained by formula (1) is shown by a solid curve. A pseudo-sun light source consisting of a xenon lamp was used as the light source. In the RFID tag 102 of shape B, which is provided with a photodetector 21 in the opening 51 in addition to the opening 50, the sensor code S showed more than twice the change in irradiance (mW/cm ² ) compared to the RFID tag 101 equipped with the photodetector shown in FIG. 3(b), and was more sensitive.

11(a) and 12(a), the direction of change in the sensor code S associated with a change in irradiance (mW/ ^cm2 ) is opposite between the RFID tag 101 (shape A) and the RFID tag 102 (shape B). This is a phenomenon that occurs because the influence of the photodetector 21 connected to the impedance matching unit 30 differs between the RFID tag 101 (shape A) and the RFID tag 102 (shape B), and is explained as follows.

The size and shape of the opening of the impedance matching section conductor pattern 31 are designed to be optimal as the impedance matching value of an RFID tag placed in free space (in air). The photodetector 21 electrically connected to such an impedance matching section 30 acts as a negative factor in the impedance matching of the RFID tag. The sensor code S is a value related to the impedance compensated by the auto-tuning mechanism of the IC chip 20 that corresponds to the change in impedance near the impedance matching section 30. The sensor code of an RFID tag equipped with an IC chip 20 in free space (in air) is usually around 250, and varies depending on changes in the surrounding environment.

In RFID tags in which the photodetector 21 is electrically connected to the impedance matching section 30, the sensor codes S are all large values that are significantly different from the value of 250. This is because the electrically connected photodetector 21 acts as a negative factor in the impedance matching of the RFID tag.

3(a) and (b) in the RFID tag 101 of shape A, the photodetector 21 shown in Fig. 3(a) and (b) is supported on both sides by the photodetector lead pins 40 and has a structure floating in the air, so there is no influence of contact with the insulating substrate 10, which is a dielectric, and the impedance matching section insulating substrate 11, and the change in resistance of the photodetector 21 due to light irradiation becomes a factor that affects the impedance of the impedance matching section 30. As shown in Fig. 5, 6, and 11, under conditions where the irradiance (mW/ ^cm2 ) is higher and the resistance value of the photodetector 21 is lower, the sensor code S decreases, and at the same time, the value of the reader RSSI (dBm), which is information related to the strength of the signal transmitted from the IC chip 20 to the RFID reader/writer, increases.

In the RFID tag 102 of the shape B, the photodetector 21 shown in FIG. 3(c) is connected to the opening conductor pattern 33 having an annular opening 51, which is provided adjacent to the impedance matching part conductor pattern 31 having the opening 50 of the impedance matching part 30, as shown in FIG. 2. The photodetector 21 is arranged so as to straddle the notch of the opening conductor pattern 33 having the opening 51. Under the condition where the irradiance (mW/cm ² ) of the light incident on the photodetector 21 is higher, the resistance value of the photodetector 21 becomes lower, and the opening conductor pattern 33 having the opening 51, which was electrically separated by the notch below the photodetector 21, becomes more conductive. As a result, the imaginary part (reactance) of the complex impedance originating from the annular opening 51 consisting of the opening conductor pattern 33 in close contact with the insulating substrate 10 and the impedance matching part insulating substrate 11, which are dielectrics, increases, and the sensor code S increases.

The electrically connected photodetector 21 acts as a negative factor when the impedance matching section 30 of the RFID tag installed in free space (in air) is designed to have an optimal impedance, and it is also possible to design it to be optimal when the photodetector 21 is electrically connected. The size and range of change of the sensor code S can be controlled by the ratio of the area of the opening 50 to the area of the annular opening 51, and the size of each area. The size and range of change of the sensor code S can also be controlled by the number, size, and arrangement of multiple openings formed around the opening 50.

[Specific examples]
Next, specific examples of the embodiment of the present invention will be described. The present invention is not limited to these specific examples. The materials and devices used in the specific examples are as follows.

[material]
(1) Aluminum foil/PET composite film: manufactured by PANAC; AL-PET9-100, aluminum foil thickness 9 μm, PET film thickness 100 μm.
(2) IC chip 20: manufactured by AXZON; Magnus-S3, size 1.6BSC x 1.6BSC, thickness 0.35 mm.
(3) UHF band RFID tag A: Avery Dennison Smartrac; Temperature Sensor Dogbone, IC chip: Axzon Magnus-S3, frequency band: UHF 860-960 MHz, IC chip: IC chip 20 (Magnus-S3).
(4) Adhesive: Bond Ultra Multi-Purpose SU, manufactured by Konishi Corporation.
(5) Elastic Ag paste: XE181G manufactured by Namics Corporation.
(6) Photodetector shown in FIG. 3(a): manufactured by Nanyang Senba Optical & Electronic Co., Ltd.; photoresistor CdS cell GL3516, size φ3 mm, peak sensitivity wavelength 540 nm.
(7) Photodetector shown in FIG. 3(b): manufactured by New Japan Radio Co., Ltd.; silicon phototransistor NJL7502L, lead pin type, peak sensitivity wavelength 560 nm.
(8) Photodetector shown in FIG. 3(c): manufactured by New Japan Radio Co., Ltd.; silicon phototransistor NJL7502R, small and thin package type, package dimensions 1.6×1.3×0.65 mm, peak sensitivity wavelength 590 nm.
(9) Absorption type fixed ND (neutral attenuation) filter (for visible light): manufactured by Sigma Koki Co., Ltd.; AND-50s-70, AND-50s-50, AND-50s-40, AND-50s-30, AND-50s-20, AND-50s-13, AND-50s-10, AND-50s-05, AND-50s-01.
(10) Blue LED: Generic LED Co.; 3 W, emission wavelength 390 nm.
(11) Green LED: Generic LED Co.; 3 W, emission wavelength 550 nm.
(12) Red LED: Generic LED Co.; 3 W, emission wavelength 620 nm.

[Device]
(13) UHF band RFID reader/writer: manufactured by Takaya Co., Ltd.; UTR-S201, specific low power radio station type, transmission frequency 916.8 MHz to 923.2 MHz (18 channels), transmission output 10 dBm (10 mW) to 24 dBm (250 mW), interface USB interface board TR3-IF-U1C.
(14) UHF band external antenna (linearly polarized type): Manufactured by Takaya Co., Ltd.; UTR-UA1709-1.
(15) 355 nm nanosecond pulse laser light source: Inngu laser; Pulse 355-3A, 355 nm, 3 W, repetition rate 30 kHz, pulse width 13 ns.
(16) Galvano scanner: manufactured by MASTER LASER; Model GH2D10C f-θ lens focal length 100 mm, control software EzCAD manufactured by BJJCZ.
(17) Simulated sunlight source: ABET; 10500.
(18) Power meter: Ophir; Power Meter Display Nova.
(19) Power meter head: Ophir; high sensitivity thermal sensor 12A-P-SH.
(20) Single-board microcomputer: Arduino SRL; Arduino Nano

[software]
(21) Homemade software for UHF RFID reader/writer: Using Takaya Co., Ltd. UTR communication protocol ver. 1.15 and serial port control module EasyComm, the RFID reader/writer was controlled and data was acquired from Microsoft Excel. The reader RSSI was acquired from the IC chip 20 (Magnus-S3) according to the Takaya Co., Ltd. UTR communication protocol. The sensor code S was acquired from the IC chip 20 (Magnus-S3) according to the information on memory bank (RESERVED), word address (C _h ), and number of bits (9) published in the RFmicron document (AN002F40: Reading Magnus (registered trademark)-S Sensors).
(22) Graph processing software: WaveMetrics; Igor Pro

[How to create and measure RFID tags]
The RFID tag (shape A) 101 shown in FIG. 1 was produced as follows.
An impedance matching section 30 consisting of an approximately quadrilateral impedance matching section insulating substrate 11 (X-axis length L3: approximately 14 mm, Y-axis width W1: approximately 6 mm) including an impedance matching section conductor pattern 31 having an opening 50 and an IC chip 20 was cut out from a UHF band RFID tag A (IC chip 20). The impedance matching section insulating substrate 11 is provided with an impedance matching section conductor pattern 31 (X-axis direction: approximately 14 mm, Y-axis direction: approximately 4.7 mm) made of aluminum on the positive side in the Z-axis direction, the impedance matching section conductor pattern 31 having an approximately quadrilateral hole-like opening 50 (X-axis direction length: approximately 7.9 mm, Y-axis direction width: approximately 2.7 mm, four corner radius: approximately 0.5 mm).

Next, the insulating layer of the UHF band RFID tag A (IC chip 20) that covered the impedance matching part conductor pattern 31 having the opening 50 of the impedance matching part insulating substrate 11 was removed to expose the impedance matching part conductor pattern 31 made of aluminum.

The antenna conductor pattern 32 in Fig. 1 was created by laser cutting an aluminum foil/PET composite film by focusing 355 nm laser light with an f-θ lens and scanning with a galvanometer scanner. The antenna conductor pattern 32 has an approximately quadrilateral opening 50 with a Y-axis width W1 of approximately 6 mm, similar to the opening 50 of the impedance matching section 30, which has an approximately quadrilateral hole-like opening (X-axis length: approximately 7.9 mm, Y-axis width: approximately 2.7 mm, corner radius: approximately 0.5 mm) and a notch (X-axis length: approximately 4 mm).

The impedance matching part insulating substrate 11 with the impedance matching part 30 cut out from the UHF band RFID tag A (IC chip 20) and the antenna part conductor pattern 32 created by laser cutting were fixed with adhesive so that the positions of the openings 50 were aligned. At this time, the aluminum foil side of the aluminum foil/PET composite film pattern created by laser cutting was the side of the impedance matching part conductor pattern 31 where the impedance matching part 30 was exposed.

The two photodetector lead pins 40 of the photodetector 21 were fixed to both ends of the exposed impedance matching section conductor pattern 31 of the impedance matching section 30 in the X-axis direction with elastic Ag paste to establish an electrical connection.

In the measurement device 70 shown in Figure 4, the output of the RFID reader/writer 71 was set to 24 dBm (250 mW), and a linearly polarized UHF band external antenna was used for the antenna 72. The RFID tag 73 was fixed to a polystyrene foam base plate 74 with tape and placed at a position of communication distance CL for measurement. Polystyrene foam is a low dielectric constant material that contains many voids inside, and does not affect the wireless communication characteristics between the IC chip 20 of the RFID tag 73 and the RFID reader/writer 71. The communication distance CL was defined as the distance between the surface of the antenna 72 and the IC chip 20 of the RFID tag 73.

Antenna 72 is a linearly polarized UHF band external antenna, and was installed so that the linear polarization was parallel to the floor surface. In the measurement, RFID tag 73 was held on a polystyrene foam base plate 74 with the polarization direction aligned so that the longitudinal direction of RFID tag 73 was parallel to the floor surface. A circularly polarized type UHF band external antenna for RFID reader/writer 71 can also be used for communication with RFID tag 73. When a circularly polarized antenna 72 is used, the communication distance is shorter than when a linearly polarized antenna 72 is used, but communication with RFID tag 73 is possible by irradiating radio waves from a wider angle.

The irradiance of light from the light source 75 arranged on the opposite side of the RFID reader/writer 71 was changed using a neutral density filter 76 with various transmittances, and the light was irradiated onto the photodetector 21 of the RFID tag 73. The irradiance (mW/cm ² ) at this time was measured with a power meter, and the RFID reader/writer 71 wirelessly received information (sensor code S and reader RSSI) from the IC chip 20 when the photodetector 21 of the RFID tag 73 was irradiated with the irradiance (mW/cm ² ) at that time, and created a calibration curve relating to the relationship between the sensor code S and the irradiance (mW/cm ² ) and the relationship between the reader RSSI (dBm) and the irradiance (mW/cm ² ). A pseudo-sunlight source consisting of a xenon lamp was used as the light source 75. When irradiating the RFID tag 73 with light, the front and back surfaces of the impedance matching section 30 were covered with black vinyl tape so that only the photodetector 21 was irradiated with light.

[Evaluation of RFID tag optical sensor characteristics]
Graphs showing the relationship between the sensor code S and the irradiance (mW/cm2) of light incident on the photodetector in an RFID tag 101 (shape A, L1=10 cm, L2=5 cm, photodetector shown in FIG. 3(a): photoresistor CdS cell) and graphs showing the relationship between the reader RSSI and the irradiance (mW/ ^cm2 ) of light incident on the photodetector are shown in FIG. 5 and FIG. 6, respectively. The calibration curves shown by solid curves in FIG. 5 and FIG. 6 ^were obtained by curve approximation using formula (1) with graph processing software.

The data in Fig. 5 and Fig. 6a are plots for a case where the communication distance CL is 37 cm and the tag is installed at the center of the antenna 72 of the RFID reader/writer 71. The data in Fig. 5 and Fig. 6b are plots for a case where the communication distance CL is 11 cm, but the tag is installed 11 cm below the center of the antenna 72 of the RFID reader/writer 71, and the power that the RFID tag 73 receives from the RFID reader/writer 71 is low.

5, the relationship between the sensor code S and the irradiance (mW/ ^cm2 ) is consistent and does not depend on the installation position of the RFID tag 73. In contrast, the relationship between the reader RSSI and the irradiance (mW/ ^cm2 ) depends on the installation position of the RFID tag 73. This is because the reader RSSI, which is a value related to the power received by the RFID reader/writer 71 from the RFID tag 73, changes depending on the distance between the RFID reader/writer 71 and the antenna 72 of the RFID reader/writer 71, obstacles present between them, and further the orientation of the RFID tag 73 and the antenna 72 of the RFID reader/writer 71.

[Antenna length dependence of information from RFID tag optical sensor]
7 is a graph showing the sensor code S (FIG. 7(a)) and the reader RSSI (FIG. 7(b)) when the antenna length L1 is changed in a UHF band RFID tag (shape A) 101. The sensor code S, which is information related to the impedance of the impedance matching section 30 of the RFID tag 101, remains constant, whereas the reader RSSI, which is information related to the strength of the signal received by the RFID reader/writer 71 from the RFID chip, shows a significant decrease as the antenna length L1 becomes shorter. This result also shows that in order to quantitatively measure the irradiance of incident light, it is an effective method to utilize the change in impedance value specific to the impedance matching section 30 of the RFID tag 101.

[LED light responsiveness of RFID tag optical sensor]
The passive RFID tag light sensor according to the embodiment of the present invention is intended to be used in a system in combination with a passive LED tag light source, so the responsiveness to LED light was confirmed. A blue LED (emission wavelength 390 nm), a green LED (emission wavelength 550 nm), and a red LED (emission wavelength 620 nm) were used as the LED light. The LED light was irradiated by periodically turning on and off the voltage application to the LED light source by controlling the opening and closing of the relay unit with a TTL signal from a one-board microcomputer (Arduino). When irradiating the RFID tag with LED light, the front and back surfaces of the impedance matching section 30 were covered with black vinyl tape so that light was irradiated only to the photodetector 21.

8, 9, and 10 show the response behavior of the sensor code S and the reader RSSI when the light irradiation from a blue LED (light emission wavelength 390 nm, 24.0 mW/cm ² ), a green LED (light emission wavelength 550 nm, 6.0 mW/cm ² ), and a red LED (light emission wavelength 620 nm, 2.5 mW/cm ² ) to an RFID tag 101 (shape A, L1 = 5 cm, L2 = 5 cm, photodetector shown in FIG. 3(a): photoresistor CdS cell) is turned on and off. A rapid and reproducible response to light irradiation was observed for all wavelengths of LED light. In terms of the irradiance (mW/cm ² ) and the amount of change in each information, a highly sensitive response characteristic was obtained for the green LED and red LED, which are wavelengths to which the photodetector 21 is highly sensitive. This is effective in detecting blood in urine or occult blood, etc., by detecting changes in transmittance at wavelengths that are hardly affected by absorption by the yellow urine itself.

[Passive RFID tag optical sensor equipped with the optical detector shown in FIG. 3(b)]
In the above passive RFID tag optical sensor, a CdS cell, which is a photoresistor, is used as the photodetector 21 shown in Fig. 3(a). The silicon phototransistor of the photodetector 21 shown in Fig. 3(b) is of the same lead pin type as the CdS cell, and is therefore suitable for replacing the CdS cell.

Fig. 11 shows the relationship between the sensor code S and the reader RSSI and the irradiance in the RFID tag 101 (shape A, L1 = 10 cm, L2 = 5 cm, photodetector shown in Fig. 3(b), communication wavelength CL = 15 cm). A pseudo-sun light source consisting of a xenon lamp was used as the light source. Although there was a difference in the range of change, the changes in the sensor code S and the reader RSSI associated with the change in the irradiance (mW/ ^cm2 ) of the incident light were shown, similar to the case where a CdS cell was used as the photodetector 21.

[Passive RFID tag optical sensor equipped with the optical detector shown in FIG. 3(c)]
Fig. 2 shows a plan view and a cross-sectional view of an RFID tag 102 (shape B) equipped with a photodetector 21 shown in Fig. 3(c). In the RFID tag 102 of shape B, the photodetector 21 shown in Fig. 3(c) is connected to an opening conductor pattern 33 having a ring-shaped opening 51 provided adjacent to an impedance matching portion conductor pattern 31 having an opening 50 of an impedance matching portion 30. The RFID tag 102 (shape B) was created as follows.

In the creation of the RFID tag 102 (shape B), the impedance matching part insulating substrate 11 having the impedance matching part 30 consisting of the IC chip 20, the impedance matching part conductor pattern 31, and the opening 50 was the same as that of the RFID tag 101 (shape A). The antenna conductor pattern 32 shown in FIG. 2(d) was created by laser cutting an aluminum foil/PET composite film by focusing 355 nm laser light with an f-θ lens and scanning with a galvano scanner. The antenna part conductor pattern 32 has a Y-axis width W1 of approximately 6 mm, an X-axis length (approximately 7.9 mm) similar to the opening 50 of the impedance matching part 30, and an opening with a Y-axis width (approximately 3.9 mm) wider than the opening 50.

By joining the opening 50 of the impedance matching section 30 with the upper position of the opening of the antenna conductor pattern 32 shown in FIG. 2(d), an opening 51 is formed as shown in the plan view of FIG. 2(b). The photodetector shown in FIG. 3(c) was electrically connected to the cutout portion of the opening of the antenna conductor pattern 32 using elastic silver paste, thereby creating an RFID tag 102 of shape B.

Fig. 12 shows the relationship between the sensor code S and reader RSSI and the irradiance in an RFID tag 102 (shape B, L1 = 7.5 cm, L2 = 7.5 cm, photodetector shown in Fig. 3(c), communication distance CL = 25 cm). A pseudo-sun light source consisting of a xenon lamp was used as the light source. The RFID tag 102 of shape B, which is provided with a photodetector 21 in opening 51 in addition to opening 50, showed a change in the sensor code S with a sensitivity more than twice as high as that of the RFID tag 101 equipped with the photodetector 21 shown in Fig. 3(b) in response to a change in irradiance (mW/ ^cm2 ).

The present invention provides a flexible, thin, passive RFID tag optical sensor that can quantitatively measure the irradiance of incident light, functions as an irradiance meter, and does not require batteries or an external power source. This sensor can be suitably used in wireless sensing in combination with various light sources.

Although the embodiments have been described above, the technology of the present invention is not limited to the above embodiments. Various modifications and improvements are possible, such as combinations or substitutions with part or all of other embodiments.

REFERENCE SIGNS LIST 10 insulating substrate 11 impedance matching portion insulating substrate 20 IC chip 21 photodetector 30 impedance matching portion 31 impedance matching portion conductor pattern 32 antenna portion conductor pattern 33 opening portion conductor pattern 40 photodetector lead pin 40C photodetector collector lead pin 40E photodetector emitter lead pin 50 opening 51 opening 70 measuring device 71 RFID reader/writer 72 antenna 73 RFID tag 74 base plate 75 light source 76 neutral density filter 101, 102 RFID tag

Claims (3)

An insulating layer;
an impedance matching section provided on the insulating layer, the impedance matching section including a conductor pattern having an annular opening and an IC chip electrically connected to the conductor pattern in the vicinity of the opening;
an antenna conductor portion extending in a first direction and a second direction from the impedance matching portion;
an RFID tag having a photodetector electrically connected to an end of the conductor pattern having the annular opening in the first direction and an end of the conductor pattern having the annular opening in the second direction, the photodetector forming a parallel circuit with the IC chip ;
A change in irradiance of light incident on the photodetector is detected as a change in a reader RSSI transmitted from the IC chip to an RFID reader/writer by wireless communication.
Passive RFID tag optical sensor. An insulating layer;
an impedance matching section including a first conductor pattern provided on the insulating layer and having a first annular opening, and an IC chip electrically connected to the first conductor pattern in the vicinity of the first opening;
a second conductor pattern provided adjacent to the first conductor pattern of the impedance matching portion and having a second annular opening;
an antenna conductor portion extending in a first direction and a second direction from the impedance matching portion;
an RFID tag having a photodetector electrically connected to the second conductor pattern;
The light incident on the photodetector is detected as a change in a reader RSSI transmitted from the IC chip to an RFID reader/writer by wireless communication.
Passive RFID tag optical sensor. A method for determining the irradiance of light using the passive RFID tag optical sensor according to claim 1 or 2 , comprising:
A method for determining the irradiance of light, comprising: creating in advance a calibration curve showing the relationship between the irradiance of light incident on the photodetector and the change in the value of the reader RSSI caused by the light of the irradiance; and determining the irradiance of the light incident on the photodetector from the measured value of the reader RSSI using the calibration curve.