  Coatings comprised of carbon nanotubes are very black; that is, characterized
by low reflectance over a broad wavelength range from the visible to far
infrared. Arguably there is no other material that is comparable. This is
attributable to the intrinsic properties of graphene as well as the morphology
(density, thickness, disorder, tube size) of the coating. The need for black
coatings is persistent for a variety of applications such as baffles and traps
for space instruments. Because of the thermal properties, nanotube coatings are
also well suited for thermal detectors, blackbodies and other applications
where light is trapped and converted to heat. We briefly describe a history of
other coatings such as nickel phosphorous, gold black and carbon-based paints
and the comparable structural morphology that we associate with very black
coatings. In many cases, it is a significant challenge to put the blackest
coating on something useful. We describe the growth of carbon nanotube forests
on substrates such as metals and silicon along with the catalyst requirements
and temperature limitations. We also describe coatings derived from carbon
nanotubes and applied like paint. Another significant challenge is that of
building the measurement apparatus and determining the optical properties of
something having negligible reflectance. There exists information in the
literature for effective media approximations to model the dielectric function
of vertically aligned arrays. We summarize this as well as other approaches
that are useful for predicting the coating behavior along with the refractive
index of graphite from the literature that is necessary for the models we know
of. We provide an appendix of our best recipes for making as-grown, sprayed or
other coatings for the blackest and most robust coating for a chosen substrate
and a description of reflectance measurements.
