The light transmission in a monocular or binocular refers to the amount of light that passes through the device and reaches the viewer's eye. The higher the light transmission, the brighter and clearer the image will appear.

Light transmission is affected by various factors, such as the quality of the optics, the coatings on the lenses, and the diameter of the objective lens. Manufacturers often include coatings on the lenses to reduce glare, improve contrast, and increase light transmission.

Light transmission is measured using a device called a spectrophotometer. This device measures the amount of light that enters the objective lens and the amount of light that exits the eyepiece. The difference between these two values gives an indication of the amount of light that is transmitted through the device. Light transmission is usually expressed as a percentage, with higher percentages indicating better light transmission.

It's important to note that light transmission is not the only factor that affects image brightness and clarity. Other factors, such as magnification, field of view, and eye relief, also play a role in the overall image quality. Therefore, it's important to consider all of these factors when choosing a monocular or binocular.