Absolute reflectance measurement is used to evaluate the spectral properties, film thickness, angular variation or other characteristics of samples such as semiconductors, thin films and optical elements.
The absolute reflectance measurement accessories are designed to measure the absolute reflectance of a specular reflecting sample (metals, semi conductors, display screens or other materials) while varying the incidence angle (all models). Several models are used to measure the transmittance of a sample (without diffuse transmittance). Absolute reflectance and transmittance are measured by rotating the sample stage to select an angle of incidence for the light to the sample and setting the detector to measure the reflected light or transmitted light.
Diffuse reflectance can also be measured by placing the sample at the rear of the integrating sphere with an incidence angle of 0º. Diffuse transmittance can be measured by placing the sample at the entrance of the integrating sphere with the incident angle set to 0º.
Effects of Polarization when Measuring Absolute Reflectance
Light from a spectrophotometer that employs a grating is always polarized. The ratio of the intensities of the S and P polarized light varies according to the wavelength and also differs from one grating to another. Greater angles of incidence result in greater difference in the intensity of the S and P polarized light making measurement less accurate. If the absolute reflectance is measured at a high angle of incidence, a polarizer can be set to 45° to minimize the difference in polarization.
