How to Calculate Reflectance

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Reflectance is a measure of the incident electromagnetic radiation that is reflected by a given interface. It is closely related to reflectivity but reflectance is more applicable to thin reflecting objects. Reflectance can vary for thin objects due to variations in the surface thickness and approaches the reflectivity as the surface becomes thicker.The reflectance may be calculated by comparing the amount of reflected radiation to the amount of incident radiation.

The Steps

    Calculate reflectivity. Reflectivity can be calculated as p(y) = Gr(y)/Gi(y) where p is the reflectivity, y is the wavelength of the light, Gr is the reflected radiation and Gi is the incident radiation.

    Calculate reflectance from the reflectivity. Reflectance is the square of the reflectivity so q(y) = (Gr(y)/Gi(y))^2. where q is the reflectance, y is the wavelength of the light, Gr is the reflected radiation and Gi is the incident radiation.

    Determine the units of measure for reflectance. The incident and reflected radiation must be measured in the same units so their ratio has no units. Reflectance is therefore a dimensionless number without units.

    Interpret the reflectance value. The reflected quantity of radiation must be non-negative, and the incident radiation will be positive. The reflected radiation can never be greater than the incident radiation, so reflectance ranges from 0 to 1 such that 0 indicates no radiation was reflected and 1 indicates that all of the light was reflected.

    Calculate the reflectance for specific conditions. For example, say that a polished gold surface subjected to direct radiation with a wavelength of 480 nanometers reflects about 60 percent of that radiation. In this case, the reflectance would be q(y) = (Gr(y)/Gi(y))^2 = .6^2 = .36, or about 36 percent.

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