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Transmission, also referred to as transmittance, usually denoted by T [non-dimensional], of light by a layer of the medium with the thickness zT is defined as a non-dimensional ratio:
| T = Φ(z + zT) / Φ(z) | (1) |
where Φ(z) is the light power at z.
Strictly speaking, the term transmission might be viewed as referring primarily to the process of passing of light through a medium. Hence, the transmittance, i.e. a property of the medium, might be regarded as a more correct term here. However, both terms are used to describe the optical property of the medium.
Optical density, usually denoted by OD [non-dimensional], of a layer of a medium with the transmission T is defined as follows:
| OD = - logT | (2) |
where log is the logarithm with the base of 10. For example, an optical density, OD, of 1 corresponds to transmission, T, of 0.1. Note that according to the Lambert law, OD is the product of the attenuation coefficient, c, and the layer thickness, zT.
| OD | = - (1 / ln10) lnT | |
| = (1 / ln10) czT | (3) |
The product czT is referred to as the optical thickness or optical depth of a layer of the medium with thickness/depth zT. The optical thickness, usually denoted by τ, is the layer thickness expressed in units of the attenuation length: (1/c). Since the attenuation length is the average distance between successive interactions of a photon with the turbid medium, i.e. the free pathlength for attenuation, the OD equals the average number of such interactions (Berrocal E et al 2007).
| CITATION: Jonasz M. 2006. Transmission, optical density, and thickness (www.tpdsci.com/Tpc/TOdOt.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 18-Jan-2006 Modified: 19-Oct-2007 Reviewed: PENDING |
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