Porro Prism System

TM 9-258

normal erect image is produced with reflection in a

(2) A second type of Porro system is known as

horizontal plane while inverted, reverted image is

the Abbe system (fig. 4-10). It consists of a single

produced with reflection in a vertical plane.

system which is three prisms in one or it can be made in

c. Porro Prism System.

the form of two prisms (fig 4-21). It is substantially the

same in principle as the first Porro type but the

(1) Each element of the Porro prism system is a

reflections occur in different order. It likewise produces

right-angle reflecting prism placed to bend rays of light

an inverted, reverted image.

through an angle of 1800. One element is arranged to

bend the rays horizontally and the other to bend the rays

veertically; one element reflects the rays into the other

(fig 4-9). With one prism in horizontal position and the

other vertical, the Porro prism produces an inverted,

reverted image. The Porro prism system is used as the

erecting system in many fire-control instruments; it is

commonly used in binoculars. The sharp corners of

these prisms are removed making the ends round. This

reduces the weight and lessens the chance of breakage.

Figure 4-10. Abbe system.

Amici or Roof-Angle Prism.

(1) This is a one-piece prism which may be

considered as made up of a right-angle reflecting prism

(fig. 2-43) with the hypotenuse face or base replaced by

two faces inclined toward each other at an angle of 900

(fig 4-11). The latter two faces form the "roof" and give

this prism its name. When inserted in the optical system

of a telescope, it bends the light rays within the

instrument through an angle of 900 while, at the same

time, inverting and reverting the image.

Figure 4-9. Porro prism system.

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