1. 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 re-emits radiative energy. With many internal reflections, a cavity coated with reflective white paper can look “blacker” to our eye than a sheet of black paper! References: https://www.glenbrook.k12.il.us/qbssci/phys/Class/light/u1212a.html http://lasp.colorado.edu/cassini/questions/Electromagnetic%20Spectrum.htm