Optical density

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In optics, density is a unitless measure of the transmittance of an optical element for a given length at a given wavelength λ:^[1]

$OD_\lambda = \log_{10} O = - \log_{10} T = - \log_{10} \left ({I \over I_0} \right )$

$O$	= the per-unit opacity
$T$	= the per-unit transmittance
$I 0$	= the intensity of the incident light beam
$I$	= the intensity of the transmitted light beam

The higher the optical density, the lower the transmittance.

Defined in terms of unit length, and expressed in decibels, the density is termed the transmission loss rate For example, an optical density of 0.3 corresponds to a transmission loss of 3 dB per cm. Defined without regard to the length of the sample, the density is a synonym for absorbance. Neutral density filters are typically quantified this way. Some filters, notably welding glass, are rated by shade number, which is 7/3 times the optical density. A shade number of 14 is regarded as safe for direct observation of the sun.