RONCHI SOURCES
The Diffuser
Importance and construction of a diffuser.
Copyright – P. J.
Smith
But permission is
given to distribute this material in unaltered form as long as it is not sold
for profit.
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The diffuser
The Foucault test is very
sensitive to errors from uneven illumination.
While the Ronchi test is less affected, it is not immune from similar
problems. In any case, one option when
Ronchi testing is to move the grating extremely close to COC where it performs
more like a Foucault test so it is well worth ensuring even light distribution.
This can be ensured if the
source is succeeded by a diffuser which is absolutely essential in most of the
test variations [1]. An impatient first time user of the Ronchi
test may be tempted to try undiffused light.
Resist the temptation and take a few more minutes to set up a better
test. Luckily this may be done very
successfully with readily available materials.
It is very easy to ensure
sufficiently even illumination. Only
when testing extremely low F:NO surfaces will you run into problems.
Some use one or more layers
of frosted sticky tape in front of the light source. This simple solution works reasonably well. With very powerful sources, the tape will
melt or burn but this is no problem with normal visual set-ups. I tend to think of the tape as a makeshift
but useable solution.
Some what better than
sticky tape is two layers of tracing paper.
I much prefer a piece of
ground glass. By definition, someone
figuring an optical surface will have grinding compounds available and it is
easy to simply grind a small piece of glass on one or both surfaces. 240 grit is probably best but not
essential. Window, picture frame, or
microscope glass all work well. Simply
cut off some pieces about 15 mm square and grind against some other glass for
about 1 minute.
With most diffusers, it is best to grind both sides. The diffuser is best placed a few mm behind
the slit.
One very successful
combined diffuser and slit to be described later uses a piece of glass ground
on one surface, the other polished surface is painted and scribed with a
slit. This MUST only be ground on
one side and the other left well polished. Beware that some picture frame glass has a stippled surface.See the Scribed Slit.
If a diffuser is made to
place behind a grating it is best done by fine grinding both surfaces of the
glass.
Emergency
alternatives
Other substitutes are two
layers of tracing paper or about four layers of ‘transparent’ sticky tape. This is best if of the ‘frosted’ type. If not, deliberate marking of the sticky
layer with the fingers before sticking the four layers together works
reasonably in an emergency.
Another viable diffuser is
two layers of tracing paper.
A glass diffuser lasts
longer and stays cleaner, but these other substituted do work.
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