Optical Transmission and Light Scatter
by Roland Christen
The subject of optical transmission keeps coming up in the newsgroups, and
various bogus numbers are being bandied about. To try to give some order to
the chaos, I have posted a discussuion based on real numbers.
The following message was posted by Bill Burnett of the Internet Telescope
Exchange who imports various Russian reflecting scopes:
"Contrary to the very popular opinion that refractors are the closest
approach to the ideal image, the off-axis parabolic reflector (OAPR) design
actually exceeds the performance of like aperture and focal ratio
refractors. Both are 100% unobstructed systems. However, the OAPR design
has only 2 reflective surfaces and allows for an energy throughput of 93% or
more while all refractors are in the 88 to 90 percent range."
My response to this is as follows:
In a 6" lens, each element is approx. 10 to 12mm thick. The Ohara catalog
gives transmittance values for 10mm thickness of the common crowns, flints
and SD glasses normally used in achromats and apos (1.0 = 100%):
These values are from the latest Ohara glass catalog. The Schott values are
in very close agreement.
Lenses have always been prized for their low scatter. That's why they are
used in coronagraphs. In this application aluminized mirrors are worthless
because the light scatter they produce pretty much overwhelms the faint
Anti-reflection coatings have been studied by many investigators. These
coatings are ultra-thin compared to evaporated aluminum coatings and
produce almost no scatter. Even when more than 50 layers are placed on the glass,
such as in our Maxbright diagonals, the scatter is an order of magnitude
lower than in an aluminum mirror coating.
Aluminum coatings produce scatter because the aluminum particles grow in
columnar fashion, and the spacing between particles is on the order of the
wavelength of light, and the height is many wavelengths of light. Part of
the light impinging on the coating gets absorbed in the space between the
particles, part gets scattered back in all directions, and a very small
amount gets transmitted thru the back of the mirror.
You can estimate the amount that gets absorbed and scattered by simply
subtracting the light that gets reflected from the incoming light minus the
light that goes through the coating. Typically, less than .01% gets through
the coating, but only 88% gets reflected back (SiO -aluminum). That means
some 12% gets absorbed or scattered. The amount of scattered light can be
seen if you illuminate a mirror with a bright light, and note the amount
of back reflection well away from the axis of the mirror. As you approach the
axis, the amount of light scattered to your eye gets very large. Some
amateurs have done this with the sun illuminating the surface, but I would
not recommend that since the sun will burn a hole in your forhead as you
approach the mirror's optical axis.
Roland Christen, ASTRO-PHYSICS
Return to Roland Christen Images and Essays.