The control of light transmission is important to the patient with low vision: too much or too little visible light can reduce visual function, while invisible (short wave length) light may be detrimental to a susceptible retina. Tint alone gives no indication of either visible or invisible transmission. Transmission percentages usually are indicated by the manufacturer.
Several categories of light-absorbing lenses are used specifically for low vision. These are lenses that:
- reduce the intensity of light;
- transmit or block specific wavelengths;
- absorb or block ultraviolet and/or infrared transmission;
- have photochromic properties to vay the transmission of visible light in response to the ambient level of light;
- are coated with a multilayer, anti-reflective coating to reduce surface reflections;
- polarize light (horizontal plane), or
- enhance contrast.
Light-absorbing lenses are available in a variety of frame styles including wrap-around, side shield frames, and lenses that clip on, flip up, or slip in behind the user's own glasses.