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View Full Version : Why do some lenses (binoculars, rifle scopes) have a green or redish color to them?


bernse
12-30-2002, 11:13 AM
Not when you look through them, but if you look at them, some seem to have a color to them. Why is this? I assume they are coated with something? I seem to recall seeing this mostly on "higher end" products.

fauxpas
12-30-2002, 11:17 AM
hmmmm....perhaps Night Vision is equipped within it?

BuckleberryFerry
12-30-2002, 11:18 AM
Good question; my glasses seem rather a parchment yellow...any clues?

~Ferry

bernse
12-30-2002, 11:23 AM
Originally posted by fauxpas
hmmmm....perhaps Night Vision is equipped within it?

Not in the instances I am thinking of, they are just for general "daytime" use, although probably of high quality.

casdave
12-30-2002, 11:44 AM
I've seen this kind of coating on iridium coated sunglasses and visors.

Having used both in their intended sprot and motorcycle environments, they improve contrast compared to say plain old smoke shades.

When looking through a scope or bins you get a reduction of light, you are effectively spreading the same light from a few seconds of arc to maybe tens of seconds of arc, hence the percieved light reduction.

The human eye becomes less sensitive to colour and contrast at lower light levels so these coating would help.

That's my WAG.

CookingWithGas
12-30-2002, 12:00 PM
See the posting by scr4 in another thread from this board (http://boards.straightdope.com/sdmb/showthread.php?threadid=151439&highlight=lens+AND+coating). It has to do with the optical phenomena related to thin coatings on glass. The coatings are not red or green.

CalMeacham
12-30-2002, 12:03 PM
Many bonoculars, camera lenses, and the like often have a bluish or purplish color if you look at them from the side. It's the result of an anti-reflection coating applied. In many cases the coating is no more complex than a quarter-wave thickness of Magnesium Fluoride (Mag Fluoride = MgF2), but such cioatings can be quite complex. The purpose is to minimize the "fresnel reflection" that occurs any time you have an interface between two media of differing refractive index.

For a typical glass the index is about 1.5. The reflection coefficient at normal incidence is given by ((n-1)/(n+1))**2, or 4%. Passing thropugh both sides of a lens gives you an 8% loss of light. If you have a system like a simple set of binoculars, with effectively six interfaces, that takes you down to about 78% your initial light -- a considerable loss. Cating the lens reduces that loss considerably.

There are other reasons for coating, as well -- wanting to eliminate unwanted wavelengths, selectively passing particular wavelengths, protecting other coatings on the lens. Each coating costs money, but there's usually a reason for it.

bernse
12-30-2002, 12:10 PM
OK. I did a search on google and came up with this:

http://www.hugediscountmall.com/product.asp?productid=203

Note this may not be the best "quality" of a unit but the pic is exactly what I mean. Is there such a thing as "ruby coated" lenses?

bernse
12-30-2002, 12:19 PM
Found this from: http://www.amateurastronomer.com/faqpages/coatings.html

Back to those "ruby coated" lenses then. What's up with that? Well, from what I can tell, ruby coated lenses are pure marketing gimmick. If you think about it for a minute you should realize that if a bright "ruby" red light is being reflected from the lens, then that much red light is not going through it. This results in an image that is disproportionately green, yellow and blue, giving an overall greenish cast to the view through the binoculars.

Looks like its just a gimmik, and a bad one at that.

CookingWithGas
12-31-2002, 12:14 PM
I think the filter isn't red, it isn't filtering out and reflecting red light, it just scatters red light so it looks red when you are looking at it. It's an artifact of the way light behaves on the coating, not the color of the coating. I'm pretty sure that you do not see a greenish cast when looking through the binocs. CalMeacham can provide expert testimony on this.