OD (Optical Density)

The Optical Density (OD or D(λ)) is the attenuation of light that passes through an optical filter. The higher the OD value, the higher the attenuation. The mathematic expression of Optical Density D(λ) is the logarithm to the base ten of the reciprocal of the transmittance and is given by the listed equation (where τλ is the transmittance). In other words, the Optical Density is a measure that indicates how many decimal places the transmission shifts at the required wavelength. But according to the above described standards the Optical Density alone is not sufficient enough to guarantee protection against laser radiation. It is important that the Optical Density will also remain high enough in case of a direct laser hit on the filter.

This requirement seems to be trivial, but there are indeed some interaction effects of filter and laser radiation, which can cause some reduction of the Optical Density. In order to make sure that the protection remains stable even under direct laser illumination, laser safety goggles (filter and frame) are subject to standardised laser safety tests performed by independent and accredited laboratories. The awarded scale number gives information about the minimum Optical Density which will remain in case of an impact. In principle there are two different strategies for selective filtering of light – absorption and reflection.

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