Abbreviated OD.  The logarithm to the base ten of the reciprocal of the transmittance at a particular wavelength, where: Dλ = log10 (1/τ λ) where τ λ is the transmittance at the wavelength of interest. Symbol: D(λ), D λ or OD. Optical Density is commonly used to describe the protection factor provided by a filter (such as used in eyewear, viewing windows, etc.) at a specific wavelength. Each incremental unit of OD (1, 2, 3….etc.) represents a 10x increase in protection.