The light source is an ordinary light bulb, and monochromatic light is obtained by allowing a beam of light to pass through a color filter. The monochromatic light is directed through a cell containing the sample, and The light that penetrates hits the photoelectric cell. The instrument is adjusted to yield a light transmission corresponding to 100 percent with the cell containing a “blank sample.” The “blank sample” is a portion of distilled or deionized water that has been treated in the same manner as regular samples. A schematic diagram of the essentials of a photoelectric colorimeter is shown in Fig. 11.3, and a typical instrument is shown in Fig. 11.4.