RU2122185C1 - Process measuring radiation flux - Google Patents

Abstract

FIELD: measurement technology, manufacture of photometric and radiometric dives of various assignment incorporating photodetectors based on photoresistors. SUBSTANCE: process measuring radiation flux involves feed of supply voltage to circuit switching on photoresistor and recording signal proportional to radiation flux, evaluation of minimum value of resistance of illuminated photoresistor, feed of bias voltage to it depending on permissible value of dissipated electric power. Bias voltage is maintained constant in process of action of legitimate or background radiation on pad of photoresistor. EFFECT: linearization of light characteristic of photoresistor and increased accuracy of measurements in wide range of level of recorded signal and background noises. 2 dwg

Description

 The present invention relates to the field of measurement technology and can be used to build photometric and radiometric devices for various purposes, containing photodetectors based on photoresistors.

 Various types of photoresistors are known which are cooled and operated at room temperature. Compared with photocells and photodiodes, they cover a wider spectral region from filter violet to far infrared. However, they are used relatively rarely in measuring photometric devices due to the restrictive range of the linear conversion of the radiation flux into an electrical signal.

To receive pulsed and modulated radiation, the most widely used method of measurements is the separation of the load by direct and alternating current (Aksenenko M.D., Baranochnikov M.L., Smolin O.V. Microelectronic photodetectors. - M.: Energoatomizdat, 1984, p. .70-82) comprising the supply voltage U _n for a chain containing photoresistor R _f and a limiting resistor R _n, and the transmission efficiency of the transmitting signal _with the current U through the separating device (capacitor or transformer) in the load circuit for the amplification and EGISTRATION (FIG. 1a). The disadvantages of this method of measurement with photoresistors are the dependence of the conversion coefficient S _in (volt sensitivity) on the level of the measured optical signal itself and the magnitude of the constant illumination, which lead to a dynamic and static change in the on mode of the photoresistor and the non-linearity of its conversion characteristic.

 Photoresistors are also widely used to register quasicontinuous radiation or signals with a wide frequency spectrum, including a constant component. In this case, different implementations of the balanced method of measurements using photoresistors are used. As an analogue, we can consider the method of photometric measurements (Hoffman K. Photodetector device. - AUSZUEGE AUS DEN OFFENLEGUNGSSCHRIFTEN N 45 01.94), which consists in the fact that the photoresistance is included in the arm of the resistive bridge, the voltage from the AC source is supplied, and the differential AC signal is recorded using frequency selective methods. This method has advantages when registering weak, slowly changing optical signals, but also when it is used, non-linearity of sensitivity with a large dynamic range of changes in the useful signal or background illumination is manifested to the same extent.

Closest to the technical nature of the present invention is the method of photometric measurements, we have chosen as a prototype (Ishanin G.G. Radiation receivers of optical and optoelectronic devices. - L .: Engineering, Leningrad. Department, 1986, p.31 -35). It consists in applying a supply voltage to the photoresistor R _f and a load resistor R _n , registering the difference signal ΔU _{c of} the photoconductor circuit and an additional balancing circuit R ₁ , R ₂ to compensate for constant bias and stationary background illumination, in which R ₁ can be the first resistance use a second similar photoresistor (see Fig. 1b).

The bias voltage on the photoresistor is usually chosen from the condition U _cm ≤ (P _max • R _f ) ^1/2 , taking into account the maximum allowable dissipated electric power P and its own resistance R _f , and the load resistance R _n (it is current limiting) is usually equal to its own. When using a bridge or differential switching circuit, only the voltage drop across the loading resistor and the influence of constant background illumination are compensated.

где
R_т - сопротивление фоторезистора в отсутствие освещения (темновое сопротивление);
k = (ΔR/ΔФ) - - отношение изменения сопротивления ΔR = (R_т - R_ф) к вызвавшему его приращению потока излучения ΔФ.
Из него следует, что даже при постоянстве k из-за изменения первого сомножителя в знаменателе вольтовая чувствительность изменяется. Для типичного неохлаждаемого фоторезистора из сульфида цинка с темновым сопротивлением R_т = 50 кОм при напряжении смещения 15В вольтовая чувствительность составляет S_В = 1500 В/Вт. При равенстве R_т и R_н коэффициент k будет не менее 10⁷ Ом/Вт, что приведет к отклонению от линейности на 1% уже при потоке 0.1 мВт. При величине порогового потока 10^-7 Вт и отношение сигнал/шум ≥ 10 диапазон линейной работы составит не более 100. С другой стороны для таких резисторов типичная величина отношения, характеризующая максимальное изменение сопротивления, составляет R_т/R_ф ≥ 1.2 Для фоторезисторов на основе селенида и сульфида кадмия этот параметр достигает нескольких тысяч. Нелинейности характеристики преобразования в большей степени проявляется для более чувствительного экземпляра приемника. Диапазон работы традиционных схем при отступлении от линейности (2-5)% редко достигает 100. Кроме того, в отличие от фотодиодов и вакуумных фотоэлементов вольтовая чувствительность схемы непосредственно зависит от величины напряжения питания.For volt sensitivity, when the signal is taken from R _n relative to a common point, we can write the expression

Where
R _t - the resistance of the photoresistor in the absence of lighting (dark resistance);
k = (ΔR / ΔФ) - is the ratio of the change in resistance ΔR = (R _t - R _f ) to the radiation flux increment ΔF that caused it.
It follows from it that even with constant k, due to a change in the first factor in the denominator, the volt sensitivity changes. For a typical uncooled zinc sulfide photoresistor with a dark resistance of R _t = 50 kΩ at a bias voltage of 15 V, the voltage sensitivity is S _B = 1500 V / W. If R _t and R _{n are} equal _{, the} coefficient k will be at least 10 ⁷ Ohm / W, which will lead to a 1% deviation from linearity already at a flow of 0.1 mW. With a threshold flux of 10 ^-7 W and a signal-to-noise ratio ≥ 10, the linear range will be no more than 100. On the other hand, for such resistors the typical ratio, characterizing the maximum change in resistance, is R _t / R _f ≥ 1.2 For photoresistors based on cadmium selenide and cadmium sulfide this parameter reaches several thousand. The nonlinearity of the conversion characteristics is manifested to a greater extent for a more sensitive receiver instance. The range of operation of traditional circuits when deviating from linearity (2-5)% rarely reaches 100. In addition, unlike photodiodes and vacuum photocells, the volt sensitivity of the circuit directly depends on the magnitude of the supply voltage.

 The technical result of the claimed invention consists in linearizing the light characteristics of the photoresistor and improving the accuracy of measurements in a wide range of changes in the level of the recorded signal and background illumination.

 The specified result is achieved in a method for measuring the radiation flux, which includes supplying a supply voltage to the photoconductor switching circuit and registering a signal proportional to the radiation flux, in which the minimum value of the resistance of the illuminated photoresistor is estimated, bias voltage is applied to it based on the allowable value of the dissipated electric power, and in the process the effects of useful or background radiation on the area of the photoresistor, the bias voltage at its own resistance is supported by standing.

 Based on an analysis of the physical properties and a number of typical schemes for switching photoresistors on, we found that the use of the traditional model of "linear change in resistance" from the magnitude of the flux or irradiation does not adequately describe the behavior of the photoresistor under illumination, especially in the case of a strong change in intrinsic resistance. On the other hand, a study of a number of industrial photoresistors showed that the linear nature of the increase in photoconductivity remains in a much wider range of flares than linearity for the dependence of the change in resistance. Based on this, we first showed that the constancy of the conversion coefficient in a wide range of flares can be realized by providing a constant bias voltage on the photoresistor during operation.

 For this, a non-standard inclusion of a photoresistor or an additional device can be used to compensate for a decrease in the bias voltage due to an increase in the supply voltage in traditional circuits. Moreover, in most cases, the influence of instability of the power source and its noise is automatically removed.

An example implementation of the method is a photodetector, the circuit of which is shown in FIG. 2. It comprises a photoresistor proper R _f, and the operational amplifier opamp resistors R _1, R _2, R _axes. To select a bias voltage of U _cm ≤ (P _max • R _min ) ^1/2 , the minimum value of the intrinsic resistance R _{f min is} determined at the maximum exposure Φ _max . Its value is set by the voltage drop across the resistor R ₁ and is determined by the ratio U _cm = (U _p • R ₁ / (R ₁ + R ₂ ). To maintain it constant, regardless of the resistance value R _f , the well-known property of the operational amplifier is used to maintain a minimum potential difference between the inverting ("-") and non-inverting ("+") inputs. In this case, the voltage at the op-amp inputs is (U _p - U _cm ). For stable balancing of the device by direct current and suppression of common mode noise on the power supply circuit, it is advisable to equalize R ₂ = R _os and R _f = R _{1. The} volt sensitivity for such a photodetector can be represented by the expression
S _in = U _cm • R _OS • α = R _OS • α • {U _p • R ₁ / (R ₁ + R ₂ )},
Where
α is the ratio of the increment of conductivity Δσ = σ _f - σ _t = α • ΔF to the increment of the flux ΔF, and σ _f = 1 / R _f and σ _t = 1 / R _t are the conductivities of the illuminated and non-illuminated photoresistance, respectively. The product U _cm • α represents the current sensitivity, which, in contrast to photodiodes or photocells, is directly proportional to the bias voltage. Another advantage of the circuit is that it combines the functions of a load circuit and a pre-amplifier.

 The proposed method can be useful for measurements using bolometers and thermistors with a large range of changes in their own resistance.

Claims (1)

 A method of measuring the radiation flux, including applying a supply voltage to the photoconductor switching circuit and registering a signal proportional to the radiation flux, characterized in that the minimum resistance value of the illuminated photoresistor is estimated, a bias voltage is applied to it based on the allowable value of the dissipated electric power, and during the exposure to useful or background radiation to the photoresistor pad, the bias voltage at its own resistance is kept constant.

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