point where the diverging rays from the
source are refracted to produce a parallel
beam of light (figure 9.17).
Theconcave(negativeordiverging)lens
is thinner in the middle than it is at the
edges. This causes a parallel beam of light to
diverge. The focus is called a virtual focal
point; it is the artificial point at which the
diverging rays would meet if they were traced
backward as straight lines through the lens
(figure 9.18).
Because refraction takes place at the
surface of the material, with the direction of
the ray unchanged between surfaces, part of
this transmitting material can be removed
without affecting the optical control. A
convex lens with sections of the glass
removed produces a lens that is thinner and
lighter in weight.
TheFresnellens, which is based on this
principle, consists of a series of concentric
lens sections regressed into a planar array
(figure 9.19). It is named for the French
physicist Augustin Jean Fresnel (1788–
1827), who developed the lens for use in
lighthouses. In luminaires, the Fresnel lens
produces a concentrated beam of light while
also reducing the brightness of the source,
providing a degree of glare control.
The Fresnel lens has a short focal
length. A light source located at this focus
produces a single, concentrated beam of
light with parallel rays. When the light source
is positioned at points other than the primary
focus of the optical system, the lamp-refrac-
tor combination produces either a spread or
asymmetric distribution (figure 9.20).
Glasses and plastics that incorporate a
regular pattern of small prisms or other
refractive elements are called prismatic
lenses. They do not concentrate the distribu-
tion of light the way Fresnel lenses do; pris-
matic lenses spread the distribution of the
source but also reduce its luminance, pro-
viding a degree of glare control.Total Internal Reflection
Total internal reflectionoccurs when light
passes into a transparent medium, such as
glass or plastic, at an appropriate angle and
travels inside the medium repeatedly reflect-
ing from side to side. Edge lighting and light
transmission through rods are examples of
this phenomenon.Fiber optics
Withfiber optics, light entering one side of a
glass or plastic fiber of optical quality is
transmitted to the other end by the process
of total internal reflection. Light rays that
strike the core at the acceptance angle are
reflected back and forth inside the core and
travel to the other end of the fiber in a zigzag
path of successive reflections.
In use, a single, large-diameter fiber is
impractical because it lacks flexibility. To
increase flexibility, a large number of small-
diameter fibers are clustered together in a
bundle. In order to prevent light leaking from
one fiber to another, each is coated with a
transparent sheath that has a lowerrefrac-
tive indexthan the fiber (figure 9.21). The
sheathing process protects the surfaces of
the fiber and allows the bundle to be embed-LIGHT CONTROLFigure 9.21Enlarged section of optical fiber.