Image converter has been developed which uses opaque photocathode for improved efficiency. Device is easier to fabricate than previous semi-transparent photocathode converters and uses compounds from Groups 3-5 that are responsive to wave-lengths between ultraviolet (approximately 100 nm) and near infrared region (approximately 1000 nm)

Hallam, K. L.

Johnson, C. B.

English

NASA Technical Reports Server

August 1973	 .	
-	 B73-10255 
•	 NASA TECH BRIEF 
Goddard Space Flight Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
Commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
A Magnetically Focused Image Tube Employing an Opaque Photocathode 
The problem: 
Optical images in infrared or ultraviolet spectra are 
beyond perception of the human eye. To see them, the 
observer normally uses an image converter that repro-
duces these spectra in visible light. This device converts 
ultraviolet and infrared optical images into electron 
beams which are projected on special screens making 
them visible. Unfortunately, existing image converters 
utilize relatively inefficient semi-transparent photo-
cathodes. These converters are difficult to manufacture 
and their performance varies greatly from unit to unit. 
Tube Axis 
I	 Electrode 
Opaque Rings 
	
Photocathode	 (Electron Lens) 
I Envelope 
	
l pe__.._.2\\	
/ Electrons 
of Focuse
Solenoid 
Solenoid 
DC Power 
I	 \ Screen 
 i..—r--i 
Ar
  Focus \ Magnetic 
Optical ,fTF:f
Coil Axis 
Light 
Axis 4h Output 
Light 
Input
.
	
Image Converter
The solution: 
An image converter has been developed which uses 
an opaque photocathode for improved efficiency. The 
device is easier to fabricate than the previous semi-
transparent photocathode converters and uses com-
pounds from Groups Ill-V that are responsive to wave-
lengths between the ultraviolet (approximately 100 nm) 
and the near infrared region (approximately 1000 nm). 
How it's done: 
The image converter (see. figure) is a vacuum tube 
which includes a flat, opaque photocathode electrode at 
one end and a flat electron target electrode (screen) at 
the other end. The screen may be a phosphor surface for 
a direct optical readout of the electron beam image 
derived from the photocathode, or a gain/storage eléc-
trode read out by an electron beam. Both the electrode 
and the screen are within the line of sight of each other. 
Their planar faces are laterally displaced relative to each 
other so that an optical image on the emitting face is not 
obstructed by the screen. In response to the optical 
image focused on its surface, the photocathode emits 
an electron beam. This beam is then focused by the 
electron lens onto the phosphor screen. 
The electron lens establishes a constant, dc, homo-
geneous electric field having a longitudinal vector along 
and parallel to the optical axis. It also provides a 
constant, dc, homogeneous magnetic field having a 
longitudinal vector between the photocathode and 
screen along and parallel to the magnetic focus coil 
axis. The magnetic coil axis is tilted at an angle U from 
the optical axis. 
The constant, homogeneous electric field is estab-
lished by several evenly spaced, parallel, annular, metal 
electrode rings on the inner circumferential wall .of the 
tube. The dc power supply whose negative and positive 
terminals are, respectively, connected to the photo-
electrode and the screen, also provides a different 
.
(continued overleaf) 
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=19730000255 2020-03-17T07:14:29+00:00Z
intermediate potential on each electrode ring at several 
points along the voltage divider. The voltage at each of 
these points is proportional to the distance of the 
particular electrode ring from the photocathode. 
In operation, the image on the optical axis is 
focused by the optical lens onto the photocathode 
surface. The photocathode emits electrons that are 
accelerated by the homogeneous constant electric field 
and focused by the magnetic field produced by the 
solenoid along the tilted axis. The electrons are uniform1y, 
accelerated along the tilted axis by the combined actions 
of the homogeneous, constant, electric and magnetic 
fields. The magnetic field also imparts cyclotron motion 
to focus the electrons on the screen. When the electrons 
strike the phosphor screen, they produce a visible image. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Goddard Space Flight Center 
Code 207.1 
Greenbelt, Maryland 20771 
Reference: B73-10255
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Inquiries concerning non-
exclusive or exclusive license for its commercial de-
velopment should be addressed to: 
Patent Counsel 
Goddard Space Flight Center 
Code 204 
Greenbelt, Maryland 20771 
Source: C. Bruce Johnson and 
Kenneth L. Hallam of
Bendix Corp.
under contract to
Goddard Space Flight Center
(GSC-1 1602)
. 
. 
B73-10255	 Category 02


A magnetically focused image tube employing an opaque photocathode

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