Light Transmission and Lenses

The term Light Transmission in regards to a lens can have two meanings. Generally we mean the percentage of visible light that penetrates a lens. For a normal pair of safety spectacles or prescription glasses the Visible Light Transmission (VLT) is about 85%. Laser filter lenses have some absorbers added, that change to color and the VLT of the lens. It is usually desirable to maintain a VLT as high as possible, but there are situations when both the laser light and the visible light has to be attenuated.

When we discuss the light transmission of a specific laser wavelength, it is under normal circumstances, our ambition to filter out as much as possible of that specific wavelength. The ability of a lens to filter out a specific wavelength or wavelength range is called its Optical Density. The relationship between the Light Transmission and the Optical Density at a specific Wavelength can be described with the following analogy.

With an optical density of zero, the light transmission would be 1.0. In comparison, an optical density of 6 would result in the light transmission being .000001

It is obvious, that a higher optical density is preferred to reduce light transmission.