The STAGE LIGHTING Guide

Types of Light

How do we choose which type to use in each position? As lighting equipment is so robust choice is further complicated, in that in addition to the range in today's catalogue, many earlier models are still in use. However lighting instruments group into families and it is convenient to consider our requirements in terms of what each family offers in terms of beam size, beam shape, and beam quality.

ISO Symbol for a theatrical flood light

Floods

Floodlights are the simplest of all theatre luminaires, comprising of little more than a lamp and reflector in a box that can be panned from side to side and tilted up and down. As they have no lenses, the output characteristics of the floodlight are determined solely by the reflector and lamp type. The light is therefore suitable for lighting skies and cloths, it is not selective enough for lighting actors.

Floodlights are available in two variations - symmetric and asymmetric

Traditional symmetrical theatre floods use 'domestic' shaped lamps in a bowl shaped reflector, giving a uniform distribution of light in all directions. Most modern theatre floodlights use linear quartz halogen lamps with a symmetrical half-pipe shaped reflector. The light is distributed equally above and below the horizontal axis of the lamp and, to a much lesser extent, equally to each side of the lamp.

Optical system of a modern symmetric stage floodlight Selecon HUI. A modern symmetric stage floodlight

Asymmetric floods, often called Cyc {Cyclorama} lights are a special type of the floodlight, used for illuminating back drops and scenery. They use a specially shaped asymmetric reflector in order to produce a light beam that spreads much further in one direction than the other relative to the horizontal axis of the lamp. This allows for a more even spread of light down the cyc cloth or back drop, or up the cloth if the cyc light is used as a groundrow. Like floodlights, there is also some light distributed to the sides of the lamp.

Optical system of a modern asymmetric stage cyclorama light Selecon LUI. A modern asymmetric stage cyclorama light

Focus Spots - Fresnels and PCs

ISO Symbol for a theatrical fresnel spot light

Fresnel spots

The fresnel (pronounced: 'fren-el') lens is unique in its design and is easily identified by the stepped concentric rings that form the surface of the lens.

The fresnel lens is named after its inventor Augustin Fresnel (1788-1827) who developed the lens for use in lighthouses to solve the problems presented by the basic plano-convex lens which was less efficient, too heavy and prone to cracking.

The fresnel lens has since become one of the most popular lenses used in luminaires for stage lighting, largely for the same reasons.

Profile and section of a fresnel lens from a stage spotlight


Optical system of a fresnel stage light

The fresnel luminaire is easily identified by looking at the lens from the outside of the luminaire where the concentric rings are easily seen.

Fresnel luminaires produce a soft edged beam of light that is brightest in the centre and gradually darkens toward the edges. This characteristic makes blending the light beams between adjacent fresnel luminaires into a continuous pool of light of even brightness quite easy.

The fresnel luminaire is the workhorse of all theatre luminaires. Fresnels are very versatile luminaires that are often used for stage colour washes, as well as for selective highlighting. The ease of blending the light beam from one fresnel with that of an adjacent fresnel makes them quick to point and focus onto the stage ready for use.

 

ISO Symbol for a theatrical Prism Convex spot light

PC Spots

Plano-Convex (PC) lenses are flat on the back (plano) and curve outward (convex) on the front.

The glass may be completely clear or the flat side may have a textured surface, the latter sometimes being called pebble-convex lenses. The textured surface softens the beam a little to improve the overall beam quality.

Older PC lenses tended to produce a rainbow around the edge of the light beam and project the filament outline in the centre of the light beam. Their glass was also prone to cracking. Modern PC lenses have largely eliminated these problems.

Profile and section of a plano convex lens from a stage spotlight


Optical system of a PC stage light

PC luminaires produce a light beam with a 'crisp' well defined edge with less light scatter than the softer edged fresnel luminaires.

The light beam characteristics of the PC spotlight make it ideal for dramatic highlights when focused to a narrow spot or for more general colour washes when focused as a wider flood.

Adding a light diffuser, such as Rosco 114 Hamburg Frost, changes the light beam to make it soft much like the light beam of a fresnel.

PC luminaires are ideal for use as tightly focused specials for highlights such as when you want to pick out a single performer for dramatic effect. They are also suitable for use in front of house positions.

Typical beam spread of a focus spot

Plano-Convex (PC) and Fresnel luminaires belong to the same family of "focus spots" with the only difference between the two being the type of lens that is fitted. Both types use a spherical reflector which, in conjunction with the single lens, provides a low cost optical system albeit a little less efficient than the more complex optical systems used in profile luminaires. The beam angle of focus spots is adjustable over a wide range, typically from a narrow spot of 4-10 degrees to a flood of 60-65 degrees. This adjustment is achieved by moving the lamp and reflector relative to the lens which is fixed in position on the body of the luminaire. Some models achieve this movement by means of a locking knob at the bottom of the luminaire that you loosen then slide back or forward while other models have a screw thread system with a knob at the back and/or front of the luminaire that is rotated to move move the lamp tray. Moving the lamp closer to the lens increases beam width towards its widest flood setting while moving it away from the lens reduces the beam width towards its narrowest spot setting.

An accessory called a barndoor is usually fitted to the front of focus spots to provide a means of controlling the edges of the beam.

 

ISO Symbol for a theatrical profile

Profile Spots

Fixed beam profile spots

In a profile spot, the lamp and the reflector remain stationary while the lens is movable (whereas in the focus spot, the lens is stationary and it is the lamp and reflector that move). The lens movement in a profile spot controls the beam quality; the lenses are capable of producing a very hard precise edge which can be gradually softened by progressive movement of the lens tube. Control of beam size and shape in standard profile spots is achieved by adjustments at the central point of the optical system known as the gate. At this point, all profile spots have four shutters which can be used to make any size of four sided shape. There is a slot with runners which accept either an iris diaphragm to give a full range of circular beam sizes, or a metal mask to produce any required beam shape.

Optical system of a base down Fixed Beam Profile stage light

The profile spot is so called because it will project a profile of whatever two-dimensional shape is placed in the gate runners; and that profile can be projected to any required degree of hardness/softness by movement of the lens. A mask for use in the gate is known as a GOBO and because of the intense heat at this point in the lantern, gobos must be made from heat resistant material.

Moving the lens to adjust the edge quality also produces some variation in beam size, and so alternate adjustment of the shutters/iris and lens is often required to achieve the best performance from the spotlight. The development of subtle diffusers, such as Rosco 119 'Light Hamburg Frost', Rosco 132 'Quarter Hamburg Frost' and Rosco 140 'Subtle Hamburg Frost' allow lenses to be set for a hard edge and then softened with a filter. This is faster than softening with lenses, and makes more efficient use of the spotlights optics.

Most profile spots also have an adjustment allowing subtle changes to be made to the positioning of the lamp within the reflector. This allows the beam to be finely tuned between peak, where the centre of the beam is more intense, or flat where the beam has an even intensity.

The shuttering and masking devices in profile spots convert a lot of the unused light to heat and so shutters should be used to trim the beam edge rather than cut it down to size. This means selecting an instrument with the appropriate lens for the throw distance from the stage. Modern fixed beam profiles are available in beam angles such as :-

19deg - projected beam diameter will be approximately quarter of the throw distance
26deg - projected beam diameter will be approximately half the throw distance
36deg - projected beam diameter will be approximately equal to the throw distance
50deg - projected beam diameter will be approximately twice the throw distance

Zoom profile spots

Zoom Profiles are profiles that utilise a precision optical system that comprises of an ellipsoidal reflector and two or more lenses to provide a light beam that can be focused to a soft or hard edge, zoomed from a wide flood to a narrow spot, and accurately shaped by framing shutters inserted into the gate. In a zoomspot two lenses are adjustable in relation to the lamp and to each other to enable the beam width and focus to be adjusted. The beam width is at its widest when the lenses are closest together and at its narrowest when they are at their furthest apart.

Optical system of an axial zoom Profile stage light

Fixed beam profile spots usually have only one lens, although some designs use two lenses, but in either case only the beam focus is adjustable and the lenses are optimised for the specified beam width.

To maximise their efficiency zoom profiles are designed to be used within a certain range of beam angles typical ranges are 25-50 degrees or 18-32 degrees.

Zoom Profiles vs Fixed Beam Profiles

Zoom profiles allow the size of the light beam to be readily set to the size needed for any particular purpose, minimising the light and energy wastage that may otherwise result from large shutter cuts. This also eliminates the need for lighting designers to calculate the exact beam angles required for each luminaire so the appropriate luminaires can be rigged for each show. The down-side is that the extra lenses and variable beam width makes zoom profiles a little less efficient. The wider the zoom range the less optimal the optical system becomes. This is why manufacturers make a series of zoom lens systems with limited zoom ranges rather than a single lens system that zooms all the way from pin spot to wide flood. When luminaires are routinely moved to different positions within a theatre, or toured to different theatres, the versatility of zoom profiles makes them the obvious and popular choice for multi-purpose venues and schools.

Fixed beam luminaires allow the manufacturer to optimise the optical system to achieve the best performance from the luminaire, at the specified beam width, to provide a more efficient luminaire with superior light beam characteristics that is simpler to use. When the luminaires are permanently rigged at the same positions in a theatre the need to calculate beam angles from one show to another is largely eliminated making it practical to select luminaires based more on efficiency and light quality than versatility.

Axial profile spots

Many of the latest profile spotlights use an axial mounted lamp rather than the traditional 'base down' design.

Optical system of an axial zoom Profile stage light

Optical system of an axial zoom Profile stage light

Compact filament lamps such as the HPL575 used in conjunction with a coated glass reflector that can remove much of the infra-red energy from the beam, produces a highly efficient spotlight. These spotlights are often called 'coolbeam' spotlights and can provide the light output equal to a conventional 1000W spotlight using only a 600W lamp. The efficient heat management from the glass reflector, provides a cooler gate temperature, providing increased life to shutters, iris and gobos.

 

ISO Symbol for a parcan

Beamlights and Parcans

Most lighting instruments produce a conical beam so that the spread widens as the throw increases. Beamlights use a parabolic reflector to produce a near parallel beam which is more intense than a lens spotlight of the same wattage.

In the parcan the optics are fully contained within the glass envelope of the lamp. Various Lamps providing angles of a squashed near-parallel beam are available. The intensity produces a depth-enhancing haze in the air, so intense that it is effective even with deep colours. The basis of most rock lighting today.

 

Follow Spots

Follow spots (often referred to as 'limes', because of the lime gaslight they originally used) are basically profile spotlights with a rather more sophisticated optical and mechanical design. They have to be particularly well balanced to allow smooth continuous control of pan and tilt by an operator, 'following' an actor on the stage. Control of focus and iris (allowing the size of the spot to be quickly adjusted) must lie within easy reach of the operators hands. The ability to quickly adjust the colour of the spot is achieved with a 'colour magazine' mounted to the front of the light, these often allow the operator to select between 5 colours and open white.

To achieve high intensity output, follow spots often use discharge lamps and are focused to a very hard edge.

 

LED Technologies

The lighting families that we have already discussed (floods, focus spots, profile spots and beamlights) have assumed the use of traditional incandescent lamps as the source of illumination. Recent developments in LED {Light Emitting Diode} technology, have permitted these devices to be bright enough to be used as lighting sources.

LED lights used for stage lighting all fall into the same families as their traditional incandescent counterparts, the only significant difference been changes to the light source and reflector design.

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