Light Loss

TM 9-258

b. When testing lenses for accuracy of grinding and

elements have been coated produces brighter, effective

images.

polishing, a test lens having the desired curvature is

used. The test lens is placed in contact with the lens to

be tested, care being taken that both surfaces are

2-46. Parallax.

perfectly clean and dry. When the lenses have been

a. Parallax is apparent displacement of one object

squeezed together, if the lens being tested is perfect, the

with respect to another due to the observer's change of

air film between the two lenses will be of uniform

position (A, fig 2-72).

thickness and the color will be uniform all over the

b. Parallax in a telescope sight is observed as a

surface. Irregular bands or patches of color show that

relative displacement (apparent movement) between the

the surface of the tested lens is not perfect and point out

reticle and the field of view. Parallax is present when the

clearly the parts of the lens needing attention.

reticle is displaced from the image plane; in other words,

it is present when the image is not formed at the reticle

2-45. Light Loss.

but is located either in front of or behind the reticle as in

a. Whenever light rays strike the surface of any

B, figure 2-72. Parallax can be eliminated either by

lens or prism, a certain amount of light is lost by

moving the objective lens, thus shifting the image, or by

absorption and reflection. Light is absorbed by every

moving the reticle to coincide with the image. In a high-

element it strikes or travels through. The more elements

power instrument (above 4X), it is necessary to provide

in the optical system, the more light is lost by absorption

means of moving the objective to eliminate parallax if the

and reflection.

instrument is to be used at all ranges, for example, from

b. Light loss by reflection from surfaces has been

50 feet to infinity. The engineer's transit has such an

objective-focusing device as do all target-type telescopic

greatly reduced by coating the surfaces of elements

sights used on rifles.

which are intended only to refract light (paras 4-13 thru

4-16). The additional light transmitted by instruments in

which various

2-55