Transmittance and Reflectance


The radiant energy when it is measured in terms of monochromatic components, it becomes the function of the wavelength. So when this quantity comes into visual sensation then it becomes the term with the adjective spectral, like spectral radiance. But there are some ratioswhich are commonly used with same term but differ in the effective field. Examples- Transmittance, Reflectance etc.
Transmittance is the ratio of the transmitted radiant or luminous flux to the incident radiance of the luminous flux.
Transmittance is denoted as T.

Here transcript t refers to the transmitted and subscript i refers to the incident flux.
The total transmittance (T) of a medium or any object has two components; namely regular transmittance (Tr) and the diffuse transmittance (Td).

Suppose radiant flux or luminous flux travels through a sample. Now according to the Snell’s Law the exit angle can be predicted from the Entry angle. The transmittance is referred as regular transmittance.

When the radiant flux or luminous flux travels through this sample and gets scattered due to the roughness of the surface, the transmittance is referred as diffuse transmittance.
In the photometric the luminous transmittance of the medium is dependent on the spectral composition of the radiating source.
When we want to determine the photometric transmittance, the specification of the source is necessary.
Again photometric transmittance can be measured in two ways, one way is the photometrically and second one is spectroradiometrically.

Here the specified radiating source should be used when employing photometric technique.
Photometric transmittance (Tv) can be computed from the knowledge of the spectral transmittance T(λ) and relative spectral distribution of the specified source Φ(λ) as follows :

V(λ) is the relative Photopic luminous Efficacy.


The definition of the reflectance is the ratio of the reflected radiant or luminous flux to the incident radian or luminous flux.

Here transcript r refers to the reflected and subscript i refers to the incident flux.
The total reflectance (ρ) of an object can be categorized into two types, one is specular reflectance, denoted by ps and another one is diffuse reflectance, denoted by ρd.

When the radiant flux or luminous flux reflection is not scattering or diffusing then it is called specular reflectance. Example: reflection by a mirror.
But in case of diffuse reflectance the reflection of the radiant flux or luminous flux is in scattering manner then this type of reflectance is diffuse reflectance. Example: reflection by cinema screen.
To determine the photometric reflectance of an object we have to take the source as the specified source. The photometric reflectance can also be determined by photometrically or electroradiometrically.

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