Monochromaticity ˇV The waves from a laser are of almost exclusively one wavelength. As a result, the light from a laser is monochromatic (of one colour). Ordinary light, on the other hand, consists of a mixture of wavelengths. Conventional light source produces a continuum, whereas, laser output has very narrow linewidth. Laser light is available in all colours from red to violet, and also far outside the conventional limits of the optical spectrum.
Collimation ˇV Rays in a laser beam are collimated, i.e. all rays are almost parallel to each other . As a result, a laser beam is highly directional .On the contrary, the light from an ordinary light source will spread out in all direction and it is impossible to form a beam by itself. Laser beams have small angular spread.
Coherence ˇV Ordinary light, such as that from a lamp or a fire, is ˇ§incoherentˇ¨, and consists of a mixture of wavelength radiating in all directions. Waves from an incoherent light source are continually being interrupted by each other and restarted out of phase. Laser light, on the other hand, is ˇ§coherentˇ¨, and consists of almost exclusively one wavelength, with all its waves travelling in the same direction and ˇ§in phaseˇ¨ with each other . Waves are in phase with each other when the troughs and peaks of one wave coincide with the troughs and peaks of the other.
Spectral brightness ˇV A finite light source can be defined by its beam divergence D W , source size A, bandwidth D n , and the spectral power density P( n ) (watts per Hz of bandwidth). Spectral brightness b n , a measure of how concentrated the optical power is, is defined as P( n ) / A D W D n , power flow per unit area, unit bandwidth, and steradian.
