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Facts About Laser Eye Protection

Light Transmission and Optical Density

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 by the following chart.

Optical Density Light Transmission
0 1
1 0.1
2 0.01
3 0.001
4 0.0001
5 0.00001
6 0.000001

As an example, an Optical Density of six (OD:6) allows only one millionth of the original light to be transmitted through the filter lens. This high level of protection is often needed because of the power of the laser  as well as the humans eye's ability to further focus the power of the beam on the retina.

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