Rapid-response electro-optical, light exposure control system, will maintain the light reaching a camera film or other light-sensitive detector at essentially constant level, despite wide variations in the brightness of the light source. The system permits detailed photographic or photoelectric recording of the phenomenon over a range of brightnesses

Kuehl, D. K.

Zwillenberg, M. L.

English

NASA Technical Reports Server

December 1968	 Brief 68-10502 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Rapid-Response, Light-Exposure Control System 
Polarizer P1 
 
Beam Splitters 
Light Kerr 
Device	 k4:TCell	 !Jfl1 ,i" ,,/ 	 I Variable Sensitive I Light I Source 
-, ', v—
Photodiodes 
It is often desirable to study luminous phenomena 
or light sources whose brightness varies over a wide 
range during the period of interest. Examples of such 
phenomena include metal combustion, exploding 
wires, pulsed gas discharges, laser operation, and 
nuclear explosions. In many of these phenomena, 
brightness changes, which occur within milliseconds 
or even microseconds, often exceed the exposure 
latitude of presently known photographic materials or 
the linear ranges of light-sensitive detectors, resulting 
in either underexposure or overexposure during the 
period of interest. 
One technique that has been used to overcome such 
a problem requires the use of photographic film which 
incorporates several emulsions. of different sensitivi-
ties. The cost of such film and its processing is quite 
high, a disadvantage which is of particular significance 
when large amounts of film are required, as in high-
speed cinephotography. The types of photoelectrically 
actuated electromechanical aperture or shutter-speed 
control, which are used in many commercially avail-
able automatic cameras to control film exposure, 
cannot be used, since such exposure-control devices 
,have a relatively slow response (of the order of 
seconds), while in many situations exposure control 
with response times of milliseconds or microseconds is 
required.
A rapid-response (on the order of a few micro-
seconds) elect ro-optical, light-exposure control sys-
tem, which has been devised, will maintain the light 
reaching a camera film or other light-sensitive detector 
at essentially constant level, despite wide variations in 
the brightness of the light source. This system permits 
detailed photographic or photoelectric recording of 
the phenomenon under study over a range of many 
orders of magnitude in brightness. The system in-
cludes a pair of crossed light polarizers, Pi and P2, with 
a Kerr cell between them. These three components are 
positioned in the path of light from the variable light 
source to be studied to the light-sensitive device 
(camera or photoelectric detector). Light from the 
source is transmitted to polarizer Pt through beam 
splitters which reflect some of the light to high-voltage 
vacuum photodiodes. The function of the beam split-
ters is to direct light to the photodiodes in such a way 
that the brightness of the light reaching them is 
propprtional to the brightness of the light reaching 
polarizer P1. Circuitry that is used to control the 
voltage across the Kerr cell as a function of the bright-
ness of the light from the source is not shown in the 
schematic. In this circuitry, the Kerr cell is connected 
in series with a variable resistor, RL, across a voltage 
source (a dc, high-voltage power supply). A series 
chain consisting of the photodiodes and two resistors, 
(continued overleal) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19680000458 2020-03-12T06:03:41+00:00Z
RD and RM, are connected across the Kerr cell. A 
high-resistance chain of three equalizing resistors is 
connected across the photodiodes to compensate for 
differences in dark current among these diodes. RL, a 
series of switch-selected resistors, controls the sys-
tem's sensitivity. The function of RD is to control the 
minimum voltage that is applied across the Kerr cell. 
The voltage across RM (a low resistance compared to 
RD) is used to monitor changes in current through 
RM, which are related to the changes in brightness of 
the light source. When the brightness of the light 
varies between minimum and maximum levels, the 
effective resistance of each photodiode varies between 
infinity and zero. Thus, the voltage across the Kerr 
cell varies between a maximum and a minimum, and 
the plane of polarization of light from Pi varies ac-
cordingly between 900 and a minimum angular value. 
Therefore when the brightness of the light from the 
light source is below a preassigned level, all of the light 
which passes through Pi is transmitted through P2 to 
the light-sensitive device. As the brightness increases, 
the voltage applied to the Kerr cell decreases automati-
cally, so that only a portion of the light which passes 
through Pj is transmitted to the light-sensitive device.
The net result is that the intensity of the light received 
by the light-sensitive device is essentially at a con-
stant level. 
Note: 
Details may be obtained from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP68 10502 
Patent status: 
This invention is owned by NASA, and a patent

application has been filed. Royalty-free, nonexclusive

licenses for its commercial use will be granted by

NASA. Inquiries concerning license rights should be

made to NASA, Code GP, Washington, D.C. 20546.

Source: M. L. Zwillenberg
and D. K. Kuehl

of United Aircraft Corporation

under contract to

NASA Pasadena Office

(N P0-10238)
. 
Brief 68-10502	 Category 01


Rapid-response, light-exposure control system

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