US3672781A

US3672781A - Contrast photometer

Info

Publication number: US3672781A
Application number: US16115A
Authority: US
Inventors: Earl S Rosenblum
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1970-03-03
Filing date: 1970-03-03
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27

Abstract

AN APPARATUS AND METHOD FOR INSTRUMENT DETERMINATION OF THE EFFECTIVENESS OF MILITARY OBSCURATION SYSTEMS BY THE DETERMINATION OF THE RATIO OF CLOUD REFLECTANCE TO COLD TRANSMITTANCE AS RELATED TO THE VISUAL CONTRAST DETERMINED BY A CONTRAST PHOTOMETER. THE METHOD AND APPARATUS CAN ALSO BE UTILIZED TO OBSERVE INDUSTRIAL SMOKE CLOUDS TO ASCERTAIN WHETHER OR NOT PARTICULATE MATTER OF THE CLOUD EXCEEDS THE MAXIMUM ALLOWABLE AMOUNT WITHIN POLLUTION CONTROL STANDARDS. AFOREMENTIONED INSTRUMENT DETERMINATION AND UTILITY IS ACCOMPLISHED BY AN APPARATUS AND METHOD WHICH EMPLOYS AN OPTICAL SYSTEM HAVNG AN OSCILLATING MIRROR MEANS THEREIN AND A PHOTOMULTIPLIER MEANS THEREIN? THE MIRROR MEANS BEING USED TO RECEIVE LIGHT WAVES FROM THE SOURCE BEING STUDIED BY THE PHOTOMETER AND TO TRANSMIT THE LIGHT WAVE TO A PHOTOMULTIPLIER MEANS TO CONVERT THE LIGHT WAVES INTO ELECTRICAL IMPULSES.

Description

June 27, 1972 ROSENBLUM 3,672,781

CONTRAST PHOTOME'IER Filed March 3, 1970 5 Sheets-Sheet 1 INVENTOR Earl 51 Rosenb/um 04 ,4 ATTOR Y June 27, 1972 E. s. ROSENBLUM CONTRAST PHOTOMETER Earl .5. Rosenb/um June 27, 1972 E. s. ROSENBLUM 3,

CONTRAST PHOTOMETER Filed March 3, 1970 5 Sheets-Sheet 3 Fig.

Ilk

"H II INVENTOR Earl S. Rosenblum June 27, 1972 E. s. ROSENBLUM 3,

CONTRAST PHOTOMETER Filed March 3, 1970 5 Sheets-Sheet 4 my? W a BY ATTOR tzYJ June 27, 1972 Filed Marbh s, 1970 5 Sheets-Sheet 5 PHUTOMUU/fi/[fi 5 SYA/(H/PO/VOUS MOTOR INVENTOR Earl 5. Rasenblum W- 5 o ll 4' l United States Patent Ofli 3,672,781 Patented June 27, 1972 3,672,781 CONTRAST PHOTOMETER Earl S. Rosenblum, Lexington, Mass, assignor to the United States of America as represented by the Secretary of the Army Filed Mar. 3, 1970, Ser. No. 16,115 Int. Cl. G01n 21/22, 21/26 U.S. Cl. 356-201 9 Claims ABSTRACT OF THE DISCLOSURE An apparatus and method for instrument determination of the effectiveness of military obscuration systems by the determination of the ratio of cloud reflectance to cloud transmittance as related to the visual contrast determined by a contrast photometer. The method and apparatus can also be utilized to observe industrial smoke clouds to ascertain whether or not particulate matter of the cloud exceeds the maximum allowable amount within pollution control standards. Aforementioned instrument determination and utility is accomplished by an apparatus and method which employs an optical system having an oscillating mirror means therein and a photomultiplier means therein; the mirror means being used to receive light waves from the source being studied by the photometer and to transmit the light waves to a photomultiplier means to convert the light waves into electrical impulses.

DEDICATORY CLAUSE The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

SPECIFICATION My invention, which was conceived and reduced to practice under U.S. Government contract DA18-03S- AMC706(A) between the Department of the Army and GCA Corporation, relates to an apparatus and method for determining the ratio of cloud reflectance to cloud transmittance by observing a military obscuration system, such as a chemical or an industry generated smoke cloud, with a contrast photometer by a means of a remote measurement technique involving the use of a pair of reflectance contrast targets located on the side of the cloud opposite to the photometer; one of the target pair being substantially black and the other being substantially white.

The prior art method employed to evaluate the obscurance provided by a military obscurance system was visual observation by a human. Such observation, of course, incorporated the human error and relied on human judgment in ascertaining the degree of obscurance. Regarding industrial smoke clouds, particulate pollutants were measured in the prior art by nephelometers and transmissivity photometers or by collecting the particulate matter and measuring the physical characteristics thereof.

The principal object of my invention is to provide a reliable and effective method and apparatus for measuring the obscurance provided by a military obscurance system or the amount of particulate pollutant in an industrial smoke cloud.

Other objects of my invention will be obvious or will appear from the specification hereinafter set forth.

FIG. 1 is a view showing the utility of my contrast photometer.

FIG. 2 is a pictorial view of my contrast photometer.

FIG. 3 is a sectional view of my contrast photometer.

FIG. 4 is a view of the mechanically mounted embodiment of the oscillating mirror of my contrast photometer.

FIG. 5 is a view of the electro-magnetically operated oscillating mirror of my contrast photometer.

FIG. 6 is a block diagram of the electronic circuitry of my own contrast photometer.

FIG. 7 is an electrical circuit diagram of the electronics of my contrast photometer.

FIG. 8 is an electrical diagram of the mirror driveroscillator-reference (chopper) section of my contrast photometer.

FIG. 9 is a block diagram of the heater arrangement of my contrast photometer.

FIG. 10 is an electrical circuitry diagram of the typical operational amplifier wiring arrangement for my contrast photometer.

FIG. 11 is a view of the plug connection for the heaters of my contrast photometer.

FIG. 12 is a view of the plug connection for the power supply of my contrast photometer.

FIG. 13 is a schematic diagram showing the mirror drive in a typical view of the image plane for my contrast photometer.

My invention as shown in FIGS. 1 to 13 will now be described in detail as follows.

The conventional method for evaluating effectiveness of obscuration agents involves finding a number called the Total Obscuring Power (TOP) for the agent. The TOP is measured in a laboratory experiment which gives results which do not always correspond to the obscuring effectiveness of munitions deployed in the field.

There are a number of reasons for the lack of agreement between TOP and obscuring effectiveness; most important of which is the fact that, while TOP varies with the ability of an agent cloud to attenuate light signals passing through it, it is not a good measure of the clouds ability to scatter sunlight and other light coming from all directions into the eye.

It has been known for several decades that visual detection of an object depends on the brightness difference between the object and its background and also on the average brightness seen by the eye. If the eye sees a low average brightness, a target only slightly brighter or darker than its background might be detected. On the other hand, if the eye sees a high average brightness, a target only slightly brighter or darker than its background could not be detected.

Thus, at one extreme, an agent which does not attenuate light signals at all, does not decrease target-background brightness differences, andscatters considerable light into the line of sight; thereby increases the average brightness seen. Such an agent would have a TOP of zero, but it might obscure appreciably under certain conditions. At the other extreme, an agent which attenuates signals without scattering might have a large TOP, but it would obscure very inefficiently.

Since agents both attenuate and scatter light, a method and/or apparatus to determine obscuration effectiveness should account for both properties; preferably in a form related simply and directly to the agents ability to prevent detection by observers.

My apparatus, method, and system measures a property, called the obscurance of an agent or industrial smoke cloud, which does have the aforementioned characteristic of light attenuation and scattering. Obscurance measures directly the ability of a cloud to restrict visibility. A munition which generates a smoke cloud having a given obscurance will decrease the detectability of any target more effectively than will another munition generating a smoke cloud having a smallr obscurance, when the two munitions and generated smoke clouds are tested under the same conditions.

It is possible that Munition A may obscure more effectively than Munition B under one set of conditions, while under other conditions, such as with the sun making a different angle with the line of sight, Munition B may obscure better. This difference is possible because cloud particles of different materials or different size distributions have different scattering properties as the directions of illumination and view vary. Measurements of obscuranee detect the aforementioned difference, while TOP does not.

As indicated above, mans ability to visually detect an object depends on the objects brightness, the angle it subtends at the eye of the observer, the brightness of the background surrounding it, and the level of illumination to which the eye is adapted. Furthermore, the brightness difference between the object and background of a first object-background pair may be the same brightness between the object and background of a second objectbackground pair and yet the two objects will not be equally visible if the average brightness of the first object-background pair is darker than the average brightness of the second object-background pair; the darker the objectbaokground pair will be more visible.

The important factor is the relative contrast between the object and background. If the brightness of a target is B, and the brightness of another target or the background to the first target, is B, their relative contrast is defined as universal contrast C and is determined by the equation:

wherein For target-background pairs having a given target size, at a given eye-adaptation level, all pairs having the same value of C or m will be equally visible. Near the lower limit of detectability, an object slightly darker than its background will be equal in visibility to another object slightly brighter than its background, if the absolute value of the contrast is the same in both cases, even though it is negative in one case and positive in the other. It is to be noted that, for a given contrast constant, the difference in brightness between an object and background is increased if the background becomes brighter.

*In order to obscure effectively, an obscuration agent must decrease the target-background brightness difference and increase overall brightness.

Brightness reduction by a smoke cloud is accomplished by attenuation of the light passing through it. All sources of white light have their apparent brightness decreased in the same proportion. If the agent produces a truly black smoke, the smoke cloud attenuates without adding scattered light. In such a smoke cloud, the difference between the target and background brightness is decreased, but the background brightness itself is decreased in the same proportion, and there may be no change or an increase in contrast. Unless the whole visible field becomes dark enough to cause a significant change in eye-adaptation level, the smoke will give no reduction in detectability.

On the other hand, a white smoke, which attenuates light passing through it, also scatters light into the eye from all sources of illumination, particularly the sun or moon. Reduction in brightness difference is accompanied by an increase in apparent brightness of both the target and background which results in a significant decrease of contrast and visibility.

As stated above, the minimum detectable contrast is a function of the object angular size and eye-adaptation level. Such detectable contrast is called the limen and will be indicated by the symbols G or m Under most condi- 4 tions of viewing, the limen is a small number. This implies that E and B are nearly the same in aforementioned Equations 1 and 2 and also that EZ B in aforementioned Equation 3, from which it follows that Equation 4 gives a convenient means of relating test results .given in terms of the modulation contrast m, to the results of tests with human observers, which are nearly always given in terms of the universal contrast C.

The most significant aspect of my invention is that, for a given set of source-target-agent-observer locations, at any instant of time, there is a single evaluation, parameter, the obscuranee, for each munition which permits an absolute evaluation of the contrast-reducing ability. Using the obscuranee parameter, munitions can then be ranked according to such parameter, and the effectiveness of each determined under various field conditions. Similarly, industrial smoke can be ranked according to the degree of air pollution being generated, or the degree of atmospheric turbulence measured in micro-meteorological studies by using a smoke cloud as an indicator means.

Contrast-reducing ability can be expected to change with different relative positions of my apparatus used to determine it, and there will be a set of obscurances for each munition. Obscurance will also change as a result of changing meteorological conditions and varying particle size distributions. Accordingly, the relative ranking of munitions and industrial smoke clouds may be different for different cloud particulate geometries and different weather conditions.

Obscurance, as stated above, is determined from measurements of contrast, or equivalently, from measurements of visual brightness. Instruments for measuring brightness are termed photometers, and a large variety of photometers, of two general classes, have been described in the literature; the two classes being visual and photoelectric photometers. Measurements by visual photometers consist of visual comparison of an image of the desired field with an image of a field having a known brightness, whereas photoelectric photometers give electric outputs having a magnitude which can be related to the brightness of the field viewed by the photometer.

Brightness measurements with a visual photometer is an inherently slow process, because the determination of contrast requires measurement on two fields and gives meaningful results only if the two measurements are made nearly simultaneously. Since the brightness of both targets viewed in field tests of obscuration agents can be expected to vary rapidly and continuously throughout the tests, visual photometers would not be expected to give useful measurements.

While photoelectric photometers have a wide range of response times, such photometers can be selected to respond rapidly to varying signals. In addition, photoelectric photometers have the additional useful property of giving an output which can be recorded continuously in the form of a permanent record from which brightness, contrast, and obscuranee can be calculated at a later time.

For the aforementioned reasons, the design of my apparatus to determine smoke cloud obscurance was based on a photoelectric photometer. A photomultiplier was selected, because it has a fast response and an amplified output which is linear with the intensity of the incident light, requires only a simple power supply and can be purchased inexpensively with a spectral response that is matched readily to that of the human eye.

In solving the problem of making contrast measure ments between a target pair having rapidly-varying brightness, two systems can be utilized as follows: (1) Two targets of different inherent reflectivities may be presented alternately to a photometer having a fixed line of sight or (2) Two fixed adjacent targets may be viewed alternately by; photometer having an alternating or rotating line of Slg t.

Above system (1) has the advantage that the photometer design is relatively simple, and a fixed line of sight minimizes spurious modulations; such modulation being caused by turbulent agent-cloud brightness variations. On the other hand, system 1) requires an independent source of target movement and a synchronizing signal which originates at the target for optimum detection sensitivity. Since target-detector distances may be quite large in the field use, this involves additional power and signal cables that may be quite inconvenient. Also, the photometer is designed with only a moderately small fieldof view so that it can have adequate sensitivity in a compact unit without sophisticated or expensive components. At field use ranges of hundreds of feet, this system requires rather large targets, and large rotating or oscillating targets in the field necessitates massive, cumbersome, and powerconsuming construction.

Above system (2), my preferred embodiment, permits the use of simple, stationary targets, and all power and synchronizing signals are located at one station. The optical axis of the photometer of system (2) can be oscillated or rotated in a number of manners, all of which can be accomplished with negligible power. The oscillation of the line of sight can result in a residual small signal modulation caused by optical asymmetry inside the instrument and in the transmission of different transmissive and reflective properties of the smoke cloud along the two lines of sight. Residual small signal modulation caused by optical asymmetry was minimized by careful design.

None of the commercially available photometers has a satisfactory combination of speed, sensitivity, spectral response, field of view, oscillating optic axis, compactness, and low cost. Therefore, my apparatus was conceived and reduced to practice to overcome the above stated problems and to satisfy the long felt need for an apparatus to measure military smoke cloud obscurance and industrial smoke cloud air pollution.

In using my apparatus, as shown in FIG. 1, the photom-. eter, as shown in FIGS. 1, 2, and 3 at 1, is sited and focused on contrasting

targets

3 and 4 by means of eyepiece 6, shown in FIG. 3; target 3 being substantially white and target 4 substantially black.

Targets

3 and 4 are located on the side of smoke cloud 2, generated by a conventional smoke munition 5, opposite to photometer 1. Any conventional multi-recorder, such as a six channel multiplexed routine automatic data conversion portable digital recorder, is connected to a terminal 7, electrical power is supplied to the synchronous motor for operating the oscillating mirror by connecting plug 8 to any conventional AC outlet and placing the mirror oscillator in operation. Photomultiplier 18 is supplied by a conventional high voltage supply through. terminals 50 by turning switch 10 to the on position, and the sensitivity of the photomultiplier is controlled by rheostat 9.

Light passes through elbow telescope 11 which incorporates the optics shown in insert A in FIGS. 3, 4, and 5, a conventional light filter 12, such as a Wratten filter, an objective lens 13, filter wheel 14 having a plurality of filters mounted thereon for operation at different light levels, switch means 29 to select an appropriate filter, a housing to accommodate optical system components, eyepiece 6, field stop 17, and photomultiplier 18.

A number of alternative mechanisms for alternating the field of view to be seen by the photomultiplier can be utilized as follows:

( l) Rotating optical wedge (2) Tuning-fork chopper (3) Rotating eccentric mirror (4) Oscillating mirror Mechanisms (1) and (2) permit a straight-through optical system, while mechanisms (3) and (4) require a bend in the optical axis which will result in a different sensitivity to horizontally and vertically-polarized incident light.

However, it is somewhat more complicated mechanically to construct an instrument with mechanisms (1) and (2). Mechanisms (1) and 3) cause the field of view to rotate which requires a target that is essentially circular, and mechanisms (2) and (4) give an oscillating field of view which requires a target that is extended horizontally but not vertically, a somewhat simpler geometry for field use. Mechanism (4) is my preferred embodiment, and the heart of this configuration is the oscillating mirror. Mechanism (4) consists of a front surface mirror 19 rigidly mounted on a stainless steel torsion bar 20 which is restrained at its extreme ends by mounts 27. Mounted on the mirror is a small nylon rod, not shown in the drawing, which bears against a rotating eccentric plate 21 driven by a synchronous motor 22. To minimize dynamic and friction problems, a 300 r.p.m. motor was selected which gives a signal which oscillates at 5 Hz. The position of the eccentric plate 21 on the driving shaft 23 can be adjusted to set the mirror 19 to an angle of 45 degrees with the axis of the telescope 11 which turns the light path approximately degrees; the preloading of the torsion bar is also governed by this adjustment. Telescope objective lens 13 and mirror 19 focus an image on the image plane 24, in which is machined an aperture 25 which defines the field of view through the relation:

Aperture diameter Field of view in TELdlZLnS- W Light passing through aperture 25 diverges and falls on the photocathode 26. The cone of light oscillates slightly as the mirror oscillates through an angle determined by the eccentricity of the eccentric plate so that the images of the two targets fall on different portions of the photocathode, which may have different sensitivities. To minimize the effects of sensitivity variation, photomultiplier 18 is mounted at such a distance from the aperture that there is considerable overlap between the cones of light to cover a large enough area and! still fall entirely within the bounds of the photocathode. The preferred embodiment of mechanism (4) is shown in FIGS. 4 and 5; FIG. 4 consisting of two views, the upper view being a top view and the lower view the front view. Alternative to using a synchronous motor drive for mirror 19, a conven tional electro-magnetic drive can be used as shown in FIGS. 5 and 8 wherein structures 28 are electro-magnetic coils.

An index wheel, not shown in the drawing, can be mounted on the motor shaft to allow the operator to position the mirror at a known point so that the instrument can be accurately pointed at a target or standard refiectance surface during calibration or for brightness measurements before a field test.

The photomultiplier has a conventional S4 spectral response and a conventional light filter, such as a Wratten No. 106 filter, not shown in the drawing, mounted behind field stop 17 that gives an overall spectral response closely matching that of the photop-ic eye.

Eyepiece 6, not shown in detail in the drawing, consists of a simple lens and a mirror to form a virtual image of the image plane for the human eye; the image plane being painted white; the field stop appearing as a black dot, accurately defining the area viewed by the photomultiplier. With a light filter behind the image plane, a somewhat brighter image is seen in the eyepiece.

While the design parameters used in the optical system of my apparatus are as follows:

(1) Objective lens, 2 /2 in. dia., 7 in FL.

(2) Field stop aperture, 0.025 in. dia.

(3) Angular field of view:

Calculated=0.02S/7 =.0036 rad=l2.5 min. Measured=.0045 rad=l5 min.

(4) Angular shift of image=26 min.

(5) Rate of oscillation of field of view=5 Hz.

these parameters are adjustable within the skill of the art to suit any given application for my device and method.

The oscillating mirror in the photometer causes first the white portion of target 3 to be imaged on the photocathode, then the black portion of target 4 which results in two signals being produced; a peak signal and a modulation signal. The peak signal, designated DC, is proportional to the maximum brightness detected by the photometer; whereas the modulation signal, designated AC is proportional to the logarithm of the difference between the maximum and minimum brightness detected when the photometer scans across the target. If B is the maximum brightness and B is the minimum brightness, then DC=KB and out= ln[ max min) 1 where c and d are constants applicable to the particular logarithmic amplifier used as set forth in subsequent equations and k is the calibration constant of the photometer.

The modulation signal, AC is linearly proportional to the brightness difference m= u-lax min) P( 0+ 1 out) =e( 0+ 1 out) wherein a =d/c and a =1/c.

a and a are the calibration constants of the logarithmic amplifier of the contrast photometer and are determined from a least squares fit to the calibration curve. The contrast is defined as max. i-nin.) max.+ min.

which can then be written in terms of output voltages, DC, and AC as 2DC AC With m the inherent or background contrast, and m the contrast with the obscuration agent present, the modulation contrast transmittance 1 is defined by m m max. min. x maX.' min. o

A0... 1 2120-110... x in wherein the subscripts o and x refer to measurements of the target performed without an obscuring agent and with an obscuring agent, respectively.

Finally the obscurance R is given by wherein R is the average inherent reflectivity of the test target. The ratio of the obscurance to the average refiectivity, R/R is then the quantity calculated to determine the effectiveness of the obscuration agents or pollution caused by particulate pollutants.

Signal processing by the electrical circuitry shown in FIGS. 7 and 8 is most clearly understood by considering the schematic diagram presented in FIG. 6. Light waves 30 passing through lens 13 and striking mirror 19 are transmitted through aperture 25 of field stop 17 to photomultiplier 18. Photomultiplier 18 sensitivity is matched to the intensity range of the input signals of the light wave by a conventional matching circuit 31 through varying a high voltage supply 32 by means of input voltage adjustment 33. The output signal from photomultiplier 18 is transmitted to pre-amplifier 34 and the peak to peak value of the wave form and the base to peak value of the wave form of the pre-amplifier output signal detected as the peak signal and modulation signal respectively. The peak signal is transmitted to peak signal detector means 35 and subsequently to output amplifier 36 for amplifiication to a suitable level to be recorded by the previously discussed digital recorder connected to terminals 37. The modulation signal is filtered by filter means 38 and transmitted to log amplifier 39 to be compressed logarithmically to produce a linear output over a signal range of 1000:1. Amplifier 39 signal is filtered by active filter means 40 and transmitted to phase sensitive detector means 41 and to output amplifier 42 for amplification to a suitable level to be recorded by the previously discussed digital recorder connected to terminals 37. The modulation signal is detected by means 41 synchronously with a reference signal from a scanner supply having a phase shift means 43 and a chopper driver means 44 connected therein. Mirror means 19 is operated by oscillator driver means 45. Although both the peak and modulation signals can be recorded in analog form on a simple dual-track recorder, the signals are also suitable for digital recording.

The time constants and rates of data acquisition depend on the requirements of the evaluation procedure. The basic data acquisition rate must be higher than the maximum significant frequency contribution to the variance of the contrast or obscurance, if the total variance is to be measured reliably.

The logarithmic amplifier in the circuit is quite temperature sensitive. Since small gain changes due to ambient temperature variation produce, in a circuit with logarithmic response, output changes corresponding to large apparent changes in the input, and since the entire optical and electronic package is mounted in a single enclosure, a thermostatically controlled heater circuit, as shown schematically in FIG. 9 and including thermostat means 46, relay means 47, and heater means 48 and 49, is provided inside the entire enclosure to permit operation of the electronic system at approximately 22 C. It is well within the skill of the art to adjust the heater circuitry to suit any necessary operating temperature requirement for any given application.

The circuitry described above and shown schematically in FIGS. 6 and 9 is shown in detail in FIGS. 7, 8, and 10 to 12. It is considered that one of ordinary skill in the art will fully understand the operation as shown in FIGS. 7, 8, and 10 to 12. in the light of the above schematic description and no further explanation is necessary. While each component in the electronic circuitry of my apparatus is conventional, the circuitry design is novel, unobvious, and solves the aforementioned prior art problems. The optical and electro components of my invention can be changed and modified within the skill of the art to adapt the invention to any visible or invisible electro-magnetic wave application.

It is obvious that other modifications can be made of my invention, and I desire to be limited only by the scope of the appended claims. 1

I claim:

1. An apparatus for measuring the degree of obscuration created by a smoke cloud selected from the group of smoke clouds consisting of chemical smoke clouds and industrial smoke clouds comprising an optical system fixedly mounted within a telescope means, said optical system comprising a light filter means located adjacent to the large diameter end of the telescope, an objective lens means located adjacent to the filter means and an eyepiece means, an oscillating mirror means located adjacent to the eyepiece means and behind said lens means, a photomultiplier means and an aperture located in a field stop means, said mirror means being positioned to receive light waves from a distant light source and to transmit the light waves to said photomultiplier means through said aperture means, said eyepiece means located means, said eyepiece means serving to focus the optical system on said light source, a filter wheel means having a plurality of filters mounted thereon, means for selecting from said plurality of filters a predetermined filter for a predeter mined light source, said filter wheel means being located between the oscillating mirror means and the aperture means located in the field stop means, said field stop means being located between the filter wheel means and said photomultiplier means; said photomultiplier means converting light waves received from the optical system into electrical impulses; means to measure the electrical impulses from the photomultiplier means as a peak signal and a modulation signal; a first amplifier means to amplify the peak signal for recordation; a filter means to filter the modulation signal; a log amplifier means to logarithmically compress the modulation signal; an active filter means to filter the logarithmically compressed modulation signal; a second amplifier means to amplify the modulation signal for recordation and a recorder means to record the peak signal and the modulation signal.

2. The apparatus of claim 1 wherein the oscillating mirror means has a mounting system comprising a stainless steel torsion bar rigidly mounted on the mirror, said torsion bar being restrained at the outer ends by mount means fixedly attached to a frame means of the telescope means; and a nylon rod means fixedly mounted on the mirror means, said nylon rod means bearing against a rotating eccentric plate to oscillate the mirror means.

3-. The apparatus of claim 2 wherein the eccentric plate is driven by a synchronous motor.

4. The apparatus of claim 2 wherein the eccentric plate is driven by an electro-magnetic circuit means.

5. The apparatus of claim 1 wherein the photomultiplier means has a sensitivity matched to the intensity range of the input signals of a light Wave, said matching being produced by a circuit comprising a matching circuit means; a high voltage means; and a voltage adjustment means.

6. The apparatus of claim 1 wherein the means to measure the electrical impulses from the photomultiplier means is a resistance-capacitance peak detector circuit and a phase-sensitive chopper detector circuit to measure the peak signal and the modulation signal respectively.

7. The apparatus of claim 1 wherein the recorder means is a six channel multiplexed automatic data conversion digital recorder.

8. A system for measuring the degree of obscuration created by a smoke cloud selected from the group of smoke clouds consisting of chemical smoke clouds and industrial smoke clouds comprising apparatus having an optical system fixedly mounted within a telescope means, said optical system comprising a light filter means located adjacent to the large diameter end of the telescope, an objective lens means located adjacent to the filter means and an eyepiece means, an oscillating mirror means located adjacent to the eyepiece means and behind said lens means, a photomultiplier means and an aperture located in a field stop means, said mirror means being positioned to receive light waves from a distant light source and to transmit the light waves to said photomultiplier means through said aperture means, said eyepiece means located at the end of the telescope means opposite to the lens means, said eyepiece means serving to focus the optical system on said light source, a filter wheel means having a plurality of filters mounted thereon, means for selecting from said plurality of filters a predetermined filter for a predetermined light source, said filter wheel means being located between the oscillating mirror means and the aperture means located in the field stop means, said field stop means being located between the filter wheel means and said photomultiplier means; said photomultiplier means converting light waves received from the optical system into electrical impulses; means to measure the electrical impulses from the photomultiplier means as a peak signal and a modulation signal; a first amplifier means to amplify the peak signal for recordation; a filter means to filter the modulation signal; a log amplifier means to logarithmically compress the modulation signal; an active filter means to filter the logarithmically compressed modulation signal; a second amplifier means to amplify the modulation signal for recordation and a recorder means to record the peak signal and the modulation signal and a pair of contrasting target members for reflecting light waves which result in the peak signal and the modulation signal, one member of the target pair being substantially White and the other member of the target pair being substantially black.

9. A method for measuring the degree of obscuration created by a smoke cloud selected from the group of smoke clouds consisting of chemical smoke clouds and industrial smoke clouds comprising the steps of focusing a telescope means on a pair of contrasting target means, locating said target means on a side of the smoke cloud opposite to the telescope means; oscillating a mirror means to selectively reflect light waves emanating from each member of the pair of target means through the smoke cloud to the optical system to detect the transmissive and reflective properties of the smoke cloud; transmitting the light waves from the optical system to a photomultiplier means to convert the light waves into electrical impulses; matching the photomultiplier means sensitivity to the intensity range of the light Wave signals; splitting the electrical impulses into two components, one component being a peak signal and the other component being a modulation signal; detecting the peak signal to determine the peak signal characteristics of the smoke cloud; logarithmically compressing the modulation signal; detecting the modulation signal to determine the modulation signal characteristics of the smoke cloud; recording the peak signal and the modulation signal; and determining the obscurance by means of the equation O (1-rm) wherein R is the obscurance, R, is the average inherent reflectivity of the target, and

wherein AC, is the modulation signal, DC is the peak signal, and subscripts 0 and x are measurements of the target without a smoke cloud intervening and with a smoke cloud intervening respectively.

References Cited UNITED STATES PATENTS 2,198,971 4/1940 Neufeld 356-208 X RONALD L. WIBERT, Primary Examiner F. L. EVANS, Assistant Examiner U.S. Cl. X.R.