The Optical Communications Reference

1.1

Chapter 1.1

Geometrical optics

Menn

1.1.1. Ray optics conventions and

practical rules. Real and virtual

objects and images

Electro-optical systems are intended for the transfer and

transformation of radiant energy. They consist of active

and passive elements and sub-systems. In active ele-

ments, like radiation sources and radiation sensors, con-

version of energy takes place (radiant energy is converted

into electrical energy and vice versa, chemical energy is

converted in radiation and vice versa, etc.). Passive ele-

ments (like mirrors, lenses, prisms, etc.) do not convert

energy, but affect the spatial distribution of radiation.

Passive elements of electro-optical systems are fre-

quently termed optical systems.

Following this terminology, an optical system itself

does not perform any transformation of radiation into

other kinds of energy, but is aimed primarily at changing

the spatial distribution of radiant energy propagated in

space. Sometimes only concentration of radiation

somewhere in space is required (like in the systems for

medical treatment of tissues or systems for material

processing of fabricated parts). In other cases the ability

of optics to create light distribution similar in some way

to the light intensity profile of an ‘‘object’’ is exploited.

Such a procedure is called imaging and the corresponding

optical system is addressed as an imaging optical system.

Of all the passive optical elemen ts (prisms, mirrors,

filters, lenses, etc.) lenses are usually our main concern. It

is lenses that allow one to concentrate optical energy or

to get a specific distribution of light energy at different

points in space (in other words, to create an ‘‘image’’).

In most cases experienced in practice, imaging systems

are based on lenses (exceptions are the imaging systems

with curved mirrors).

The functioning of any optical element, as well as the

whole system, can be described either in terms of ray

optics or in terms of wave optics. The first case is usually

called the geometrical optics approach while the second

is called physical optics. In reality there are many situa-

tions when we need both (for example, in image quality

evaluation, see Chapter 2). But, since each approach has

advantages and disadvantages in practical use, it is im-

portant to know where and how to exploit each one in

order to minimize the complexity of consideration and to

avoid wasting time and effort.

This chapter is related to geometrical optics, or, more

specifically, to ray optics. Actually an optical ray is

a mathematical simplification: it is a line with no thick-

ness. In reality optical beams which consist of an endless

quantity of optical rays are created and transferred by

electro-optical systems. Naturally, there exist three kinds

of optical beams: parallel, divergent, and convergent (see

Fig. 1.1.1). If a beam, either divergent or convergent, has

a single point of intersection of all optical rays it is called

a homocentric beam (Fig. 1.1.1b,c). An example of

a non-homocentric beam is shown in Fig. 1.1.1 d. Such

Fig. 1.1.1 Optical beams: (a) parallel, (b,c) homocentric and

(d) non-homocentric.

Practical Optics; ISBN: 9780124909519

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