Stage lighting Instruments are usually classified by either their LENS structure or their REFLECTOR type.   Sometimes a type of instruments are called by a brand name...(an example is that old timers refer to all ellipsoidal spotlights as Lekos).like some people calling every refrigerator a "fridgadair".

Lens type:  Usually stage instruments use either a single or double PLANO-CONVEX Lens or a FRESNEL lens.  A possible third lens type is a variation on the Plano-Convex lens which cuts away a portion of the plano side of the lens in a "step series" and is called a "step lens".  In fact the Fresnel is a step lens with the convex part to the lens cut away in a step series.

The Purpose of whatever lens is used is to BEND light, and for stage purposes to bend it so that the light rays travel in a straight line relative to the instrument

The "FOCAL LENGTH" is distance from the lens at which light rays converge into a point.  When considered along with the diameter of the lens, a shorter focal length will produce a wider pool of light......Thus a 6 ( diameter)by 9 (focal length) lens will produce a wider pool of light than a 6 x 12 (6" diameter and a 12 inch focal length).  However the longer focal length has a further "throw" or usable distance.  Thus a 6x9 is useful in closer hang locations and a 6x12 useful at a distance further from the stage.  In fact for a time this was the way that most theatre instruments which used the plano-convex lens and an ellipsoidal reflector were grouped or identified.....6x9, 9x12, 6x22, etc

We can complicate this by using a ZOOM feature which varies the distance between the two Convex lenses, thus altering the beam shaping or focal length of the lens structure. (see your text)

Thus one heard "give me a 6x9 Leko" as way of describing a Ellipsoidal reflector spotlight with a 6 inch diameter lens and a 9 inch focal distance.

Most modern style spot lights use an Ellipsoidal type of reflector.  For a discussion of the focal points inside an ellipse reflector see your text.  The major advantage of the ellipsoidal reflector is that it captures more light, and focuses it at a point outside the reflector where it again begins to diverge.  If we place the plano surface of a plano-convex lens at this convergence point, we can, using the lens make the great majority of the light into straight line beams from the instrument.

Another advantage is that the light may be manipulated at or near this convergence point.  This quality is what allows us to have sharp shutter cuts, gobos or cookies to project patterns.  Remember however that modifications at this convergence point are "backwards" as the light is again altered as it goes through the lens!

Newer instruments such as the Source 4 and Shakespeare 600 series add another item to the spotlight.  This is a mirror, called a "cold mirror" that is made of Pyrex glass covered with a dichroic coating which reflects the visible light spectrum, but allows the infrared (heat) spectrum to pass though it...resulting in a "Cooler" beam of light which is less wearing on the color media, the gobos, cookies, and shutters....not to mention the actors claim to be cooler on stage.  Oh course the technician should remember that this heat has to go somewhere if it is not in the beam.....and that means it is transferred to the instrument via a heat sink and dispersed into the air.....the heat sink gets VERY hot.

One other way that some instruments are classified is by the type of lamp (bulb) that they use.

You text gives numerous illustrations of base types, globe types, and filament styles as well as some INTERNAL reflector types on pages 338 and 339 (new edition)

Qualities of lighting Instruments:

Soft vs Hard:  Instruments throw either a "hard edged" or "soft edged" pool of light.  The Fresnel Spotlight is designed to throw a SOFT edged pool of light. while the Ellipsoidal can be focused from soft to hard or hard to soft.

BEAM vs FIELD:  a pool of light has two areas.......the beam (the brightest part) and the field, the part where there is still light, but it is not as bright.  As a "rule of thumb" the field angle is about twice the beam angle.  The Field angle is where the light drops off to 50% of that of the beam angle.

IN CLASS we will demonstrate field and beam angle, soft vs hard pools and the qualities of the instruments in our inventory as well as accessories like safety cables, c-clamps, color media holders, gobo holders, barn doors, snoots, top hats, and cookie sheets.

We will also examine other lighting types such as the "runts" and farcyc, Scoops, Broads, Striplights Pars, and follow spots.

While we do no own Intelligent or Kinetic instruments, students will be expected to know the general principles of these types of instruments (pgs 352-354) as well as the uses of color changers, rotating gobos, etc.