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Optical density is the absorbance of an optical element for a given wavelength λ per unit distance: In spectroscopy, the absorbance A is defined as , where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and is the intensity of the light before it enters the sample (or incident light intensity). ...
The wavelength is the distance between repeating units of a wave pattern. ...
Where: -
| l | = the distance that light travels through the sample (i.e., the sample thickness), measured in cm | | Aλ | = the absorbance at wavelength λ | | T | = the per-unit transmittance | | I0 | = the intensity of the incident light beam | | I | = the intensity of the transmitted light beam | Although absorbance does not have true units, it is quite often reported in "Absorbance Units" or AU. Accordingly, optical density is measured in ODU, which are equivalent to AU cm−1. In the fields of optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. ...
The higher the optical density, the lower the transmittance. Optical density times 10 is equal to a transmission loss rate expressed in decibels per cm, e.g., an optical density of 0.3 corresponds to a transmission loss of 3 dB per cm. The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power) relative to a specified or implied reference level. ...
Optical density is sometimes defined without regard to the length of the sample; in this case it 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. In photography and optics, a neutral density filter or ND filter is a grey filter;. In theory, a neutral density filter reduces light of all wavelengths or colors equally, much like inexpensive sunglasses. ...
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