# Why do some lenses (binoculars, rifle scopes) have a green or redish color to them? **URL:** \ **Category:** Factual Questions\ **Created:** [December 30, 2002, 5:13pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701 "2002-12-30T17:13:36Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![bernse](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@bernse](https://boards.straightdope.com/u/bernse)\ **Post date:** [December 30, 2002, 5:13pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/1 "2002-12-30T17:13:36Z")
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. ---
**Author:** ![fauxpas](https://avatars.discourse-cdn.com/v4/letter/f/e480ec/32.png) [@fauxpas](https://boards.straightdope.com/u/fauxpas)\ **Post date:** [December 30, 2002, 5:17pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/2 "2002-12-30T17:17:57Z")
hmmmm…perhaps Night Vision is equipped within it? ---
**Author:** ![BuckleberryFerry](https://avatars.discourse-cdn.com/v4/letter/b/5f9b8f/32.png) [@BuckleberryFerry](https://boards.straightdope.com/u/BuckleberryFerry)\ **Post date:** [December 30, 2002, 5:18pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/3 "2002-12-30T17:18:51Z")
Good question; my glasses seem rather a parchment yellow…any clues? ~Ferry ---
**Author:** ![bernse](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@bernse](https://boards.straightdope.com/u/bernse)\ **Post date:** [December 30, 2002, 5:23pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/4 "2002-12-30T17:23:15Z")
> [@](#): > > \*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. ---
**Author:** ![casdave](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@casdave](https://boards.straightdope.com/u/casdave)\ **Post date:** [December 30, 2002, 5:44pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/5 "2002-12-30T17:44:37Z")
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. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [December 30, 2002, 6:00pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/6 "2002-12-30T18:00:07Z")
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. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [December 30, 2002, 6:03pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/7 "2002-12-30T18:03:32Z")
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. ---
**Author:** ![bernse](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@bernse](https://boards.straightdope.com/u/bernse)\ **Post date:** [December 30, 2002, 6:10pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/8 "2002-12-30T18:10:36Z")
OK. I did a search on google and came up with this: [http://www.hugediscountmall.com/product.asp?productid=203](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? ---
**Author:** ![bernse](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@bernse](https://boards.straightdope.com/u/bernse)\ **Post date:** [December 30, 2002, 6:19pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/9 "2002-12-30T18:19:16Z")
Found this from: [http://www.amateurastronomer.com/faqpages/coatings.html](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. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [December 31, 2002, 6:14pm UTC](https://boards.straightdope.com/t/why-do-some-lenses-binoculars-rifle-scopes-have-a-green-or-redish-color-to-them/145701/10 "2002-12-31T18:14:26Z")
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.