JP3157463B2 - Photoelectric sensor and color sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a photoelectric sensor and a color sensor having an AC amplifier circuit composed of an operational amplifier.

[0002]

2. Description of the Related Art FIG. 6 shows a so-called color sensor for performing color discrimination using a photoelectric sensor. The color sensor is provided with light emitting elements 1, 2, and 3 that emit red, blue, and green light, respectively.
It is called PU. The light projecting operation is performed on the device X based on the light projecting signals f1, f2, and f3 output (high level) from (8) (see FIG. 8). Further, a light receiving element 4 is provided to receive the reflected light from the device under test X for each light emitting operation, and each light emitting signal f1, f
The light receiving signal a is output (low level) from the light receiving circuit 5 in synchronization with 2 and f3 (see FIG. 8). Further, since the light receiving signal a is minute, an amplifying means 6 for amplifying the light receiving signal a is provided. The light receiving signal a is amplified by the amplifying means 6 and then sent to the CPU 8. Then, the CPU 8 performs color discrimination based on the light receiving signal a.

The amplifying means 6 is constituted by connecting a plurality of amplifying circuits 7 shown in FIG. Each of the amplifying circuits 7 is a non-inverting type AC amplifying circuit including an operational amplifier A, and has a coupling capacitor C and a resistor R on its positive-phase input side for removing a DC component which is a disturbance light component.
And a differentiating circuit composed of

[0004]

However, the coupling capacitor C is indispensable as an AC amplifying circuit for removing a DC component. However, after the light receiving signal a is inputted, the electric charge accumulated at the end thereof is received. Therefore, after rising of the light receiving signal a, ringing due to overshoot and undershoot is induced in the output signal b of the operational amplifier A (see FIG. 8). In particular, in the case where the AC amplifier circuits are connected in multiple stages as in the above configuration, the ringing becomes considerably large in the final output. Therefore, in order to perform the next light emitting operation, it is necessary to wait after the ringing is attenuated, that is, in the above configuration, until the output signal b is stabilized at Vcc. I couldn't do that. Accordingly, in the color sensor, the light emitting and receiving operations for red, blue, and green are continuously performed on the object X. However, since the intervals cannot be increased more than a predetermined value, for example, When the object X is moving, the measurement points for each color are shifted, and there is a problem that accurate color determination cannot be performed. Specifically, when it is assumed that the amount of reflected light for each color is the same, the measurement point is shifted with respect to each light emitting operation due to the movement of the object X, and as a result, as shown in the lower part of FIG. Voltage differences d and e occur between the light receiving signal a and the output signal b, which should normally be at the same level, and erroneous color discrimination.

On the other hand, in order to solve the above-mentioned problem in the color sensor, as a method of increasing the light emitting / receiving operation interval, as shown in FIG.
Color filters F1, F2, F3, light receiving element 1 respectively
4, 15, 16 and amplification means 17, 18, 19,
It is conceivable to amplify the light receiving signal a for each color in parallel. That is, even if ringing occurs in the output signal b of the amplifier circuit 20 (the AC amplifier circuit shown in FIG. 7), the light emitting operation can be shifted to the next light emitting operation regardless of the ringing. You can speed it up. However, in this configuration, since it is necessary to provide a light receiving element, an amplifying means, and the like for each color, there is a disadvantage that not only the circuit becomes complicated but also the economic burden increases. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a photoelectric sensor and a color sensor that can increase a measurement interval with a simple configuration.

[0006]

In order to achieve the above object, a photoelectric sensor according to claim 1 includes a light receiving element for receiving light emitted from a light emitting element, and a light emitted from the light emitting element. In a photoelectric sensor having an AC amplifier circuit for amplifying a light receiving signal output from a light receiving element at the time of receiving light, the AC amplifier circuit is an amplifier circuit having a differentiating circuit including a coupling capacitor and a resistor at its input. And, the AC amplifying circuit is provided with discharging means for rapidly discharging the electric charge stored in the coupling capacitor by short-circuiting the resistance of the differentiating circuit, so that the light receiving signal is in a non-output state.
It is characterized in that the discharge means is operated at a predetermined timing after the discharge.

According to a second aspect of the present invention, a photoelectric sensor includes a light receiving element for receiving light emitted from the light emitting element, and amplifies a light receiving signal output from the light receiving element when receiving light emitted from the light emitting element. In the photoelectric sensor provided with an AC amplifier circuit, the AC amplifier circuit includes a feedback resistor between the output side of the operational amplifier and the negative-phase input side, and a differential circuit including a coupling capacitor and a resistor on the positive-phase input side. A non-inverting amplifying circuit equipped with an AC amplifier and a discharging means for rapidly discharging the charge stored in the coupling capacitor by short-circuiting the resistance of the differentiating circuit, and a feedback resistor in parallel. an input voltage correction means for feeding back via a capacitor for drift voltage remove the output voltage to the inverting input of the operational amplifier provided operational amplifier provided, the light receiving signal
It is characterized in that the discharge means and the input correction means are operated at a predetermined timing after the state of the power supply becomes non-output state .

According to a third aspect of the present invention, in the color sensor according to the first or second aspect, the light projecting element comprises a plurality of light projecting elements for sequentially projecting different colors to the object to be measured. Is characterized in that the color of the device under test is determined based on the ratio of each signal level output from the AC amplifier circuit according to the amount of reflected light from the device under test.

[0009]

According to the first aspect of the present invention, when the light receiving signal is output from the light receiving element, the DC component is removed by the coupling capacitor, and only the AC component is supplied to the operational amplifier. Is entered. At this time, the coupling capacitor will be charged. Then, the charged electric charges are slowly discharged through the resistance constituting the differentiating circuit together with the coupling capacitor at the same time as the output of the light receiving signal is finished. In the present invention, the light receiving signal is not output.
Discharge means operates at a predetermined timing after entering the state
Therefore, the charged electric charge is discharged at once at that time, and the input side of the operational amplifier becomes the reference potential. That is, even if an undershoot or overshoot occurs in the output signal of the operational amplifier due to the provision of the coupling capacitor, the time required for the undershoot or overshoot to converge to the reference potential can be shortened. Therefore, there is an effect that the time until the next light emitting operation is started is shortened, and the measurement interval can be shortened.

<Invention of Claim 2> In the invention of Claim 2,
The AC amplifying circuit is a non-inverting type amplifying circuit, and a differential circuit is formed on the positive phase input side by a coupling capacitor and a resistor. When the discharging means is operated in the same manner as described above, the charge of the coupling capacitor charged by the light receiving signal is discharged at a stretch. Here, even if the electric charge stored in the coupling capacitor can be discharged at once, a voltage is generated by the electric charge stored in the coupling capacitor on the positive-phase input side of the operational amplifier. Appears at the output of the amplifier. However, in the present invention, since the input voltage correcting means operates together with the discharging means, this output voltage is fed back to the negative-phase input side of the operational amplifier, and both input voltages of the operational amplifier eventually act in the same direction, and the coupling capacitor is operated. Is not amplified.

Furthermore, when the output voltage of the operational amplifier is fed back, only the AC component is fed back through the capacitor, and the drift voltage inherent to the operational amplifier, which is the DC component, is not fed back. This is nothing but the fact that the fluctuation of the drift voltage of the operational amplifier before and after the operation of the input voltage correction means can be suppressed. Therefore, in addition to the effect that the measurement interval can be shortened by discharging the electric charge charged in the coupling capacitor at once, the amplification of the voltage charged in the coupling capacitor can be prevented, and the change in the drift voltage can be suppressed. Also, there is an effect that the received light signal can be accurately measured without errors.

<Invention of claim 3> In the invention of claim 3,
Lights of different colors are sequentially irradiated on the object to be measured. Then, a light receiving signal is output from the light receiving element in accordance with the reflected light from the object to be measured for each color, and the light receiving signal is amplified by the AC amplifier circuit described above. Then, the color of the device under test is determined from the ratio of the signal level of the output signal to each of the amplified colors. Therefore, since the measurement interval can be shortened by using the above-described AC amplifier circuit,
Even if the object to be measured moves, the measurement points for each color do not shift, and the amount of reflected light can be measured accurately, so that accurate color judgment can be performed.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION

<First Embodiment> One embodiment of a photoelectric sensor according to the present invention will be described below with reference to FIGS.
The photoelectric sensor according to the present embodiment is applied to a color sensor, and a circuit diagram thereof is shown in FIG.

In FIG. 1, reference numeral 31 denotes a central processing unit (hereinafter, referred to as a CPU), which controls a light projecting / receiving operation and performs a color discrimination based on a light receiving signal. The CPU 31 supplies a red light through light emitting circuits 32, 33 and 34, respectively.
Light emitting elements 35, 36 and 37 that emit blue and green light are connected. Then, based on the light emission signals f1, f2, f3 (high level) output from the CPU 31, the light emission operation is performed on the DUT X.

A light receiving element 38 is provided to receive the reflected light of the light emitted from each of the light projecting elements 35, 36, 37 to the object X, and this light receiving element 38 is provided. Is connected to the light receiving circuit 39. On the output side of the light receiving circuit 39, there is provided an amplifying means 41 formed by connecting an AC amplifier circuit 40 to be described later in a plurality of stages.
After amplifying the light receiving signal a (low level) output from the CPU 31, the light receiving signal a is sent to the CPU 31.

As shown in FIG. 2, each AC amplifying circuit 40 is a non-inverting type amplifying circuit provided with one operational amplifier A, and has a coupling capacitor C1 and a resistor R
1 is connected. The other side of the resistor R1 is connected to a power supply (Vc
c) connected to 42; That is, the light receiving signal a output from the light receiving circuit 39 is differentiated and then input to the positive-phase input side of the operational amplifier A. As a result, the DC component which is a disturbance light component is removed, and only the AC component is input to the operational amplifier A. The resistor R1 forms a differentiating circuit on the positive-phase input side of the operational amplifier A,
It has a function of flowing a bias current to the operational amplifier A. A feedback resistor R is connected between the negative-phase input side of the operational amplifier A and the output side.
2 is inserted, and a voltage dividing circuit is configured by the feedback resistor R2 and the resistor R3 such that the voltages on the negative-phase input side and the positive-phase input side of the operational amplifier A become substantially equal in the non-input state.

An analog switch SW1 is provided in parallel with the resistor R1, and is closed based on a discharge start signal g (high level) output from the CPU 31. Further, a capacitor C2 for removing a drift voltage and an analog switch SW2 are connected in parallel with the feedback resistor R2.
Is provided, and the analog switch SW2 is connected to the CPU 3
1 is closed on the basis of the correction signal h. Note that the analog switch SW1 corresponds to the discharging means according to the present invention, and the analog switch SW2 corresponds to the input voltage correcting means according to the present invention.

Next, the operation of the present embodiment will be described with reference to the timing chart shown in FIG. When the light emission signals f1, f2, and f3 are continuously output (high level) from the CPU 31, the light emission elements 35, 36, and 3
Light (red, blue, green) is emitted in order from 7. Then, three light receiving signals a are continuously output (low level) from the light receiving circuit 39 based on the reflected light from the device X for each light. Each light receiving signal a is sent to the positive-phase input side of the operational amplifier A via the coupling capacitor C1. At this time, the DC component which is a disturbance light component is removed by the coupling capacitor C1, and only the AC component is sent to the operational amplifier A.

The coupling capacitor C1 is charged when the light receiving signal a is output (low level) from the light receiving circuit 39. Then, when the light receiving signal a becomes non-output state (high level), the stored charge is discharged through the resistor R1. However, in the case of the present embodiment,
Since the discharge start signal g is output from the CPU 31 at a predetermined timing after the light receiving signal a becomes non-output state, the analog switch SW1 is closed, and the positive-phase input side of the operational amplifier A is directly connected to the power supply 42. Then, the coupling capacitor C
Since the charge stored in 1 is discharged at once, the positive-phase input side of the operational amplifier A becomes the reference potential (Vcc). Therefore, after the light receiving signal a is in the non-output state, the coupling capacitor C1 does not repeat charging and discharging, and the signal i input to the positive-phase input side of the operational amplifier A becomes the light receiving signal a.
Becomes non-output state, the coupling capacitor C1
Is discharged to converge to the reference potential.

Further, the discharge start signal g is output from the CPU 31, the correction signal h is output from the CPU 31, and the analog switch SW2 is closed. Therefore, even if a voltage is generated on the positive-phase input side of the operational amplifier A due to the electric charge accumulated in the coupling capacitor C1 and the voltage appears at the output of the operational amplifier A, the voltage is inverted via the capacitor C2. It is fed back to the phase input side. Therefore,
Acts in a direction in which both input voltages of the operational amplifier A become equal,
The voltage charged in the coupling capacitor C1 is prevented from being amplified by the operational amplifier A. That is, as shown in FIG. 3, the overshoot based on the voltage charged in the coupling capacitor C1 is removed from the output signal b of the operational amplifier A.

Since the output signal b of the operational amplifier A becomes the reference potential by closing the analog switch SW2, the discharging of the coupling capacitor C1 is also accelerated. That is, when the AC amplifiers 40 are connected in multiple stages as in the present embodiment and the analog switch SW2 is not provided, the output signal b of the AC amplifier 40 is charged to the coupling capacitor C1. Overshoot of the voltage occurs, which prevents discharge of the electric charge accumulated in the coupling capacitor C1 on the downstream side (the next stage of the AC amplifier circuit). Therefore, when the AC amplifier circuits 40 are connected in multiple stages, the discharge cannot be substantially accelerated by merely closing the analog switch SW1, and the effect can be exhibited by closing the analog switch SW2. become.

Further, when the voltage appearing at the output of the operational amplifier A is fed back, only the AC component is fed back through the capacitor C2, and the drift voltage inherent to the operational amplifier A, which is the DC component, is fed back. Not done. This means that the change of the drift voltage of the operational amplifier A before and after the analog switch SW2 is opened and closed due to the closing of the analog switch SW2 can be suppressed. To explain this in more detail, when only the DC component is considered, when the analog switch SW2 is closed without the capacitor C2, the feedback resistor R2 is disabled and the amplification factor before and after the analog switch SW2 is opened and closed changes. become. However, if the capacitor C2 is provided, the feedback resistor R2 still works effectively even if the analog switch SW2 is closed, and the analog switch SW2 is closed.
The amplification factor before and after opening and closing does not fluctuate. Therefore, by providing the capacitor C2, the analog switch SW
The fluctuation of the drift voltage before and after the opening and closing of No. 2 can be suppressed.

The timing for opening the analog switches SW1 and SW2 is as follows.
To open. This is the analog switch SW
When the analog switch SW1 is opened after the analog switch SW1 is opened, the feedback resistor R2 becomes effective by opening the analog switch SW2, thereby preventing the switching noise generated when the analog switch SW1 is opened from being amplified. is there.

As described above, in this embodiment, the electric charge stored in the coupling capacitor C1 can be discharged at a stroke, and the positive-phase input side of the operational amplifier A can be used as the reference potential. Therefore, even if an overshoot or the like occurs in the output signal b of the operational amplifier A, it can be converged in a short time (in FIG. 3, the overshoot converging in a short time is removed by the input correction means. .). Therefore, it is possible to shorten the time T until the next light projection operation is started, and to shorten the measurement interval. Moreover, the operational amplifier A
Is output to the negative-phase input side via the drift voltage removing capacitor C2, so that the voltage charged in the coupling capacitor C1 can be amplified to prevent overshoot, and the drift voltage can be prevented. Can be suppressed, and the light receiving signal a can be measured accurately without error. As a result, in the color sensor, even if the object X to be measured moves, the amount of received light can be accurately measured without shifting the measurement point, and color determination can be performed.

<Second Embodiment> In a second embodiment, a case where the photoelectric sensor is applied to a multi-optical axis photoelectric sensor will be described. As shown in FIG. 4, this multi-optical axis photoelectric sensor has a plurality of pairs of light emitting and receiving elements 51 and 52 arranged side by side. A light receiving circuit 54 is provided. Further, each of the light emitting and receiving elements 51 and 5
2 in order to perform light emitting and receiving operations in a certain cycle in order,
Gate circuits 55 and 56 are provided.

The light receiving signal a output from the gate circuit 56 in synchronization with each light emitting / receiving operation is amplified by the amplifying means 57 as in the first embodiment, and then sent to the CPU 58. As in the first embodiment, the amplifying means 57 is configured by connecting a plurality of stages of the non-inverting type AC amplifying circuit 40 shown in FIG.
Each of the AC amplifier circuits 40 includes the analog switches SW1 and SW2 and the capacitor C2 for removing the drift voltage, as described above.

Therefore, also in the present embodiment, the discharge start signal g
Thus, the electric charge accumulated in the coupling capacitor C1 is discharged at a stroke to make it a reference potential, so that the convergence time such as an overshoot generated in the output signal b of the operational amplifier A can be shortened. Furthermore, since the voltage charged in the coupling capacitor C1 can be fed back to the negative-phase input side of the operational amplifier A via the drift voltage removing capacitor C2, the voltage charged in the coupling capacitor C1 is amplified. And the fluctuation of the drift voltage can be suppressed. As a result, each light emitting operation interval T
And the multi-optical axis sensor can detect an object moving at a high speed.

<Third Embodiment> In the first embodiment, the AC amplifier circuit 40 is a non-inverting amplifier circuit. However, in the present embodiment, the AC amplifier circuit 40 is an inverting amplifier circuit. That is, as shown in FIG. 5, the AC amplifying circuit 40 includes one operational amplifier A, and has a power supply (Vcc) 6 on its positive-phase input side.
1 is connected. On the opposite-phase input side of the operational amplifier A, a coupling capacitor C3 and a resistor R4 are connected in series to constitute a differentiating circuit. Further, a feedback resistor R5 is inserted between the output side of the operational amplifier A and the negative phase input side. Further, analog switches SW3 and SW4 are provided in parallel with the resistor R4 and the feedback resistor R5, respectively, and are closed by the discharge start signal g and the correction signal h output from the CPU 31 (see FIG. 1) as in the first embodiment. It is supposed to be.

Therefore, also in this embodiment, the discharge start signal g
By closing the analog switch SW3,
Since the electric charge accumulated in the coupling capacitor C3 is discharged at once, the convergence time of the overshoot generated in the output signal b of the operational amplifier A can be shortened. Further, by closing the analog switch SW4 with the correction signal h, the occurrence of overshoot itself can be prevented. Further, in the inverting type amplifier circuit as in the present embodiment, since the amplification degree is smaller than that in the non-inverting type amplifier circuit shown in the first embodiment, the generation of the drift voltage is small, and
The capacitor C2 for removing the drift voltage as in the embodiment
(See FIG. 2).

When the inverting amplifier circuits are connected in multiple stages, the output signal b is alternately inverted to the plus side and the minus side centering on the reference potential every time through the operational amplifier A. Therefore, if the reference potential is set to a value close to the power supply voltage of the operational amplifier A, it is not possible to obtain a sufficient output with respect to the positive side swing. For example, when the power supply voltage of the operational amplifier A is 7 V, the reference potential of the operational amplifier A is 5
Assuming that the amplitude is V, only the plus side amplitude of the output signal can be output at 2 V. Therefore, it is desirable that the reference potential of the operational amplifier A whose output is inverted to the plus side be lower than the reference potential of the operational amplifier A whose output is inverted to the minus side. Specifically, in the present embodiment, since the light receiving signal a output from the light receiving circuit 39 is at a low level, the reference potential of the operational amplifier A of the first stage (the odd-numbered stage) is set to 2-3 V, The reference potential of the operational amplifier A of the second stage, the final stage (even stage) is 5V.

If the reference potential is made different for each operational amplifier A as described above, the switching noise generated when the analog switch SW3 is operated becomes large due to the potential difference between both ends of the coupling capacitor C3.
After opening the analog switch SW3, it is desirable to open the analog switch SW4 sufficiently late. Note that the present invention is not limited to the above-described embodiment, and can be implemented with, for example, the following modifications.
These embodiments also belong to the technical scope of the present invention. (1) In each of the above embodiments, the discharging means and the input voltage correcting means include the analog switches SW1, SW2, SW3,
Although the SW4 is used, the switching operation may be performed using a transistor, a relay, or the like.

(2) In the first and second embodiments, the reference potential is Vcc (for example, 5 V). However, the reference potential may be 0 V. In this case, the other side of the resistor R1 and one side of the resistor R3 are connected to the ground of the circuit, and the output state of the light receiving signal is at a high level.

(3) In the above embodiments, the photoelectric sensor of the present invention is applied to a color sensor or a multi-optical axis photoelectric sensor. However, the present invention can be applied to various other photoelectric sensors.

(4) In the first and second embodiments, the AC amplifier circuit 40 having the discharging means and the input correcting means has been described. However, only the discharging means, that is, the analog switch SW1 is provided, and the drift voltage is removed. AC amplifier circuit in which the capacitor C2 for use and the analog switch SW2 are omitted. In this case, an overshoot occurs in the output signal of the operational amplifier A. However, since the convergence time is short as in the above-described embodiment, there is an effect that the light emitting operation interval T can be increased. There is no change. Further, also in the third embodiment, the analog switch SW4
, And may be an inverting type AC amplifier circuit provided with only the analog switch SW3.

(5) In the first embodiment, the light emission colors of the color sensor are red, blue, and green, but other colors may be used.

(6) In each of the above embodiments, the AC amplifier circuit 40 is constituted by the operational amplifier A, but may be an amplifier circuit which does not use an operational amplifier. Further, when the operational amplifier circuit is not used, it is not necessary to use a feedback type.

(7) In each of the above embodiments, the amplifying means 4
1 and 57 are configured by connecting the AC amplifier circuit 40 in a plurality of stages, but may be configured by one stage. In addition, the present invention can be implemented with various modifications without departing from the scope.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an entire first embodiment;

FIG. 2 is a circuit diagram showing an AC amplifier circuit according to the first embodiment.

FIG. 3 is a timing chart in the embodiment.

FIG. 4 is a circuit diagram showing the entire second embodiment.

FIG. 5 is a circuit diagram showing an AC amplifier circuit according to a third embodiment.

FIG. 6 is a circuit diagram showing a conventional color sensor.

FIG. 7 is a circuit diagram showing a conventional AC amplifier circuit.

FIG. 8 is a timing chart in a conventional color sensor.

FIG. 9 is a circuit diagram showing another conventional color sensor.

[Explanation of symbols]

 SW1: Analog switch (discharging means) SW2: Analog switch (input voltage correcting means) SW3: Analog switch (discharging means) SW4: Analog switch (input voltage correcting means) A: Operational amplifier R1: Resistance R2: Feedback resistance R4: Resistance R5: feedback resistor C1: coupling capacitor C2: capacitor for removing drift voltage C3: coupling capacitor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-55-10528 (JP, A) JP-A-6-273230 (JP, A) JP-A-60-144621 (JP, A) JP-A-63-1988 284430 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01J 1/42-1/46 G01J 3/50

Claims (3)

(57) [Claims] An AC amplifier for providing a light receiving element for receiving light emitted from a light projecting element and amplifying a light receiving signal output from the light receiving element when light emitted from the light projecting element is received. In the photoelectric sensor provided with a circuit, the AC amplification circuit is an amplification circuit including a differentiation circuit including a coupling capacitor and a resistor at an input thereof, and the AC amplification circuit includes a short circuit resistor of the differentiation circuit. Discharging means for rapidly discharging the electric charge stored in the coupling capacitor, and the light receiving signal is not output.
A photoelectric sensor characterized in that the discharge means is operated at a predetermined timing after the power state . 2. An AC amplifier for providing a light receiving element for receiving light emitted from the light emitting element and amplifying a light receiving signal output from the light receiving element when receiving the light emitted from the light emitting element. In the photoelectric sensor provided with a circuit, the AC amplification circuit includes a feedback resistor between the output side of the operational amplifier and the negative phase input side, and includes a differentiation circuit including a coupling capacitor and a resistance on the positive phase input side. A non-inverting amplifying circuit, and a discharging means for rapidly discharging the electric charge stored in the coupling capacitor by short-circuiting the resistance of the differentiating circuit in the AC amplifying circuit; It provided provided an input voltage correction means for feeding back via a capacitor for drift voltage remove the output voltage of the operational amplifier to the inverting input side of the operational amplifier, the light receiving signal Predetermined timing after a output state
In photoelectric sensor, characterized in that actuating said input correction means and said discharge means. 3. The light-emitting device according to claim 1, wherein the light-emitting element includes a plurality of light-emitting elements for sequentially projecting different colors on an object to be measured. A color sensor for determining a color for the object to be measured based on a ratio of each signal level output from the AC amplifier circuit in accordance with an amount of reflected light from the object to be measured.