Importance of Cosine Response of Optical Detectors in Irradiance/Illuminance Measurements

Quantification of artificial lighting is an important aspect in the field of radiometry and photometry. Optical detectors are employed for the radiometric and photometric measurements and need to be characterized in terms of linearity and angular response. This work highlights the importance of the cosine response of detectors while doing illumination measurements under different lighting conditions. It is found that the measurements under quasidirectional light sources result in erroneous values whereas the uniform distribution of illumination minimizes the measurement errors irrespective of the detector positioning. The discrepancies in the irradiance/illuminance measurements are attributed due to the cosine errors of the detectors. The analysis clearly shows the importance of ascertaining the linearity and angular response of detectors before employing them for measurement purposes. A study of angular response and corresponding measurement errors in irradiance/illuminance is also done.