Radiometrics in image evaluation

Calibration of photocathode response by photometric and radiometric procedures is discussed, and the limitations of the two methods are described. A calibration approach is recommended which uses a photometric analysis coupled with a spectral conversion factor. The spectral conversion factor is the dimensionless ratio of photocathode current resulting from the measurement source to the photocathode current that would result if a standard source were used. The problem with photometric measurements is that they have become less accurate with the development of greatly enhanced near-infrared sensitive photocathodes because the input energy cannot be characterized over the spectrum of the photocathode response. Although radiometric measurements provide knowledge of the energy without wavelength limitations, the measurements are time consuming, require expensive equipment, and are limited by detector sensitivity.