RE: [APML] Hutech IDAS LPS Filter Question

["Lane, Jason R" <jason.lane@navy.mil>]

Hi Ulrich and Gary,

I've been talking to coating manufacturers lately for a project at work, so I can probably help answer this question.  The LPS is an example of an interference filter, which operates on thin film theory.  Basically, this means the coatings on the filter govern how the spectrum is filtered, and of course the coatings are designed according to the requirements of the filter.

For light pollution suppression (as well as stuff like anti-reflection coatings), the easiest coating to design is for a 0 degree angle of incidence (ie assuming the light hits the filter at 0 degrees).  Slightly more complicated is designing assuming an angle of incidence at other than 0 degrees, for example assuming the light will hit the filter at 45 degrees.  With this type of coating, performance will fall off at other angles much like Ulrich describes.  In a general sense, the filtered wavelengths depend on the thickness of coating, so you can see that at something like 20 degrees, the light has to go through a different amount of coating material and so the filtered wavelengths begin to shift (and in a related way, transmission at the design wavelength goes down) at other than the design angle of incidence.

A more complicated coating can be designed for a range of incident angles.  For example, instead of just assuming 0 degrees, you can design for 0-20 degrees angle of incidence (or similarly, 45 degrees +/- 10 degrees).  For simply designing anti-reflection coatings, this is similar to increasing the bandwidth (or range of wavelengths) at which you specify a certain transmission....for filtering it's probably more complicated because you don't want to increase the bandwidth, but rather increase the angles of incidence. 

When designing for a range of incident angles, there is a point at which the cost of the filter begins to increase exponentially.  This is due to complexity of coating, difficulty in coating the substrate, cost of materials, etc., etc.  Basically it becomes really tough to do (but can be done if you throw enough money at the problem).  If you really wanted to, you could contact a coating manufacturer who would be willing to design an interference filter that would meet your requirements, but it's going to be very expensive (especially for a one-off filter).  This is probably why IDAS recommends use of their filter for focal lengths of 100mm or more: the range of incident angles that they designed for is related to the field of view of the camera lens.  Designing the filter for use with incident angles that would be present with a 55mm lens maybe would have made for a much more expensive product.

Hope this helps,
Jason

-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Ulrich C. Beinert
Sent: Friday, April 01, 2005 9:07 AM
To: 'Discussion of Film Astrophotography'
Subject: RE: [APML] Hutech IDAS LPS Filter Question


> Thanks Ulrich.  Do you know of a LPS filter
> the WILL work with a 55mm lens?

Sorry, I don't. And as far as I know, there isn't really anything that
can compete with an IDAS anyway.

I don't know the exact physics of LPS filters, but as far as I know,
they all rely on light hitting the filter at a certain angle - maybe
someone else can tell us more?

Ulrich

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