It is well known that objects placed above, absolute zero can emit electromagnetic radiation. The wavelength range in which the object will emit radiation depends on its absolute temperature. The radiation emitted by a hot body, for example, a tungsten lamp with a temperature of 2850 K. is party invisible and is mainly in the infracted (or high) region. As the temperature of the body increases, the radiation emitted by it comes into the visible region. The Sun, whose surface temperature is about 55001K, emits radiation. A graph of its energy plotted as a function of wavelength shows a peal at ^=550mm, which corresponds to green color and is approximately in the middle of the visible region, The energy- wavelength distribution graph of a given body shows a peak at a wavelength with is inversely proportional to the absolute temperature the body The measurement of light as perceived by the human eye is called photometry. Photometry is a physiological phenomenon It is a measure of the response of the human eye to the luminous intensity of light, which is transmitted through the optic nerves and analyzed by the brain. The three important physical quantities of photometry are(I) the luminous intensity of the source,(ii) the luminous flux emitted by the source, and (iii) the luminance of the surface. The SI unit of luminous intensity (1) IS the candela is the intensity of light in a direction emitted by a source of radiation of frequency 540 x102 Hz and whose radiation intensity in that direction is (1/683) times per steroidal. If a light source emits light of luminous intensity of one candela in a solid angle of one steroidal, the total luminous flux emitted in that solid angle is one lumen (Im). A 100 watt standard temperature light bulb emits about 1700 lumens. In photometry. Luminous density is the only parameter that can be measured directly. It defined as the number of luminous intensity luminosities (Im/m2or lux) incident on a unit area of a surface. Most photometers measure this physical quantity. The luminous density produced by a source of luminous intensity is given by E= 1/1. Where 1is the perpendicular distance from the surface to the source. A physical quantity called luminance (L) is used to describe the brightness characteristics of emissive or reflective flat surface, Its unit is cd/m2(also called ‘hit’ in the industry). A good LCD computer monitor has a luminance of about 250 units.