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The indicated powers are for visible-light, continuous-wave lasers. For pulsed lasers and invisible wavelengths, other power limits apply. People working with class 3B and class 4 lasers can protect their eyes with safety goggles which are designed to absorb light of a particular wavelength.

Certain infrared lasers with wavelengths beyond about 1.4 micrometres are often referred to as being "eye-safe". This is because the intrinsic molecular vibrations of water molecules very strongly absorb light in this part of the spectrum, and thus a laser beam at these wavelengths is attenuated so completely as it passes through the eye's cornea that no light remains to be focused by the lens onto the retina. The label "eye-safe" can be misleading, however, as it only applies to relatively low power continuous wave beams and any high power or Q-switched laser at these wavelengths can burn the cornea, causing severe eye damage.

History

Foundations

In 1917 , Albert Einstein established the theoretic foundations for the LASER and the MASER in the paper Zur Quantentheorie der Strahlung (On the Quantum Theory of Radiation); via a re-derivation of Max Planck’s law of radiation, conceptually based upon probability coefficients (Einstein coefficients) for the absorption, spontaneous emission, and stimulated emission of

electromagnetic radiation; in 1928 , Rudolf W. Ladenburg confirmed the existences of the phenomena of stimulated emission and negative absorption; in 1939 , Valentin A. Fabrikant predicted the use of stimulated emission to amplify “short” waves; in 1947 , Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and effected the first demonstration of stimulated emission; in 1950 , Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping, experimentally confirmed, two years later, by Brossel, Kastler, and Winter. On 16 May 1960 , Theodore Maiman demonstrated the first functional laser at the Hughes Research Laboratories, [9] introducing a technology applied mostly used for data storage, via optical storage devices, such as the compact disk player and the DVD player, wherein a semiconductor laser, less than a millimeter wide, scans the disc’s surface; the second-most application is fiber-optic communication, and related devices, e.g. bar code reader, laser printer, laser pointer.