PHYSICS

(or wavelength). A typical value for the monochromaticity of a
He-Ne laser is approximately L1v/v = 10- 12 -10- 13 •
Coherence is the coordinated movement of several wave
processes in space and time. We can distinguish two types of
coherence: temporal coherence - when at a given point in space
there is a constant phase difference between the amplitude of the
wave at two successive instances in time; and spatial coherence ­
which is characterised with a constant time-independent phase
difference for amplitude at two different points.
For example, the degree of temporal coherence for a He-Ne
laser is t = 1/ L1v = 10- 4 s; for a ruby laser it is r = 10- 8 s (here
L1v is a bandwidth of the laser line).
Directionality is determined by the angular distribution of the
laser beam. This property is the direct result of the propagation of
two light waves in the optical cavity; only waves that propagate
normally to the resonator mirrors partici pate in the formation of
the laser beam. The angular distribution, or divergence, is defined
as:
(] = 1.22}"/D (21.17)
where D is the beam diameter and A is the wavelength.
The divergence of gas lasers is about 10/; solid-state lasers ­
10/ - 40/; semiconductor lasers - about 3°. For example, the
beam of a He-Ne laser with the diameter 1 mm has an angular
divergence about 10- 3 radians. Therefore if the surface of the
moon was illuminated from earth with a laser spot, the diameter
of the spot would be about 400 km.
Brightness of the source of electromagnetic waves is defined as
the power of radiation that is emitted from the unit at the source
surface through the unit solid angle. The solid angle is
proportional to f) 2 (where f) is beam divergence). This is why laser
beams with even a small divergence are several orders of
magnitude higher in brightness than traditional sources of light.
For example, the brightness of a high-pressure mercury lamp
(P = 100 W) is five orders of magnitude less than the brightness
of a He-Ne laser (P = 1 mW).
Applications of Lasers. Laser sources make it possible to obtain
information that is difficult or impossible to obtain with conventional
sources. Lasers are extremely useful due to their intensity,
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amount of monochromatic radiation, and the coherent nature of
their output. They can be successfully applied to diagnostics,
therapy, and surgery due to their unique properties - spatial and
temporal coherence, frequency and intensity range, monochromaticity,
and degree of control over the focal area and beam
length.
Optical and laser spectroscopy methods can be utilized for the
analysis of different parameters. For example in cattle-breeding,
the diameter, density, orientation and pigmentation of animal
hair can be quantitative determination using laser diffractometry,
microfluorometry, and microphotometry. Likewise, measurement
of light scattering can be used to investigate the size distribution
of milk particles. Using laser Doppler spectroscopy, dynamic parameters
of spermatozoa and a precise estimation of sperm motility
can be determined.
Laser-induced fluorescence of chlorophyll in vivo can be used
as a nondestructive, fast and precise estimation of the health status
of plants and the effect of different stresses.
Lasers can be used for remote sensing to monitor various
conditions of the atmosphere, such as the amount of aerosols and
gaseous air pollutants (e.g., N0 2, S02' and 0) or the determination
of gases in smoke plumes from a distance of several hundred
meters. Likewise, laser remote sensing methods are useful for
meteorological applications such as the measurement of cloud
characteristics, preci pitation, humidity, wind, and temperature.
22. PHYSIOLOGICAL OPTICS
iSOOJllII~~!'lj~l\I:;i~;::W-'.:$!'ii!li"~'~'('""":;~~~~~
22.1. THE EYE AS AN OPTICAL SYSTEM
Mammals. The eye is an extremely complex part of the body. Fig.
22.1 shows the essential parts of the eye. The front is covered by a
transparent membrane called the cornea. Behind the cornea is
clear liquid region (the aqueous humor), a variable aperture (the
iris and pupil), and a crystalline lens. The iris, which is the colored
portion of the eye, is a muscular diaphragm that controls the size
of the pupil. The iris regulates the amount of light entering the
eye by dilating the pupil when the light intensity is low and
contracting the pupil when high. Light entering the eye is focused
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