Electronic means of performing the equivalent of automatic aperture control has been devised for the new class of television cameras that incorporates a solid state imaging device in the form of phototransistor mosaic sensors

Anders, R. A.

Callahan, D. E.

Mc Cann, D. H.

English

NASA Technical Reports Server

February 1968
	 Brief 68-10028 
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. 
Electronic "Aperture Control" Devised for Solid

State Imaging System 
An electronic means of performing the equivalent of 
automatic aperture control has been devised for the 
new class of television cameras that incorporates a 
solid state imaging device in the form of phototransis-
tor mosaic sensors, as described in Tech Brief 66-
10112. The control system enables the sensors to 
detect a useful gray scale, positioned over a wide 
range of viewing light levels, without requiring either 
variation of the lens opening or automatic gain con-
trol of a video amplifier. Operation of the system 
involves controlling the effective limits over which 
time integration of the light incident on each image 
point takes place in its corresponding sensor element. 
In this system, variation of the integration period to 
give electronic aperture control is achieved by varying 
the system frame rate. Thus, a voltage controlled 
oscillator acting as the master clock for stepping the 
digitally sequenced interrogation circuits can be 
servo controlled from a separate video detector. This 
detector is matched to the entire frame so as to set 
the frame time to the proper interval for the average 
of all picture elements to reach an optimum video 
level. The upper limit of control, representing the 
highest light level to which the upper end of the sys-
tem dynamic range can be set, is an integration 
period in which total discharging is caused by that 
intensity for which steady state photoresponse is 
more significant than the transient response. The 
dynamic range that can be displayed at this setting is 
limited by any mosaic commutator inaccuracies and 
nonlinearities of subsequent video processing circuits. 
The lower extreme is a frame time setting for which 
the discharging due to junction leakage is still insig-
nificant compared to the lowest shade of gray in a 
useful range of light levels.
Control by varying the frame rate comp1icaes the 
task of image reconstruction and display. With stand-
ard real time cathode ray tube display, the lower 
frame rates required for sensing at very dim light 
levels produce objectionable shading and flicker. 
However, many television applications for which this 
method of light level control is pertinent also involve 
recording of all video data before display. There are a 
number of scan conversion techniques for displaying 
slow-scan data at viewable raster rates which can be 
employed with this control system. (See for example 
Tech Brief B67-10676 "Scan Rate Converter for Tape 
Recording and Playback of TV Pictures.") 
Notes: 
I. Two alternative methods of electronic "aperture 
control" are: (I) control of the total charge in the 
sensor for light integration by varying the bias 
pulse amplitude to vary the range of useful input 
light: this method has the disadvantage of requiring 
video gain control; (2) control of the total available 
charge by varying either the charge time (readout 
period) or the charging impedance; this alternative 
method has some objectionable image holdover 
from frame to frame. 
2. Inquiries concerning this invention may be di-
rected to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B68-10028 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
-	 (continued overleal) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19680000028 2020-03-12T05:41:50+00:00Z
Source: R. A. Anders, D. E. Callahan, 
and D. H. McCann
of Westinghouse Electric Corporation 
under contract to
Marshall Space Flight Center
(M FS- 12428) 
Brief 68-10028	 Category 01


Electronic aperture control devised for solid state imaging system

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