Percent Transmission and Optical Density
Percent transmission (%T) is the most common unit used to quantitatively express how an optical filter transmits light. Percent transmission is measured at a particular wavelength (or wavelength range) and is the ratio of transmitted light intensity (I) to incident light intensity (Io), expressed as a percentage:
- %Tλ = I/Io * 100
The diagram below illustrates the terminology typically used when discussing spectra expressed in %T. This figure is explained further in the Filter Characteristics section.
Figure 5: Transmission Nomenclature
The attenuation level (blocking level) and attenuation range (blocking range) of filters are normally defined in units of optical density (OD). Optical density uses the same units as the quantity absorbance, and is expressed as the negative log of the transmission. It should be noted that one should not try to infer optical density from a transmission plot, a common mistake made by those working with fluorescence. Expressed mathematically:
- OD= -log Tλ = -log [(%Tλ) / 100
- Example: OD 4.5 = 3 x 10 -5 T (0.003 %T)
Figure 6: Attenuation Nomenclature
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