RE: ATM Anti reflection coatings and TDE (not related)

["Chalfan, Richard D" <Richard.Chalfan@PSS.boeing.com>]

        Robert wrote:

> Why can't it "disappear" (e.g., be absorbed).  Coatings do have color
> which makes me think you are losing something ("think"ing in this case
> means "wondering out loud" since I really don't know how they work).
> 
There is absorbtion, but it mainly occurs within the glass and is not noticeable for 
small lenses of good quality optical glass.  Coatings are so thin and so transparent that 
they do not sensibly contribute to absorbtion.  Remember that absorbing the light doesn't 
mean it goes away, it is simply converted in form - mainly a slight heating of the le
ns.

It is also important to note that absorbtion is really not a significant issue for the 
observer.  You really can not detect, from the eyepiece, the loss of a percent or two of 
the incoming light.  Even the loss of light by reflection would not be a problem if it 
all left the optical system.  The problem for the observer is reflected light that gets 
back to the image plane to degrade the contrast (and this CAN be seen by an experienced 
observer on low contrast objects like planets).  Good lens design in the "pre-coating" 
days (not so long ago) meant being sure the curves of the glass elements did not create 
focused reflections (and ghost images) at the image plane.

There is color in the reflection from coatings because the cancellation is frequency 
dependent.  So not all frequencies of light in the reflection are equally suppressed.  
(Note too, that most coating materials are not of exactly the optimal index of refraction 
for the index of refraction of the glass they are coated on, they are close but not 
perfect).  The color you are left with is a function of the addition of the colors of the 
remaining reflected light.  Single layer Magnesium Fluoride coatings appear a deep purple 
color when the coating is optimized for visual use (because some red and blue light is 
reflected, but very little green and yellow).  

In camera lenses you often see different colors on different elements.  That is to 
balance the overall color transmission of the lens (that is a different issue than 
correction for achromatism, when a dumb camera salesman says a lens is not color 
corrected they are really trying to say that the lens color transmission has not been 
balanced.  Only very low end box-like-camera lenses are not corrected for achromatism).  
Multicoated lenses also have colored reflections, although the brightness of the 
reflected image is less than a single layer MgF coating.  

Hope this helps.

Richard Chalfan
Seattle, WA
(Yuch, cloudy here again today)