Transmittance Measurements

Understanding Hemispherical Directional Transmittance (HDT)

The hemispherical directional transmittance (HDT) represents the ratio of the total transmitted power to the total incident power. It is a function of the transmitted angle, polarization, wavelength, and the material’s intrinsic optical properties. A transmissive material may transmit electromagnetic radiation in one of the following two ways. First, as a collimated beam of light propagates through the material it may be scattered into a hemisphere of 2 p steradians upon exiting the material. Materials that exhibit this type of property (Scattered Transmittance, Ts) are called translucent.

Secondly, if the transmitted beam is parallel to the incident beam across the width of the entire beam, the transmittance is referred to as Collimated Transmittance (Tc). Materials of this type are called transparent.

Utilizing either the Cary 5000 UV/VIS/NIR reflectometer, the SOC-100 or SOC-400T IR reflectometer HDR or T measurements can be made from the UV out to the very long IR. Data is recorded directly to ASCII text files on the PC and is importable into Excel or other spreadsheet software.