Laser Light is Highly Directional
A laser beam departs from strict parallelism only because of diffraction effects, determined by the wavelength and the diameter of the exit aperture. Light from other sources can be made into an approximately parallel beam by a lens or a mirror, but the beam divergence is much greater than for laser light. For example, focused light from a tungsten filament source forms a beam, the angular divergence of which is determined by the spatial extent of the filament.
Laser Light is Highly Coherent
Wave trains for laser light may be several hundred kilometers long. Interference fringes can be set up by combining two beams that have followed separate paths whose lengths differ by as much as this amount. The corresponding coherence length for light from a tungsten filament lamp or a gas discharge tube is typically considerably less than 1 m.
Laser Light is Highly Monochromatic
Tungsten light, spread over a continuous spectrum, gives us no basis for comparison. The light from selected lines in a gas discharge tube, however, can have wavelengths in the visible region that are precise to about 1 part in 106. The sharpness of definition of laser light can easily be a thousand times greater, or 1 part in 109.
Laser Light can be Sharply Focused
This property is related to the parallelism of the laser beam. As for star light, the size of the focused spot for a laser beam is limited only by diffraction effects and not by the size of the source. Flux densities for focused laser light of 1015 W cm-2 are readily achieved. An oxyacetylene flame, by contrast, has a flux density of only 103Wcm-2.
Tuning
Some lasers can be used to emit radiation over a range of wavelengths. Laser tunability leads to applications in photochemistry, high resolution and roman spectroscopy.
Brightness
The primary characteristic of laser radiation is that lasers have a higher brightness than any oilier light source. We define brightness as the power emitted per unit area per unit solid angle.
