Abstract: A luminometer apparatus has a light-tight box (12) into which a cuvette (2) containing a sample and a reagent are located for producing a phosphorescent reaction. The phosphorescent reaction is detected by a photodetector (PD) and a shutter (3) is interposed between the cuvette and the shutter. Output signals from the photodetector are applied to a circuit arrangement (FIG. 4) which detects the phosphorescent reaction at predetermined intervals and by subtracting the successive values of light from one another so a measured peak value of light intensity is determined. The shutter is closed and the dark current signal of the photodetector is determined and subtracted from the measured peak value. A true peak intensity of the emitted light is thus derived and the concentration of the material being assayed is calculated by raising the peak intensity to the power n where n has a value between 0 and 9 predeterminedly derived for the sample.