Pechan or Z Rotating Prism

TM 9-258

g. Pechan or Z Rotating Prism. The rotating

horizontal or vertical plane (the figure shows the front of

the object and the rear of the image). This prism is

Pechan or Z prism (B, fig 4-13) consists of two prisms

useful in range finders where the angles of the ray must

separated at an angle of 45. Light rays entering this

remain constant, it is necessary to reflect light rays

prism assembly are reflected five times before emerging;

through an angle of 900 in a horizontal plane, and the

twice in one element and three times in the other

image should neither be reverted nor inverted. The

providing reversion because there are five reflections in

angles measured by the range finder are so small that; if

one plane. The incident rays and the emergent rays are

a greater number of prisms were used, any deviation in

in alinement and travel in the same directions. Two of

the angles brought about by slight bending of the tube

the external reflecting surfaces must be silvered. When

and consequent rotation of the prisms would be sufficient

this prism is rotated about its longitudinal axis, the image

to impair the accuracy of the instrument. It is necessary

rotates in the same directions with double angular speed.

h. Pentaprism or Five-Sided Prism.

to silver the two reflecting surfaces of the pentaprism as

the rays strike the reflecting surfaces at angles less than

(1) The pentaprism (A and B, fig 4-14) reflects

the critical angle and total reflection otherwise would not

light rays through an angle of 90 by two reflections from

result. It also is used in elbow telescopes to erect the

two silvered surfaces at a 450 angle to one another. It

image and produce the 90ƒ deviation.

does not invert or revert -he image if reflection takes

place in either the

4-16