# How Do Absorbance and Transmittance Relate to One Another?

Absorbance and transmittance are inversely proportional to one another. According to Beer’s law, a larger amount of light being transmitted through the sample corresponds to a smaller amount of light being absorbed by the sample.

Absorbance and transmittance are parameters that indicate how much light is absorbed and transmitted for a specified material thickness. The two parameters are calculated for monochromatic light, as materials interact differently with different wavelengths of light.

A monochromatic source of radiant power (Po) is shined on a sample of thickness (t). If the beam of radiation leaving the sample has a radiative power of P, then the transmittance of the sample is calculated as: T= P/Po. The percentage transmittance, %T, is found by multiplying this value by 100.

Absorbance is found using the formula A =2-log(%T). As the transmittance increases, the percentage transmittance correspondingly increases, causing the absorbance to decrease.

Absorbance is also called optical density, as it is an indication of how well the material is able to stop light. Materials with low absorbance in a certain wavelength are considered transparent in that wavelength. Meanwhile, materials with high absorbance in a wavelength are opaque to that wavelength. Materials can be custom tailored to be absorbent in one wavelength and transparent in another. An example is greenhouse glass, which is transparent to visible light and opaque to infrared radiation.