Abstract
By analysing two sets of atmospheric solar measurements, Laulainen and Taylor conclude that conventional Volz sunphotometry is subject to considerable error arising from apparent day-to-day variation in J0, the zero-airmass instrument response (“meter deflection”). They determined J0 from the atmospheric data by using a curve-fitting and extrapolation procedure which is very similar to the familiar graphical “Langley plot” method. However, this method is subject to large errors under conditions of time-varying or horizontally inhomogeneous turbidity. We present an example of such errors and reinspect their data. This indicates that the apparent variation in J0 was probably in large part (if not completely) due to changing turbidity conditions, rather than an actual change in instrument calibration. Thus their data do not necessarily support their conclusion that “much existing Volz turbidity data are of dubious value.” We emphasize that indiscriminate day-to-day determination of J0 by curve fitting is risky at best, and recommend the use of artificial calibration sources of known radiance as the most conclusive method to determine the response of a photometer.
