JPH0432335B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for correcting the offset of an output signal processing circuit of a photodetector including the dark current correction circuit in a spectrophotometer equipped with a dark current correction circuit. Dark current correction is performed by memorizing the photodetector output when no light is incident on the photodetector and subtracting it from the output when light is incident on the photodetector. When the sensitivity of the photodetector is controlled to keep the photodetection output on the reference light side constant during measurement, such as in a luminous flux spectrophotometer, a photomultiplier is used as the photodetection element to apply a negative high voltage. Since the magnitude of the dark current changes when the value is changed, it is necessary to constantly correct the dark current during measurement. Therefore, in this type of spectrophotometer, the dark chopper is rotated to periodically create periods in which no light is incident on the photodetector, so that the dark current signal is constantly sampled. The present invention attempts to make it possible to correct the offset of the signal processing circuit by utilizing the above-mentioned dark current correction means already provided in the spectrophotometer.

As mentioned above, the dark current correction means of the spectrophotometer can time-divisionally obtain the dark current signal from the photodetector and the photodetection signal when light is incident on the photodetector.
After suitably amplifying the output of the photodetector using a preamplifier etc., the above two types of signals are selected using a signal selection circuit.
Each signal is held in a hold circuit and input to a subtraction circuit. With this configuration, the offset that occurs between the photodetector and the signal selection circuit is corrected together with the dark current correction, but in the circuit section after the signal selection circuit, the dark current signal and the photodetection signal are The offset amounts that occur in the two do not necessarily match and are not eliminated by the subtraction circuit. Conventionally, such an offset has been manually adjusted to zero by providing a variable resistor.

The present invention attempts to automatically eliminate the above-mentioned offset, and includes a circuit section that processes the photodetection signal as well as a circuit section that operates the signal selection circuit described above to process the dark current signal. A dark current signal is also input to both, and the above-mentioned subtraction circuit output at that time is memorized, and in other words, the above-mentioned stored data is derived from the measurement output after dark current correction. The feature is that it uses subtraction. By inputting the dark current signal to the circuit section that processes the photodetection signal and the circuit section that processes the dark current, and subtracting the output of each circuit section, the difference in offset generated in each circuit section can be subtracted. An offset amount signal is obtained that is the sum of the offsets generated from the circuit to the storage circuit, and this is the offset amount signal for the entire measuring device, so this can be stored and used from the measurement output during measurement. By subtracting, a correct measurement result with offset correction can be obtained. According to the present invention, the existing dark current correction means is used, and there is an advantage that no additional parts for offset correction are required. The present invention will be explained below with reference to Examples.

FIG. 1 shows an embodiment of the invention. A is the monochromatic light flux emitted from the spectrometer. 1 is a sector mirror that divides the same beam into two beams of RS. Luminous flux R
is reflected by mirrors m1 and m2 and enters the photomultiplier 3. The luminous flux S is mirror m3, m4,
It is reflected by m5 and enters the photomultiplier 3. With this configuration, the light beams RS are alternately incident on the photomultiplier 3. 2 is a dark chopper that rotates integrally with the sector mirror 1. FIG. 2 shows the front side of the sector mirror 1 and the dark chopper 2. The dark chopper 2 is integrated with the sector mirror 1 in a manner protruding from the mirror surface portion of the sector mirror 1 in the rotational direction, and light is absorbed in the dark chopper portion and neither reflected nor transmitted. Third
Figure P shows the output signal waveform of photomultiplier 3.
is the output while the luminous flux S is incident, and R is the output while the luminous flux R is incident.Since the dark chopper 2 is integrated with the sector mirror in a way that it protrudes from the sector mirror, the outputs S and R are In between, there is a portion D corresponding to a period in which no light is incident on the photomultiplier 3. The output of this part is the dark current of the photomultiplier. The output signal of the photomultiplier 3 is sent to the analog switch section 5 via the buffer amplifier BA, where signal selection is performed. The analog switch section 5 includes three analog switches SS,
It consists of SR and SD, and in the normal measurement mode, the switch SS is made conductive while there is a pulse shown as Sp in Fig. 3, and the output signal of the photomultiplier 3 corresponding to the light flux S is selected. . The switch SR is made conductive while there is a pulse indicated by Rp in FIG. 3, and selects the output signal of the photomultiplier 3 corresponding to the light flux R. switch
SD is made conductive during the pulse indicated by Dp in FIG. 3 to select the dark current signal of the photomultiplier 3. 6 is a sample and hold circuit section including three sample and hold circuits 6S, 6R, and 6D;
holds the side optical output S of the photomultiplier 3 for the luminous flux S when the switch SS is turned on. Similarly, 6R is the photometric output R for the luminous flux R.
, and 6D holds the dark current signal D. In is an inverter that is connected to the output side of the sample and hold circuit 6D and inverts the dark current signal. Reference numerals 71 and 72 denote dark current subtraction circuits, and 71 subtracts the dark current signal D from the photometric output S held in the sample hold circuit 6S. Similarly, 7
2 subtracts D from the photometric output R. Subtraction circuit 7
The circuits 1 and 72 are adder circuits, but an inverter In is installed on the output side of the sample and hold circuit 6D.
is connected and the sign of the dark current signal D is inverted and inputted to the adder circuit, resulting in D being subtracted from S and R. The outputs of the subtraction circuits 71 and 72 are A/D converters AD1,
It is converted into a digital signal by AD2 and sent to the control circuit Com. The output of AD1 is the photometric value obtained by subtracting the dark current from the photometric value for the luminous flux S, that is, the photometric value is corrected for the dark current. Similarly, the output of AD2 is a photometric value with dark current correction applied to the luminous flux R. The control circuit Com calculates the transmittance or absorbance of the sample from these photometric values, and sends the data to a recording and display device for recording or display.

If the offset of the entire circuit is to be corrected, the correction is performed as follows. In other words, Switch SS
A pulse shown as Dp in FIG. 3 is sent to SD and switch SR, and no pulse signal is given to switch SR. In this way, the subtraction circuit 71 (dark current signal D) -
(Dark current signal D) will be calculated.
In this case, the output of the A/D converter AD1 should be 0, but the output from the analog switch section 5
It does not become 0 due to the offset of all the circuits up to . Therefore, the output data of AD1 at this time corresponds to the offset value of the entire circuit. control circuit
Com memorizes this offset value A,
In the normal measurement mode mentioned above, AD1 and
The offset is corrected by subtracting the value of A from the output of AD2. That is, when the calculations in the control circuit Com are displayed, S...photometric output for luminous flux S R... photometric output for luminous flux R D... dark current output S-D... output of AD1 in normal measurement mode R-D... output of AD2 in normal measurement mode Output A...Output (D-D) of AD1 in offset value detection (zero clamp) mode For example, the transmittance T is T = {(S-D-A)/(R-D)} x 100% In Figure 1 4 is an analog switch control circuit. In the same circuit, 4p, 4D, and 4R are detectors such as photocouplers that correspond to the reflective part, transparent part, and dark chopper 2 of the sector mirror 1 on the circumference of a disk fixed to the rotating shaft of the sector mirror 1. Sp, Dp,
Outputs a pulse indicated by Sp. In normal measurement mode, keep terminal RO at high level. Then, the NAND circuit NA1 normally has inputs (1, 0) and outputs high, and becomes low only when the Sp pulse is input. Since the terminal RO of the NAND circuit NA2 is high, one input is always low due to the inverter i, so the output is always high. Therefore, the input of the NAND circuit NA3 becomes (0, 1) and the output becomes high only when the pulse Sp is outputted from 4S, and eventually the pulse of Sp is obtained again as the output of NA3. Analog switch SS is controlled by the output signal of NA3. 4D output pulse Dp
directly controls the analog switch SD. Similarly, the output pulse Rp of 4R directly controls the analog switch SR. Therefore, the operation in the normal measurement mode described above is performed. When performing zero clamp mode (offset value detection mode), set terminal RO to low level. In this way, one of the inputs of the NAND circuit NA1 will always be low, so the output will always be high. Also NAND circuit NA2
Normally the input is (1,0) and the output is high, but
Only when the 4D output pulse Dp is input, the input becomes (1, 1) and the output becomes low. Therefore, the NAND circuit NA3 becomes high only when there is an output pulse Dp of 4D, and the pulse of Dp is also given to the switch SS, and the subtraction circuit 71 becomes (D-
D) will be output. The high and low signals applied to terminal RO are output from the control circuit Com. The control circuit Com keeps terminal RO at high level when the zero clamp key is not pressed, and sets terminal RO to low level when the zero clamp key is pressed.

FIG. 4 is a flowchart showing an outline of the control operation of the control circuit Com. Check whether the zero clamp key is pressed in a, and press YES.
Then, set the B terminal low (b), store the output data A of AD1 at a predetermined address in the memory (c), and set the input of the terminal B high (d). If the zero clamp key is not pressed in a, NO, AD1
Output data S-D of AD2 and output data R- of AD2
Read D and read the value of A from memory (S-
Perform the calculation DA)/(RD) (e).

In the above embodiment, the offset amount is detected and stored when the zero clamp key switch is pressed, but this key operation can also be automated to detect and store the offset amount at regular time intervals. You can do it like this. Also, the subtraction circuits 71, 7
The dark current signal (output of the hold circuit 6D) is also A/D converted and stored in the control circuit Com, and the dark current correction, offset amount storage,
It is also possible to perform all offset corrections.

As a method of offset correction, a shutter that can go in and out of the light flux S is prepared, and by allowing this shutter to enter the light flux S, the dark current signal is held in both the hold circuits 6S and 6D in the circuit shown in FIG. A configuration has been proposed in which the amount of offset is detected by adjusting the shutter, but this requires a shutter that can be moved in and out. According to the present invention, such additional parts are unnecessary, variable resistors for adjustment, etc. are not required, and the structure of the spectrophotometer is not complicated, and offset correction as well as dark current correction is automatically performed. It is.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of an embodiment of the present invention, Fig. 2 is a front view of a sector mirror and a dark chopper in the embodiment, Fig. 3 is a signal waveform diagram in the embodiment, and Fig. 4 is a diagram of the sector mirror and dark chopper in the embodiment. It is a flowchart of part of the operation of the control circuit. 1... Sector mirror, 2... Dark chopper, 3... Photo multiplier, 4... Analog switch control circuit, 5... Analog switch section,
6... Sample hold circuit section, 71, 72...
Subtraction circuit, Com...control circuit.

Claims (1)

[Claims] 1 Dark chopper to periodically create periods when no light is incident on the photodetector and photodetection during periods when no light is incident on the photodetector from the photodetector output during periods when light is incident on the photodetector In a spectrophotometer equipped with a dark current correction means for a photodetector consisting of means for subtracting a photodetector output, the dark current correction means calculates a photodetector output signal during a period when light is incident on the photodetector. Input the photodetector output signal during the period when no light is entering the photodetector into the circuit section to be handled,
At this time, a signal corresponding to the offset amount is obtained from the above-mentioned subtraction means, this is held, and the signal corresponding to the above-mentioned offset amount is subtracted from the output signal of the above-mentioned subtraction means during normal measurement. A spectrophotometer offset correction method characterized by performing offset correction.