Colour density value which is defined by extinction coefficient at the definite wavelength is usually measured by a photoelectric colorimeter. As the monochromatic light source of a colorimeter usually has some band width, data obtained with such a colorimeter do not obey the Lambert-Beer's law. This paper shows that the disagreement with the Lambert-Beer's law results from the difference between pure monochromatic light in definition and pseudo-monochromatic light in actual measurement, and that the correcting curve can be made by the calculation from spectrum of measured liquid and other optical elements. It was verified that the corrected data with this obey the Lambert-Beer's law.
Using above results, we assembled a prototype of colour density value measuring system which is a self-balancing type with varying light pass area. It has dual beam and a simple sliding iris which gives a standard light attenuation equal to the light attenuation in sample liquid.
Next, a line spectrum of a mercury lamp of which intensity pulsates at the double frequency of the AC power source was used as pure monochromatic light source. In this case, a particular electric circuit is necessary for taking out the difference between two pulsating signals of photoelectric detectors. The measuring system with this pure light source has been tested, and good results were obtained.