Optics Reports Publishing

In most hunting and  naturalist scenarios, high image contrast is more important than transmission in the daytime. Animals often have low contrast features. Further reduction in contrast by the optic makes animals more difficult to locate. Details of fur, antlers, plumage, etc. are more difficult to see. Naturalists and hunters often find that the object of interest is between them and sun, so the object is backlit.

Veiling glare occurs most often when facing a setting a rising or setting sun. Sometimes glare only occurs when the sun is in a specific location with respect to the field of view. Bright “flares” sometimes occur when the sun is very close to the field of view. Glare can also occur when the object is in shade and the surrounding area has bright features, such as when viewing animals under a forest canopy on an overcast day, or viewing the shaded side of birds against a bright sky. Since the best viewing times are often in the early morning and late afternoon, veiling glare can become a problem for naturlaists and hunters just when they need the optic to perform the best.

Effect of veiling glare on natural scenes: High and low contrast images.

Image contrast performance varies a lot from one manufacturer to another. Some companies have a culture of designing high contrast optics. They measure veiling glare throughout the design process. They use internal surface treatments that absorb glare and apertures (“glare stops”) that minimize stray light. Other manufacturers rely largely on offshore suppliers to design their optics and have no design process or measurement capability to assess veiling glare or enhance contrast during the design process.

Veiling glare depends a lot on the magnification and aperture of the optic. Generally, the larger the aperture and the higher the maximum magnification, the worse the glare and the lower the image contrast. On a variable magnification scope the Glare Contrast usually decreases with increasing magnification.  We measure glare for variable magnification optics at multiple magnification values. We generally do not measure glare for magnifications below 9X, because glare is usually low at such low magnification values.

The standard veiling glare measurement procedure that we follow uses a large integrating sphere and produces a value called the Glare Index, for which lower values indicate better performance (lower glare). We report a parameter that we call the Glare Contrast, which is the inverse of Glare Index (Glare Contrast = 1/Glare Index). Higher Glare Contrast values indicate higher contrast and better image performance. Glare Contrast values correlate very well with visual observations of glare in bright daylight and when using a bright light source like the sun or a street light at night. See the examples in the graph below.

Examples of very good (diamonds) and very poor (squares) Glare Contrast performance. Scopes that have higher Glare Contrast have higher image contrast under bright daylight and other adverse lighting conditions.

Scopes with Glare Contrast values below about 2 have visibly low contrast images under most daylight conditions. The images have a gray appearance in bright daylight, and vivid colors in the scene look like pastels through the scope. Scopes with values between 2 and 4 still lack good image contrast and are prone to glare when facing a setting sun. This is usually not a problem at most shooting ranges, because targets usually face south (direct sun illumination) and the contrast of paper and painted steel targets is high. For scopes that have Glare Contrast values less than about 4, a sunshade is recommended for target shooting. When Glare Contrast values are between about 4 and 8, glare problems are less frequently observed and image contrast is generally acceptable for hunting. A sunshade will probably improve image contrast in bright daylight. Values above 8 represent high contrast under nearly all lighting conditions and a sunshade is unnecessary. Colors are vivid and bright, and the image tends to “pop”.

To measure veiling glare, we use a large integrating sphere (27” diameter), tungsten halogen lamps and spectral filters that limit the measurement to the photopic response of the eye. This instrument provides uniform illumination outside the field of view and measures how much light falls inside a black target in the center of the field of view. If an optic has a glare problem, this instrument will detect it. The measurement does not indicate the specific nature of the problem (i.e., localized lens flare vs uniform contrast loss). Our equipment is capable of measuring a Glare Contrast of up to 50. The reproducibility of our Glare Contrast measurements is within +/-3%.

Unfortunately, measuring the same optic on different veiling glare instruments will result in slightly different Glare Contrast values. This is due to the differences in specific details of the target and detector used in the instrument (the international standard for this measurement does not specify these details).  Therefore, veiling glare measurements for different scopes should be compared only if measured on same instrument using the same target and detector.

Veiling glare doesn’t usually affect the observation of high contrast objects, such as paper or steel targets. These targets usually have high contrast features and high target contrast isn’t really needed to aim the rifle accurately. Plus, shooting ranges are usually set up so that the target faces the sun rather than the shooter facing the sun. In most hunting scenarios, however, high image contrast is important. Game animals have low contrast features. Further reduction in contrast by the optic makes animals more difficult to locate and details like antler points more difficult to see. Hunters often find that their target is between them and sun.
 

Figure 4: Upper: Bright light conditions in which veiling glare can reduce contrast.
Lower: Effect of veiling glare on target contrast.