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Re: [APML] AP 130/F6 LPR filter help
In a message dated 2/22/2001 10:57:38 AM Pacific Standard Time,
k010150@hotmail.com writes:
What are "cheap" vs. "expensive" filters? Are we talking about the
difference between $200 and $2000? Can we tell the difference by looking
at
the colors reflected off the coatings? I notice that binocular optics seem
to have widely varying colors coming off the objectives (red on some, green
on others).
Good question. The color given off by a low reflection coating can be faint
green, faint red or faint bluish. With plain glass, I can look at it or
thru it and get a good feel for the overall reflectivity. Unfortunately,
with a filter it is hard to look thru the glass to see how clear it is.
Take a piece of plain glass, place it next to a coated surface, and using
an ordinary green eyepiece filter, judge the intensity of the reflection of
a fluorescent light from each surface. Unfortunately, a plain clear piece
of glass will have two reflections, so you will have to somehow tilt it
slightly to move one of the reflections away from the other. Poor coatings
will have plenty of light in the reflected image. With a real high quality
coating, the reflection will almost seem to disappear. The only other way
to judge is to send it to a lab and have it scanned in a reflection
spectrophotometer.
I just looked at a red filter of unknown origin that I had in my junk box. It
has a bright blue coating on it. It looks like a single layer mag-fluoride
with a coating thickness of 1/2 wave. The reflectivity of that surface is
actually larger than an uncoated glass surface.
Binoculars have all kinds of weird coatings, none of which is designed to
increase light transmission, rather it is meant to attract the eye of
shoppers who would otherwise not know which of 39 models to choose from at K
Mart. If you're a warm person, you get the red coated ones, a cool person
would choose green or brilliant blue.
Roland Christen