SU1696894A1 - Photometer - Google Patents

Abstract

The invention relates to an opto-electronic technique and can be used for photometry of stationary light fluxes with a wide dynamic range in illumination. The aim of the invention is to improve the accuracy of measurements of the intensity of stationary light fluxes. The device contains an operational amplifier 2 with a photodiode 1. a comparator 3, a generator 4 linearly decreasing voltage, a single vibrator 5, a delay line 6, a switching element 7, an exemplary voltage source 8, a compensating current generator 9, a sampling-storage device 11 in which remembering the value of the voltage corresponding to the initial voltage on the barrier capacitance of the photodiode 1. This value is the reference for the generator 4 linearly decreasing voltage. 3 il. from 43

Description

Fig 1

The invention relates to an optoelectronic technique and can be used in the photometry of stationary light fluxes with a wide dynamic range in illumination.

A photometer is known based on a photodiode included in the feedback circuit of an operational amplifier, in which the discharge of the pre-charged barrier capacitance of the photodiode occurs under the action of the light flux. At the same time, the moment of equality of the values of the linear function describing the discharge capacitance of the photodiode to the value of some given reference function implemented by a special generator is fixed on the time axis 1

Such a photometer is characterized by a substantial dependence of the measurement accuracy on the stability of the charge level of the barrier capacitance of the photodiode (1) n) and the initial voltage of a special generator

(Uon).

The closest in technical essence to the invention is a photometer containing an operational amplifier, the non-inverting input of which is connected to the zero potential bus, the inverting input to the cathode of the photodiode, and the output to the anode of the latter and the inverting input of the comparator, the second input connected to the output of the linearly falling generator voltage, and the output - to the input of a one-shot, connected to the control inputs of the generator of a linearly falling voltage and a switching element, the switching inputs of which Keys to the exemplary supply voltage and a compensating current generator, and an output - to the cathode of the photodiode 2. The essence of this circuit is to determine the equality of moments voltages: defined by the integrated illumination of the photodiode from the output of the operational amplifier and linearly falling voltage generator. The frequency of these events recorded by the comparator, each switch of which initiates a recharge of the barrier capacitance of the photodiode and, consequently, the repetition of the cycle of operation of the device, is proportional to the intensity of the measured radiation.

The known device has an inherent measurement error due to the instability of the circuit parameters, and first of all the level of the charge capacity of the barrier capacitance of the photodiodes) and the initial voltage of the generator of linearly falling voltage (Uon) due to temperature

system drift, shot and radiation noise of the photodiode, other non-stationary external influences. When the relative change in UH is 611n, the relative

the change in the period of the output pulses of a known scheme is

6T - 6UH / (1 + UOH / UH,

and since UH Uon is adopted for a known device, 6T 6UH / 2.

0 The aim of the invention is to improve the accuracy of measurements of the intensity of stationary light fluxes.

Into a device containing an operational amplifier, the non-inverting input of which is connected to the zero potential bus, the inverting input to the cathode of the photodiode, and the output to the anode of the latter and the first input of the comparator, the second input connected to the output of the generator

0 linearly decreasing voltage, and the output - to the input of the one-shot, switching element, the first and second switching inputs of which are connected respectively to the exemplary voltage source and to the compensating current generator, and the output to the cathode of the photodiode, the shaper of the voltage level, a sampling-storage device and a delay line, while the forwarder of the level of the reference voltage is connected to the anode of the photodiode by the input, and the output to the information input of the sampling-storage device connected to the control They are input to the one-vibration output, and the output to the input of the level of the reference voltage of the generator of linearly decreasing voltage, the delay line is connected to the output of the single-vibrator, and the output to the control inputs of the generator of linearly falling voltage and switching element.

Figure 1 shows the structural diagram of the photometer; Fig. 2 is a diagram of a voltage level shaper; on fig.Z - that

5 is an option.

The photometer contains a photodiode 1, an operational amplifier (op-amp) 2, a comparator 3, a generator of linearly falling voltage, a one-shot 5, a delay line 6,

0 a switching element 7, an exemplary voltage source 8, a compensating current generator 9, a reference voltage level shaper 10, a sampling-storage device 11.

5 The photometer works as follows.

The start of the measurement cycle initiates a pulse from the output of the one-shot 5, which puts the water-discharge monitor 11 in the storage mode, and after a time interval required for

confident operation of the VHD provided by the delay line 6 connects the inverting input of the amplifier 2 through the switching element 7 to the exemplary voltage source 8. At the output of the operational amplifier 2, a negative voltage UH is set, causing the barrier capacitance of the photodiode 1 to charge. On the basis of the reference level shaper 10, the reference potential Uon is generated for generator 4, which is fixed for the duration of one measurement cycle of the VHI 11. Uon is a shift relative to UH in the positive region on the fixed interval Usdv 2 IUHI, which not only stabilizes the interval I UH, Uon I isdv, but also ensures the equality I UH t I Uonl.

At the end of the pulse of the one-shot 5, a generator 4 of the linearly decreasing voltage is started and the switching element 7 connects to the inverting input of the amplifier 2 a compensating current generator 9. At the same time, the barrier capacitance of the photodiode begins to discharge under the action of the incident light flux and the dark current of the photodiode. The compensation of the latter is made by the generator current 9. At the moment of intersection of the increasing voltage from the output of amplifier 2 and from the output of the generator

4comparator 3 is switched, which causes the one-shot to restart

5 and repeating the photometer cycle. Due to the undefined state at the output of the operational amplifier 2 at the moment of switching on the device, the first measurement cycle is not taken into account, which does not affect the overall measurement accuracy, since the frequency of subsequent cycles is constant and depends only on the intensity of the light flux.

In accordance with the proposed structure, the voltage Uon for a generator of a linearly decreasing voltage is a function of the value UH (Uon f (UH)). The type of the operator f is chosen so that the stability of the difference I UH Uon I is ensured under various external influences (fluctuations of the system parameters). In this case, the total error in the formation of the interval T, due to the instability of UH and Uon, as follows from the relation

T ki D-k2 / UH-Uon. where F is the luminous flux;

ki and k2 are the coefficients characterizing the discharge of the barrier capacitance under the action of light and the formation of a linear

the decaying voltage, respectively, tends to zero.

Possible schemes for the implementation of the driver 10 of the reference voltage level 5 are shown in Figures 2 and 3. In the case of using only an operational amplifier, the bias voltage TCm is selected on the basis of the condition Ecm Uon.

Unlike well-known photometers

0 proposed has a high measurement accuracy in a wide dynamic range due to the introduction and use of the dependence of the reference potential of the generator of linearly decreasing voltage

5 Uon from the initial charge voltage of the barrier capacitance UH during the formation of Uon. At the same time, due to the stability of the difference I UH - Uonl, the measurement error due to changes in UH and Uon, due to the temperature drift of the parameters of the electronic components of the device, the natural noise of the photodetector tends to zero. The application of the proposed device is especially effective when measuring the intensity of low light fluxes.

Claims (1)

The invention includes a photometer containing an operational amplifier, the non-inverting input of which is connected to the zero potential bus, the inverting input to the cathode of the photodiode, and the output to the anode of the latter and the first input of the comparator, the second input connected to the output of the ramp generator, and the output to the entrance of the one-shot. switching element, the first and second switching inputs of which are connected respectively to the exemplary voltage source and to the compensating current generator, and output to the cathode of the photodiode, which is different in that, in order to improve the accuracy of measuring the intensity of stationary light fluxes, the driver of the voltage level, the sampling-storage device and the delay line, while the driver of the voltage level of the reference voltage is connected to the anode of the photodiode and the output to the information input device The sample-storage unit connected by the control input to the one-shot output and the output to the input of the level of the reference voltage of the generator of the linearly decreasing voltage, the delay line is connected to the output of the one-shot of the one-vibrator, and the output is connected to the control inputs of the generator of the linearly falling voltage and a switching element. Ft.1 Fm 3