How Black is Black?
The colors perceived
by our eyes are a
small part of the
electromagnetic
spectrum.
The physical
approximation to a
blackbody is a cavity with
a small aperture and large
internal surface area. We
want our ACR cavity to be
as close to a blackbody as
possible.
Many internal reflections
within the cavity absorb
input radiation. The more
absorptive the cavity
surface, the better the
blackbody approximation.
To our eye, black is
simply an lack of
wavelengths in the
visible spectrum.
A true blackbody
perfectly absorbs
radiation and
characteristically reemits radiative energy.
References:
https://www.glenbrook.k12.il.us/qbssci/phys/Class/light/u12
12a.html
http://lasp.colorado.edu/cassini/questions/Electromagnetic
%20Spectrum.htm
With many internal
reflections, a cavity coated
with reflective white paper
can look “blacker” to our
eye than a sheet of black
paper!