DE2008329A1 - Electrical circuit with logarithmic gain function - Google Patents

Description

PATENTANWÄLTE 7 ST U TTGAr ) ... l · ² '! ^ .: ¹ : ⁹⁷⁰

DR.-ING. WOLFF, H.BARTELS 20Ό8329 S ^^

DR. BRANDES, DR.-ING. HELD telex.0722312,

RecT.-No. 122 337

EASTiIAH KODAK COMPANY, Rochester, New York State, United States by Z \ rnerika

Electrical circuit with logarithmic gain function

The invention "relates to an electrical circuit having a Exponential amplifier and with a correction network that the exponential amplifier is connected downstream and the exponentially amplified signal to an output signal of the Corrected circuit that forms a logarithmic function of the input signal, especially when used as an exponential amplifier a photoelectron multiplier is provided.

Electrical circuits of this type are grounded in optical densimeters, especially in the field of photography, used. The optical density, also known as blackening, is the logarithm of the ratio of the Intensity of the light incident on an object whose optical density is to be measured at the intensity of the light transmitted through this object. So the optical density is inversely proportional to that

009837/1941

Logarithm of the intensity of the light transmitted through the object. If the object lets through 100 ^c / o of the incident light, then the object has the optical density of Mull. If the object lets through 10 or 1 fo of the incident light, then the object has an optical density of 1 or 2, and so on

To measure the intensity of the light transmitted by an object, photoelectron multipliers are preferably used which amplify an input signal according to an exponential function, the amplification characteristic being dependent on the number of dynodes. The number of electrons triggered by the radiation incident on the cathode of the photomultiplier increases exponentially as a function of the number of dynodes of the photomultiplier. Since an exponential relationship ( y = x ⁿ ") is to a limited extent ^{similar to a logarithmic relationship (y = n x} ), the output signal of a photoelectron multiplier is approximately a linear function of the logarithm of the light incident on the cathode. However, this means that the output of the photoelectron multiplier varies approximately linearly with the optical density of an object whose transmitted radiation has been directed onto the cathode of the photoelectron multiplier.

In the U.S. Patent specification 2,492,901 describes a known electrical circuit of the type mentioned above, in which a photoelectron multiplier is used as an exponential amplifier is provided, the one voltage divider for the dynode voltages and one connected in series with this having variable resistance, the V / resistance value jsum of which is kept constant by the photoelectron multiplier flowing current can be changed by this. This known electrical circuit is for use

009837/1941

provided as a density meter with a correction network, a step-by-step correction of the exponentially amplified signal to a logarithmic function of the Input signal causes. This known correction network points to the instrument displaying output signals a voltage divider connected in parallel to the electrically biased resistors via rectifiers through which a step-by-step change in the division the resistance values of the voltage divider is effected. This known electrical circuit is in its structure very easy. However, due to the stepwise change in the exponentially amplified signal, it is functional less than corresponding, i.e. if, for example, the output signal is converted for a digital counter display will not be corresponding digital Results achieved.

In the Journal of Scientific Instruments, Volume 33 February 1966, in an article ^including Wide Hange, Recording, Logarithmic Photometer Circuit by Hariharan and Bhalla, there is a non-stepwise technique for linearizing the relationship between measured light intensity The electrical circuit proposed here is functionally corresponding, but has a relatively complicated and expensive construction.

The invention is based on the object of providing an electrical circuit with an exponential amplifier in which the output signal forms a logarithmic function of the input signal and at any time can also be converted accordingly for a digital counter.

00 9837/1841

This Ai: <f ^ a "': e is" in an electrical circuit of type mentioned at the beginning, ge ::: äii oer invention solved by da.c the correction netzv.erl: in ei;] er. Strcrr.veg des exponentially amplified signals with a varistor Experiments have shown that a varistor, i.e. a resistor with a voltage-dependent resistance value, is the property list, an exponentially amplified one that he suggests To convert the signal into a logarithmically amplified signal. If, for example, radiation through an object hits the cathode of an exponential amplifier Photoelectron multiplier then be directed the electrons released in this way exponentially by the Effect of the dynodes of the phctoelectron multiplier multiplied. Then becomes the output current of the photoelectron multiplier passed through the varistor, then the voltage built up across the varistor is linearly proportional to the optical density of the irradiated object.

In a preferred embodiment of the electrical circuit, a photoelectron multiplier is provided as an exponential amplifier for measuring the optical density, which, as is known, has a voltage divider for the synodic voltages and a variable resistor connected in series with this, the resistance value of which is kept constant is through the Photoelektronem ^r ervielfältiger current flowing from this variable, wherein the varistor for interaction .i;. is connected it to the voltage divider in order to prevent this, that the varistor beeinflußtest puncture of the Photoelektrcnenvervieliältigers preferable is that the varistor with a with him The resistance connected in series is connected in parallel to the voltage divider.

009837/19

_ 5 -

If the resistance value is selected to be somewhat large, then this will result in the varistor is electrically isolated from the dynodes to a certain extent. But if the dynode voltage changes, then the voltage occurring across the varistor and thus its resistance value also changes. As a result of special characteristics of the 7 / resistance value of the The varistor converts the dynode voltage, which occurs as an exponential function of the input signal, into an as logarithmic puncture of the input voltage occurring varistor voltage converted, so that the voltage drop across the varistor is linear with the optical. Density of the irradiated object changed

The invention is illustrated in the following description of two exemplary embodiments shown in the drawings explained in detail.

Show it:

Figd shows a block diagram of an inventive Circuit;

.Figo2 the circuit diagram of a known through the Invention improved circuit;

Pig.3 on the output voltage versus the optical density of a measured object illustrative diagram for explaining the invention;

Pig.4 is a circuit diagram of a preferred embodiment the circuit according to the invention.

The invention is explained schematically in Pig.1. One

009837/1941

Light source 10, e.g. a backlit slide, illuminated the cathode of a photoelectron multiplier 12 with an intensity whose logarithm is proportional to a parameter d, e.g. the optical density of the slide. Of the Current through the photoelectron multiplier 12 is generated by a network 13 which changes the dynode voltages kept constant. These changes in dynode voltage are exponentially related to the light incident on the cathode of the photoelectron multiplier 12; accordingly, the output current of the photoelectron multiplier is accordingly approximately analogous, but, to be precise, slightly larger than the parameter in question, so i ^ kd. Becomes the output current of the photoelectron multiplier 12 passed through a varistor 14, then a voltage e = gd appears across the varistor, which is exactly analogous to the parameter d. The reason for this is that the voltage occurring across the varistor The voltage drop does not increase as quickly as the current causing the voltage drop.

The circuit according to FIG. 2 is based on the circuit known from US Pat. No. 2,492,901. The light is thrown from a lamp 16 through a lens 18 and through an object 20, the optical density of which is to be measured, onto the cathode 22 of a photoelectron multiplier 24. The current through the photoelectron multiplier 14 is independent of the intensity of the on the cathode incident light is kept constant by adjusting the voltages applied to the dynodes 26a-26i of the photoelectron multiplier 24 accordingly. Regardless of whether the optical density of the object 20 is zero (100 $ transmittance) or one (10 ⁱ ; 5 transmittance), etc., the current through the photoelectron multiplier

009837/1941

is always kept constant by the dynode voltages can be changed in an approximate proportionality to the optical density of the object 20. this happens in that a voltage is generated across a resistor 28 by the current of the photoelectron multiplier 24 flowing to the anode 30. A feedback amplifier 32 is between points A: and B in series with a voltage source for the dynodes, namely one appropriately tapped resistor 34, switched and at a higher or lower negative potential than Puncture the optical density of the object 20 held. If the optical density of the object 20 changes e.g. from one to JTuIl, then the potential at the lattice of Amplifier 32 negative, so that the resistance of the Amplifier 32 increases' and the voltage across the tapped Resistance 34 drops to a lower Y / ert, which corresponds to the optical density Muli of the object 20, whereby the current through the photoelectron multiplier is kept constant »It controls the optical density of the object 20, e.g. from full to one, then steps the reverse, '' Yirkung a. ·

It is known that the dynodes of a photoelectron multiplier exponentially multiply the electrons released photoelectrically at the cathode. However, since . the exponential relationship between that by an optical Density on the cathode of the photoelectron multiplier incident light and the node potential is not exactly the same as the logarithmic relationship that exists between the incident light and the density the dynode voltage is one for a density Measurement of switched photoelectron multiplier with constant current is only an approximate analog of the optical Density of the transilluminated object 20. For this reason, the dynode voltage becomes practically a fraction

00 9837/1941

BATH ORIGINAL

from this, applied to a varistor 36, the parallel is connected to the tapped resistor 34. Homogeneous varistors, especially silicon carbide varistors, were found to be particularly useful. So that the varistor 36 does not retroactively adjust the voltage of the dynodes 26a-26i, a resistor 38 with a high resistance value is connected in series with the varistor 36, to isolate the varistor from the dynodes.

In Fig. 3, the dynode voltage is dashed as a function of the optical density d of the object

fc 20 shown. With increasing voltage, however, the resistance value of the varistor 36 decreases, so that the voltage drop across the varistor 36 forms the linear function of the optical density d shown by a solid line. Regarding I ¹ Ig.3, it should also be noted that the abscissa can be shifted along the ordinate. A voltmeter 40 which measures the voltage representing the optical density can be set to zero so that the effect of a rest voltage which appears on the varistor 36 when the optical density is equal to zero does not appear. Such a zero setting of the instrument is useless if the voltage indicating the optical density is zero for a digital counter

P display (000) should be converted because the voltage Zero practically never appears across the varistor 36 when the circuit of Figure 2 is in operation.

With the circuit according to FIG. 4, the output signal is a The output voltage representing the measured optical density is generated directly for a digital meter display can be converted. Since the circuit according to FIG. 4 corresponds to the circuit according to FIG. 2, parts are similar to the parts of the circuit according to FIG. 2, in the circuit according to FIG. 4 with the same reference numerals

009837/1941

• - 9 -

referred to, which are, however, provided with a prime. A transistor 32 'and a tapped dynode resistor 34' are ^{connected in series between the terminals A 1} B 'of a current source that when the current through a photoelectron multiplier 24' begins to change, the resistance of the transistor 32 changes 'changes to change the voltages applied to the dynodes and thereby keep the current through the photomultiplier tube constant. The dynode voltage is also applied to a resistor 38 'which has a high resistance value and is connected in series with a summing network 42 which has a varistor 36'. The varistor 36 'is connected to the dynode voltage via a J / resistor 38' ., "as well as being applied to a bias voltage V via a variable resistor 44. These two voltages ¹ are algebraically added and generate a resulting voltage at the varistor 36 '. Such a resulting voltage can be set to zero by means of the variable resistor 44 in order to increase the optical density This allows the output of the circuit of Figure 4 to be converted into a digital counter display which represents the optical density.

V / ie in the circuit according to Figure 2, the voltage drop indicated above varistor 36 by a 'optical density calibrated measuring instrument 40' will. . . . . .

Since the transistor '/ contact resistance, a relatively small input and the photo-electron multiplier 24' 32 have a relatively large output resistance in the base circuit of the transistor 32 is ^r nor a Widerstandaahpassungsverstärker 45 connected, which may be formed by a field effect transistor.

0098 37/1941

Claims (5)

- ίο - claims 1) Electrical circuit with an exponential amplifier and with a correction network that is connected downstream of the exponential amplifier and that exponentially amplified signal is corrected to an output signal of the circuit, which has a loga- ^ forms the rithmic function of the input signal, - characterized in that the correction network has a varistor (14) in a current path of the exponentially amplified signal. 2) Electrical circuit according to claim 1 "with a photoelectron multiplier al · -, ^. Potential amplifier, which ^{1 has} a voltage divider for the dynode voltages and a variable resistor connected in series with this, the resistance value of which can be changed to keep the current flowing through the photomultiplier constant is, thereby W indicates that the varistor (H) for a Interaction with the voltage divider (34) is switched 3) Electrical circuit according to claim 2, characterized in that the varistor (36) is connected in parallel to the voltage divider (34) with a resistor (38) connected in series with it. 4) Electrical circuit according to one of claims 1-3, 009837/1941 ■ - 11 - • characterized in that as an input signal for measuring the optical density of an object (20.) one of this object through: let radiation is provided. 5) Electrical circuit according to one of claims 1 - 4, characterized in that for the Varistor (3d) a bias voltage is provided, which is exponential with that of the varistor (3d) amplified signal is added algebraically and has such a magnitude, that with a maximum input signal (input radiation at zero optical density) that exponentially amplified with the signal Bias removes. 00 9837/1941 Lee on the side