Figure 3-11.--Comparison of optical characteristics of piano-convex lens and Fresnel lens.

The second effect that will be observed is that the

light rays, as illustrated in figure 3-11. When the rays of

motion of the bar of light from the cell center to the

light, parallel to the principal axis of a convex spherical

transition line does not appear to be smooth. At higher-

lens, pass through zones near the edge of the lens, the

than-design temperature, the bar of light disappears in

principal focus occurs at a point that is closer to the lens

the observed cell before it starts to appear in the adjacent

than the focus for rays that pass through the lens near

cell. At lower-than-design temperatures, the bar of light

the principal axis. Therefore, the light rays from a plano-

disappears into the transition line before a bar of light

convex spherical lens tend to scatter. The Fresnel lens

starts to appear in the adjacent cell. At extreme tem-

also can be formed around a suitable radius to minimize

peratures, it is possible to get blank areas or double bars

astigmatism. Astigmatism of a lens is the inability of the

of light at or near the transition line.

lens to bring all the light rays from a point on an object

to a sharp focus to form the image.

The last effect that will be observed is that the

vertical field angle (angle of the lens from top to bottom

The optical characteristics of the Fresnel lens will

as viewed from the front) is larger when the ambient

vary appreciably with a change in temperature. If the

temperature is higher than design temperature and

lens temperature is allowed to vary beyond operational

smaller when the ambient temperature is lower than the

limits, three effects will be observed. First, the size of

design temperature. To maintain design characteristics

the bar of light near the center of the lens is different

of the Fresnel lens, the lens-heating compartments are

from that which is seen near the center of the lens when

maintained at a temperature that is relatively constant.

the lens is at design temperature. Also, as the observer

The Fresnel lenses are each enclosed in a separate

moves up or down, the size of the bar of light appears

compartment in which a lenticular lens serves as the

to change as the image moves from the lens center to the

front and an optical glass serves as the back of the

transition line between cell assemblies. The transition

compartment. Hot air is circulated in the compartment

line is defined as the physical break between the cells.

under thermostatic control.

If the ambient temperature is higher than design tem-

perature, the bar of light at the center appears smaller

LENTICULAR LENS

than the design bar of light, and it blooms to a larger-

than-design bar of light at the transition line between

A lenticular lens is placed in front of each Fresnel

cells. If the ambient temperature is lower than design

lens. Each lenticular lens consists of many long, convex,

temperature, the opposite conditions occur.

cylindrical lenses placed side by side, as shown in

figure 3-12. Each individual lens has the same short

focal length. The area in which the light source (the

object) can be viewed is spread by the short focal length

of the lenticular lens. If the object consists of a multiple-

light source with spacing between the lights, the object

appears to an observer looking into the lens as a con-

tinuous band of light that fills the width of the lens. In

the GSI system, the arrangement of the lenses with

respect to the source lamps and the physical properties

of the lenses cause the source lamps to appear as a single

light image 12 inches wide and approximately 1/2 inch

high. The object appears as a continuous band of light

regardless of the observer's position in the azimuthal

range of the lenticular lens. The azimuthal range is the

angular position (expressed in degrees) in a horizontal

plane in which a pilot of an approaching aircraft can

observe the band of light. The azimuthal range of the

lenticular lens used in the Fresnel system is 40 (see

fig. 3-12). The appearance of the height of the object is

not affected by the lenticular lens.

The lenticular lens in the GSI assembly is manu-

Figure 3-11.--Comparison of optical characteristics of

factured with three different color segments to eliminate

piano-convex lens and Fresnel lens.

the need for filters and their subsequent light

3-8