The ellipsoidal reflector spotlight has a fixed ellipsoidal reflector in relation to a movable plano-convex lens assembly (barrel). The ellipsoidal reflector collects light from the source and bounces it to a second conjugate focal point. This instrument is, therefore, a type of projector. The image produced is actually a projection of the circular "gate". Patterns called "gobos" may be inserted in the shutter plane and pattern will then be projected to the stage.
The Zoom Ellipsoidal has a field angle range. It allows you to hang and instrument in a specific location and get a number of different sizes of lighting area out of it. Zooms are convenient, but what you gain in convenience you lose in light output.
The Fresnel Spotlight is a short throw, soft edged fixture that is useful for lighting areas close to scenery and lighting even washes of colour. It has a fixed fresnel lens and a moveable spherical reflector. Moving the bulb/reflector assembly closer to the lens puts the instrument in flood focus. The opposite direction puts the fresnel in spot focus. The only way to control the light output from a fresnel is to add accessories like barndoors and tophats to the gel frame in order to make cuts to the light.
A Beam Projector has no lens. It uses a parabolic reflector to reflect the source into parallel beams of light. This instrument is useful for sunlight effects.
A striplight has a number of compartments each with its own light source and circuit. By hooking thes open face instruments together, you can create a sheet of even illumination useful for lighting cycloramas and backdrops.
A Cyclight is an open-faced floodlight useful for lighting backdrops and cycloramas.
Units are designated by either their field angles in degrees or by lens diameter and focal length as in 6x12, where 6 is the diameter and 12 is the focal length of the lens in inches. A long focal length means a narrow angled, long throw instrument, whereas a short focal length means a short throw, wide angled instrument.
A Stage Lighting Instrument is defined as "anything that holds a light source in place and directs and/or shapes the beam of light".