Surface with grooves having flat bases gives directional emissivities and absorptivities that can be made to approximate a perfect directional surface. Radiant energy can then be transferred in desired directions

Howell, J. R.

Perlmutter, M.

English

NASA Technical Reports Server

July 1970	 Brief 70-10321 
.
	
NASA TECH BRIEF 
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 Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Directional Control of Radiant Heat 
.
The usual assumption in radiant heat transfer be-
tween surfaces is that the emission from any surface is 
diffuse. This is seldom true. If the surface were 
grooved, for instance, the emission would be large to 
surfaces "viewing" the interior of the groove, and 
small in other directions. 
If surfaces could be designed so that the surface 
radiative properties were strongly directional, the 
radiant energy transfer could be controlled to some 
extent. For example, a heat sink for radiant energy 
would have a larger net energy gain if it had a high 
absorptivity in the direction of a heat source and a 
low emissivity in the direction of a cold environment. 
A poor heat sink could be obtained by interchanging 
these characteristics. Similarly, a heat source could be 
designed to emit strongly to an absorber but weakly to 
the environment. 
A "perfect" emitter could be considered as a sur-
face with an emissivity of one in a desired direction and 
an emissivity of zero in all other directions. Analyti-
cally, such a surface is designed by the use of grooves 
having flat bases. The walls of the grooves are consid-
ered to be specularly reflecting with a reflectivity of 
one, while the base is assumed optically black. Based 
on these assumptions, an infinitely long specular sym-
metric groove with a flat black horizontal element at 
an arbitrary depth within the groove is analyzed for di-
rectional emissivity. These emissivities are calculated 
for various angles of groove opening and various 
depths of placement of the black surface. The results 
are then used in the calculation of radiant interchange 
between surfaces. Since the grooved surfaces absorb
and emit directionally, they can be designed to con-
trol the equilibrium temperature of the groove base 
over a wide range. 
The results show that the configuration analyzed 
gives directional emissivities and absorptivities that 
can be made to approximate a perfect directional sur-
face, i.e., a surface which has an emissivity or absorp-
•tivity of one over a prescribed range of angles, and zero 
over the remaining angles. The radiant energy can then 
be transferred in desired directions, which results in 
more efficient energy absorbers and emitters. This 
property should prove very useful in the transfer of 
radiant heat from a single source to multiple or remote 
locations. It should also find use in directed nonvisible 
beams for security or counting purposes. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B70-10321 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,229,682), and royalty-free license rights 
will be granted for its commercial' development. In-
quiries about obtaining a license should be addressed 
to NASA, Code GP, Washington, D.C. 20546. 
Source: M. Perlmutter and J. R.,Howell 
Lewis Research Center 
(LEW-90237) 
Category 03 
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.
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Directional control of radiant heat

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