DESCRIPTION
The instruments essentially comprises of :-
1. Light source – 6 V, 10 W Tungsten filament lamp.
2. IC – Regulated power supply.
3. Optical system consisting of Reflector – lens combinations.
4. Fixed aperture of incident light.
5. 5/8 glass filters are available.
6. Shutter to permit light to fall on Photodiode when required.
7. Test tube holder for housing test tube containing solutions.
8. Photodiode for detecting amount of light passing through the test tube solution.
9. Calibrated Microammeter for direct measurement of optical density and percentage transmission.

OPERATING PRINCIPLE
The operation of the equipment is primarily based on the two fundamental laws of light absorption :
1. Lambert’s Law – Which states that the proportion of light absorbed by an absorbing is independent of the intensity of the incident light, and
2. Beer’s Law – Which states that proportion of light absorbed depends on the total number of absorbing molecules through which the light passes. Thus, from these laws, it can be defined that if the light is passing through a given depth of solution of an absorbing substance in non absorbing medium then the proportion of the light absorbed is dependent upon the concentration of the absorbing substance.
3. An alternative way to find the concentration of the test is to take use of previous prepared graph showing the density of the test on the colorimeter, corresponding concentration can be read on the graph. However this method can be a serious source of error unless the graph is frequently checked by the person actually doing the analysis.