Page 164 - Modern Optical Engineering The Design of Optical Systems

Prism and Mirror Systems 147

A beamsplitter is frequently useful for the purpose of combining two
beams (or images) into one, or for separating one beam into two. A thin
plate of glass with one surface coated with a semireflecting coating, as
shown in Fig. 7.31a, can be used for this purpose, but it suffers from two
drawbacks. First, if used in a convergent or divergent beam, it would
introduce astigmatism, and second, the reflection from the second surface,
although faint, would produce a ghost image displaced from the primary
image. (Note that in parallel light neither of these objections is valid,
provided the surfaces of the plate are accurately parallel.) The astigma-
tism can be controlled by placing a weak convex surface on the second
side of the plate. The beamsplitter cube (Fig. 7.31b) avoids these diffi-
culties. It is composed of two right-angle prisms cemented together. The
hypotenuse of one prism is coated with a semireflecting coating before
cementing.
Where the weight or absorption of the cube cannot be tolerated, a
pellicle is often used as a semireflector. A pellicle is a thin (2- to 10- m)
membrane (usually a plastic such as nitrocellulose) stretched over a
frame; by virtue of its extreme thinness, both the astigmatism and
ghost displacement are reduced to acceptable values.
Obviously, the shape of the pellicle surface is determined by the shape
of the frame over which it is stretched, and an accurately plane support
is necessary. There are two less obvious features of the pellicle which
may be disadvantageous: (1) Interference between light reflected from
the two surfaces of the extremely thin pellicle can result in a trans-
mission that varies in a rippled way as a function of wavelength, and
(2) the pellicle can act as if it were the diaphragm of a microphone, and
any atmospheric vibrations can change the shape of the reflecting
surface, introducing significant changes in the imagery of the system.
This is the basis for one “talk-on-a-beam-of-light” toy.

Figure 7.31 Beamsplitters. (a) A thin parallel plate is con-
venient but may be objectionable because of ghosting and
astigmatism, unless used in parallel light. (b) Beam-
splitting cube has a semireflecting coating supplied to one
of the diagonal faces before cementing.