Fast and Accurate Method for Measuring Photon Flux in the Range 2500–6000 Å

This paper presents a technique for fast and accurate measurement of photon flux from pulsed and steady state light sources over a wide range of wavelength and flux. The technique uses a 1P28 photomultiplier (PM) tube plus a solution of rhodamine B in ethylene glycol (RhB solution). The flux to be measured is absorbed by the rhodamine solution and the resultant rhodamine fluorescence is measured by the PM tube. The solution absorbs completely all photons below 6000 Å and fluoresces with constant fluorescence spectrum and fluorescence quantum efficiency for incident wavelengths from 6000 to at least 2500 Å. These properties of the RhB solution result in a PM tube-RhB solution detecting system with constant photon sensitivity from 6000 to 2500 Å. A large range of flux can be measured by adjusting the distance between the fluorescent solution and the PM tube so as to maintain the tube current within limits of stable operation. An RhB detector with a given distance between the RhB solution and the PM tube photocathode is useful over a 1000 to 1 flux range. Two RhB detectors described in this paper had solution-photocathode distances of 1 and 5.5 cm and operated in the ranges 2×1010 to 2×1013 photons/sec and 1.6×1012 to 1.6×1015 photons/sec. The lowest flux which can be detected (2×1010 photons/sec with the detectors here described) can be reduced by using special techniques for measuring noise and signal or by using red-sensitive photomultipliers. The RhB detector has excellent long time stability. Tests made with a tungsten-filament lamp showed constant sensitivity within ±2% over a period of more than two months. With the RhB detector photon flux measurements can be made in a few minutes with high precision. The detector can also be used for measuring relative fluxes with constant wavelength sensitivity. The RhB detector is compared with the potassium ferrioxalate actinometer, one of the most sensitive chemical actinometers presently available, and shown, where applicable, to be more sensitive and easier to use than the actinometer.