Stray Light, Scattering and Flare

Stray light is exactly what it sounds like. It is, quite simply, light that strays into the image from all sorts of different sources and that is not intended to be there. It is the unwanted light.
It can enter the image in a number of different ways:

• reflected light (this is the light that is technically supposed to be entering the scope, but it follows an unintended path, as mentioned in the previous section),
• off-axis light from outside the field of view that enters the objective lens at an angle and causes reflections from glass surfaces (veiling flare, for example)
• scattered light from particles and glass impurities in the optical path
• scattered and reflected light from mechanical structures inside the scope

Optimizing stray light performance of the scope is, often, one of the more important final touches in the design. If you look into the objective lens of a high quality riflescopes, you will note that all the metal surfaces inside look like they were sandblasted and blackened in some sort of a non-flaking way. If you pay attention you might also see some slight baffles that are strategically positioned to block off some off-axis light. Bottom line is that figuring out how to deal with stray light is mostly a problem for an optical designer; however, there are a few thing a shooter can do as well, to alleviate some of the problems. No matter how much effort goes into suppressing stray light, all scopes are susceptible to it, although to varying degrees.

First and foremost, if your scope came with a sunshade (metallic tube that can get screwed into the objective bell of a scope as an extension of sorts), use it. It might get in the way on a hunting rifle, when out in the field, but on other occasions, I suggest giving it a shot. Other than the scope being slightly longer, there is really no downside: it will either have no effect on the image, or it will make it better. While the name “sunshade” is generally appropriate, often times it is equally useful, when there isn’t much sun visible at all. Anytime there are bright ligth sources outside the field of view, a sunshade is likely to help.

Keep in mind that stray light can enter the scope through both the objective lens and the eyepiece, and putting a sunshade onto the eyepiece of a weaponsight is not feasible for a number of obvious reasons. However, putting rubber eyeshield on the eyepiece can work for rifles with low recoil or simply when a riflescope is used as an observation device (it also helps in filtering out unwanted light entering your eye). Even something as simple as wearing a baseball hat can help block some unwanted illumination.

As far as scattering goes, there isn’t too much you can do about scattering except take care of the lenses: any dirt or scratches will create some light scatter, so if you want optimal performance, maintain your optics properly. Some scattering occurs simply due to imperfections in the glass, such as tiny air bubbles. However, it is not a major problem in scopes by reputable manufacturers.
Lastly, flare deserves a few words. Flare is a type of stray light that is obviously visible. A lot of stray light simply degrades the image quality without creating any distinctly visible artefacts. Flare is usually easy to detect. The most common type is the veiling flare which, true to the name, looks like a bright curtain across all or part of the image. Veiling flare is usually caused by reflections within the objective lens system (and scopes with large objective lenses are especially prone to it). Other types of flair often show up as bright floating objects within the image and, depending on the type of illumination present, can have a color aspect to them. For example, some halogen light sources produce rather characteristic purple flare in many optics that has very little to do with real life performance, since that kind of lighting scheme is never encountered outdoors.