Compact spectral irradiance probe has been designed and built which uses wedge filter in conjunction with silicon cell and operational amplifier. Probe is used to monitor spectral energy distribution of solar simulators and other high intensity sources

Davidson, G. A.

Mcwilliams, I. G.

Seward, H. H.

English

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July 1972
	
B72-10327 
NASA TECH BRIEF 
NASA Headquarters 
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 Compact Spectroradiometer for Solar

Simulator Measurements 
Cells 
The problem: 
There is a need for a compact probe to monitor the 
spectral energy distribution of solar simulators and other 
high intensity sources. 
The solution: 
A compact spectral irradiance probe has been de-
signed and built which uses a wedge filter in con-
junction with the silicon cell and the operational
amplifier. Its recording wavelength range is 400 to 750 
nm front which 23 bands can be selected by an auto-
matic stepping switch each having a 20-nm bandwidth. 
How it's done: 
The wedge filter, model VERIL B-60. used in this 
system is cut into three sections and split lengthwise 
into four parts. Two of these are cemented to form a 
double t'ilter, thus making two sets of filters. One 
• 
(continued overleaf) 
This document was prepared under the sponsorship at 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=19720000326 2020-03-17T03:29:13+00:00Z
additional filter, a Schott BG-38, is included for the 
short wavelength section around the 400 nm range. 
Radiation transmitted through the filters is picked 
up by silicon photovoltaic cells, which are a part of the 
photovoltaic readout assembly. Output of these cells 
varies linearly with wavelength. For example, at 700.nm 
wavelength with 20-nm band, a cell may have sensitivity 
of 0.5 A/W, giving a current of 18.55 IAA. At 400 rim, 
this sensitivity is approximately 0.00267 A/W, giving an 
output of 100 nA. A schematic of irradiance probe is 
shown in the figure. 
An operational amplifier with a minimum gain of 
105
 converts this cell current to an output voltage. The 
amplifier acts as an inverter such that the current tend-
ing to drive the amplifier input positive produces a 
negative output voltage. Because the desired amplifier 
output is 0.5 V, the amplifier feedback resistance should 
vary between 27 kilohms and 5 megohms for 700 nm 
and 400 nm, respectively. 
Twenty-three channels are provided for selection of 
wavelength bands within this range. These channels are 
selected with a stepping switch which connects the 
proper feedback resistance across the amplifier. 
The probe is calibrated by use of QL . 108 quartz 
iodine standard lamp operated at 6.35 A. The probe is 
placed normal to irradiance at a distance of 43 cm. 
Channels are then selected for different wavelengths 
and their output voltages are recorded with a sensitive 
voltmeter. The solar irradiance equal to one solar con-
stant should produce 500 mV on each band. Because the
-I 
irradiances of solar and standard quartz lamp are known 
on each band, their ratios are fixed. Thus, at any given 
band, the ratio of 500 mV to recorded voltage set equal 
to the proper fixed ratio determines the irradiance scale. 
With this scale calibration, the overall system errors are 
of the order of ±5%. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
NASA Headquarters 
Code KT 
Washington, D. C. 20546 
Reference: TSP72.10327 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
Patent Counsel 
NASA Headquarters 
Mail Code: GP 
Washington, D.C. 20546 
Source: H. H. Seward, I. G. Mc Williams, and 
G. A. Davidson of

Massachusetts Institute of Technology

under contract to
NASA Headquarters
(HQN-1 0683) 
B72 . 10327	 Category 03
	
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A compact spectroradiometer for solar simulator measurements

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