Limits of radiometry

Radiometry, defined as the measurement of optical radiation, has its limits. Other disciplines can measure routinely with uncertainties of 1 part in 106 (voltage), 1 part in 108 (length) or even 1 part in 1012 (frequency). Radiometric measurements in the laboratory have achieved 1 part in 104, and we are usually overjoyed when field measurements approach 1 part in 100. The limits of uncertainty are discussed with regard to optical wavelength (broadband vs. monochromatic), radiant power, and frequency (modulation, pulse characteristics, etc.). Other limits are imposed by such diverse phenomena as instrumental linearity, polarization, atmospheric effects, diffraction, coherence, `phase of the moon,' and availability and suitability of appropriate standards. The uncertainties in field measurements are not closely related to laboratory uncertainties; the closure of this gap is a current topic of study.