[rollei_list] Re: Lens coatings and veiling flare.
- From: Eric Goldstein <egoldste@xxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Fri, 10 Jul 2009 06:48:38 -0400
Richard - You're getting at why I raise the question... the science is
such that it is questionable that a single coating now could be any
more effective across the visible spectrum than a single coating then.
And absent any measurements/data, there is no reason to believe that
it is...
I like the impedance matching/horn loading analogy...
Eric Goldstein
--
On Thu, Jul 9, 2009 at 11:17 PM, Richard Knoppow<dickburk@xxxxxxxxxxxxx> wrote:
>
> ----- Original Message ----- From: "Eric Goldstein" <egoldste@xxxxxxxxx>
> To: <rollei_list@xxxxxxxxxxxxx>
> Sent: Thursday, July 09, 2009 6:05 PM
> Subject: [rollei_list] Re: Lens coatings and veiling flare.
>
>
> This is precisely the question we are trying to get smart about... we
> know that modern coatings are more durable but is there an improvement
> in flare reduction and bandwidth with modern single coatings when
> compared with the coatings of the mid to late 60s and 60s?
>
>
> Eric Goldstein
>
> I don't know how to determine that other than by measurement. The physics
> of coating are well understood and are related to transmission line and
> filter theory in electronics. Essentially, the coating is an impedance
> matching section and like a 1/4 wave section works best at one frequency.
> Also, coatings vary in effectivness with the angle of the light striking
> them since the path length varies. In theory the coating should be of a
> material with an index of refraction geometrically between the index of air
> and the index of the glass. Multiple coatings are the electronic equivalents
> of a transformer or, in distributed constants, of a horn section. This
> broadens out the bandwidth. However a single coating, at its optimum
> wavelength should have no reflection (or a very minimal one). There is
> probably some residual since the match may not be perfect. Multiple coatings
> have a minimal reflection over a fairly wide bandwidth. Since the band of
> wavelengths to which the eye is sensitive is only about an octave (and film
> not much more) a single coating will have some effect over the entire range
> but will fall off away from the optimum wavelength. A multiple coating can
> reduce reflections substantially over the entire range with little
> variation.
> Also note that coatings can be designed to _increase_ reflection and are
> thus used for mirrors. For instance, aluminum mirrors are not as efficient
> for visible light as a silvered mirror but can be made to equal the
> efficiency by suitable coatings.
> I don't know for certain what method is currently used for measuring the
> thickness of a coating but certainly there are methods available which are
> much more precise than the old method of looking at the color of the
> residual reflection from the lens while in the vacuum chamber.
> Again, there is a great deal of historical material on the Vacuum Coaters
> site.
>
> --
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
---
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