Transmission and reflection light losses are reduced by introducing light into end-window of device at an angle large enough to normal so total internal reflection occurs at both photocathode-vacuum and window-air interfaces

Grant, G. R.

Gunter, W. D., Jr.

English

NASA Technical Reports Server

May 1971	 Brief 71-10113 
NASA TECH BRIEF
	 V Ames Research Center 	 W 
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. 
Optical Enhancement of Photomultiplier Sensitivity 
The problem: 
Increase the sensitivities of commercially available 
photocathode devices to values approaching the 
theoretical limit for visible light. Semitransparent 
photocathodes commonly used in photomultipliers 
sensitive to visible light (S-il, S-20, etc.) do not 
absorb all the incident light. At a wavelength of 
400 nm, a typical S-20 photocathode will reflect 
40%, transmit 10%, and absorb only 50% of the 
light; at 700 nm, the same photocathode will re-
flect 30% transmit 50%, and absorb only 20%. 
The solution: 
Reduce light losses due to transmission and re-
flection by introducing light into the end-window of 
the device at an angle large enough to the normal 
so that total internal reflection occurs at both the 
photocathode-vacuum and window-air interfaces. 
How it's done: 
Immersion oil or optical cement is used to 
optically couple a small 7r/4-7r/2-7f/4 rad (450W 
90°-45°) glass prism to the piano-piano end window 
of a photomultiplier. The multiple reflections re-
sulting cause the photocathode to absorb more 
of the light, thus increasing photomultiplier sensi-
tivity by a factor of 1.5 to 5, depending on wave-
length.
The refractive index of the photocathode need 
not be known to determine the angle within the 
end window for total reflection. In layered media, 
n 1 sinO 1
 = n2 sinO 2 = . . . nsinO = a constant, k 
Light traveling in the layered media will be totally 
reflected whenever it encounters a layer with an 
index of n<k. Since the refractive indices of air 
and vacuum are both approximately 1.0, the con-
dition for total reflection at the air-window inter-
face and the cathode-vacuum interface are the same, 
namely, sin0 2 >(l/n 2) in the end window. Although 
little light is lost as a result of scattering at the 
two surfaces, a dark-adapted observer can see wheie 
the light is bouncing on the cathode and the ex-
ternal surface of the window by viewing the small 
amount of forward scattered light. This can be use-
ful in making adjustments. 
Factors to be considered in designing optical 
enhancement devices include the area and acceptance 
angle required, the wavelengths of interest, and the 
thickness and optical index of the photomultiplier 
window. These factors interact with each other and 
with other design parameters such as the optical 
index of the entrance optics. For example, with the 
simple glass prism shown in the figure, the ac-
ceptance cone is limited to f/6; however, with a 
prism of high-index glass, speeds of f/2 and faster 
can be attained. Sapphire may be employed for 
small devices, especially those designed for photo-
multiplier tubes having windows transparent to UV 
radiation. Special entrance optics can be adapted 
for systems involving physical constraints which pre-
vent the use of simple prisms.
(continued overleaf) 
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References: 
1. Gunter, W. D., Jr.; Grant, G. R.; and Shaw, S. 
A.: Optical Devices to Increase Photocathode 
Efficiency. Applied Optics, vol. 9, p. 251, Feb-
ruary 1970. 
2. Jennings, R. J.; Gunter, W. D., Jr.; and Grant, 
G. R.: Quantum Efficiencies Greater than 50% 
from Commercially Available Photomultipliers. 
Journal of Applied Physics, vol. 41, No. 5, p. 
2266-2267, April 1970. 
Notes: 
1. The combination of field-enhanced gain with the 
optical gain described provides even greater sensi-
tivity in the near-infrared region. (See W. D. 
Gunter, Jr., R. J. Jennings, and G. R. Grant, 
"Dual Enhancement of Photocathode Sensitivity in 
the Near Infrared," Applied Optics, vol. 7, p. 
2143, October 1968.)
2. A 14 1/2-minute sound, color film entitled "Op-
tical Enhancement of Photomultiplier Tubes—
A Method of Obtaining Large Increases in the 
Quantum Efficiency of Photocathodes" is avail-
able for loan. Requests for loan should be directed 
to the Technology Utilization Officer at Ames 
Research Center. 
3. Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B71-10113 
Patent status: 
No patent action is contemplated by NASA. 
Source: W. D. Gunter, Jr. and G. R. Grant

Ames Research Center

(ARC-10213)
. 
. 
Brief 7l-l0ll3	 Category Ol,03


Optical enhancement of photomultiplier sensitivity

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