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The spectral transmission curve also indicates if the optic filters out any colors. Filtering effects can be as important as high transmission, because reducing transmission at one wavelength relative to the others alters the natural colors of the scene. For example, high transmission in the red part of the spectrum (550-650 nanometers) improves discrimination of game animals in daytime, while high transmission in the blue (450-550 nanometers) enables glassing in low light (see below). For hunting applications, the spectral transmission curve should be nearly flat between 450 and 700 nanometers. 

Lower: Sensitivity of the eye in daytime (photopic) and low light (scotopic) conditions as a function of wavelength. Upper: examples of good and bad spectral transmission curves.

To measure spectral transmission we use a collimated metal halide discharge light source, an integrating sphere and a detector array spectrometer. This type of light source is very bright between 450-700 nanometers, but fades off at lower and higher wavelengths, which gives rise to noise in the transmission curve in the 400-425 and 700-750 nanometer parts of the spectrum. The light source is collimated and does not pass through the aiming reticle of a rifle scope: we measure just the transmission of the optics, and the measurement is not affected by obscuration by the reticle. We measure transmission for the central 13 mm of the entrance objective. Measurements are repeated and individual results are averaged. Overall, our spectral transmission measurements are accurate to within +/- 0.5%.

From the spectral measurement, the daytime (also called “photopic”) and low-light (also called “scotopic”) transmission is calculated. The wavelength response of the eye peaks in the green (555 nanometers wavelength) in daytime, and in the blue (507 nanometers) in low light conditions. The eye is much more sensitive in low light than it is in daytime. The scotopic transmission is relevant when the eye is fully dark adapted, which is rare in typical hunting and shooting situations. In most cases when sport optics are used at the edges of dusk and dawn the wavelength response of the eye is somewhere between photopic and scotopic. Nonetheless, the scotopic transmission indicates the “worse-case” transmission performance in low-light, fully dark adapted conditions.

Advertised light transmission specifications can be very misleading. For example, one well-known brand advertises “up to 95% transmission” for their rifle scopes, although that value is only for red colors at about 650 nm wavelength. Actually, the photopic transmission of this brand of scopes is only about 87%. Other manufacturers will try to exploit these measured transmission values by claiming a 1-2% advantage over a competitor or claiming that their scope has the same transmission as a much more expensive competitor.

Image brightness depends on two factors: transmission and exit pupil. The transmission of most modern rifle scopes spans a relatively narrow range of about 85% to about 95%.  The exit pupil factor can be a much stronger effect than the transmission factor, so experienced shooters will make an effort to maximize the exit pupil in low light conditions. See Optics Tutorial for more information on exit pupil effects.

In daytime conditions, differences in transmission have very little effect on visual acuity, and image contrast is more important than transmission (see below).The eye easily accommodates changes in light levels in bright daylight conditions. In daytime conditions, differences in transmission of less than about 14% cannot be discerned by the eye. In some conditions, visual acuity can actually improve slightly with lower luminance. In low light conditions, however, the eye always loses visual acuity when luminance decreases. Even still, in low light small differences in luminance of less than about 2% cannot be discerned by the eye, and therefore small differences in transmission of a few percent are really not significant. Generally speaking transmission values between 85% and 90% are acceptable, and 90% or higher are considered very good.

We always confirm our transmission measurements by conducting a visual test of the optic in low-light conditions. We perform this test during the first 30 minutes after sunset n a moonless night. Whenever possible, we compare two similar products side-by-side to confirm that no undiscovered effect impacts low-light performance.