Instrumentation for the Measurement of Luminescence
All photomultiplier tubes produce an electrical signal unrelated to that from incoming light. This is called dark current, dark counts, or background. It differs from noise in that noise affects both the light signal and the background. Background can be reduced by cooling the tube. Most modern bialkali or trialkali tubes have low background at room temperature. However, if the ambient temperature rises above 22
C the background can become very significant. A major advantage of photomultiplier tubes is that low-energy noise can be mostly eliminated
using pulse height discrimination (Figure 2.2). The main drawback of the photomultiplier tube, other than cost of producing uniform tubes, is that it is difficult to miniaturize. Therefore, photomultiplier tubes cannot be used for counting a two-dimensional array of light-producing points simultaneously.