Choose Your Weapons.

Any form of classification system tends to show up the exceptions - there are always items which fit into two categories or not quite in any. Nevertheless it is useful to have some kind of system which can give us points of reference and helps us to understand the behaviour of whatever is being classified.

Our subject being light sources, we could begin with two categories: spotlights and floodlights.

A spotlight will include some means to concentrate the light from the source into a comparatively narrow angle, generally less than 60 degrees. This may be done in either of two ways: firstly by the reflector alone (often called a beamlight), or secondly, by the use of a reflector plus a lens or lenses.

A floodlight will tend to allow the light from the source to spread over a wide area, typically more than 100 degrees. It may or may not include a reflector to utilise light that would otherwise be lost and there may be a means of reducing the spread by cutting off the unwanted portion - unlike the spotlight, there is no means to concentrate it. Again, a division in two can be made by distinguishing between a simple floodlight, basically a harder source, where a substantial amount of the light comes directly from the lamp, and a softlight where the light source is a large area - the reflector and, in the case of fluorescents, the surfaces of the tubes.

To prove my point about exceptions, there is also a category of lampheads known as focussing floodlights which share some of the characteristics of both of the above; more on these later.

Over the TV, theatre and the film industries, one type of lamphead is seen as the 'workhorse' instrument. If you restricted a lighting designer from any of these industries to only one type of lamphead, they would certainly choose the Fresnel spotlight. The name 'Fresnel' comes from the French physicist Augustin-Jean Fresnel who famously used a type of stepped lens to improve the performance of lighthouses. The design relies on the fact that it is the transitions between air and glass in a lens that have the required effect on the light. Therefore it is possible to make a lighter and thinner lens do a similar job to a much thicker one by cutting out the unwanted glass but keeping the same curve on the front surface (see diagram). This design allows a very wide angle lens to be made, which would otherwise be impossibly thick and heavy.

Fresnel lenses are typically used in lampheads together with a spherical reflector which simply redirects light going to the rear of the fitting back through a point as near to the original light source as practically possible. In an ideal world a point source of light would be used although in practice this is not possible, so the lens is generally stippled or diffused slightly to avoid projecting a pattern of the filament.

In a practical Fresnel spotlight, a beam angle of up to 60 degrees can be achieved and by moving the light source back from the lens, the beam angle can be reduced sometimes to as little as 5 degrees. When doing this, the lamphead becomes less efficient as more light is lost inside the fitting. This is not always appreciated since, as the head is 'spotted', the subject is seen to get brighter, although the total output may only be one quarter of what it was on flood.

The wide angle of the Fresnel allows its use at comparatively short throws and, on flood, a well-designed unit can produce a reasonably even intensity over the area covered. At the edge of the beam, the intensity will drop sharply but with a soft edge, which makes it easy to blend adjacent beams when lighting a large area with several units. Because it is a 'softened point' source, it can still be controlled easily using barn doors on the lamphead but gives a more flattering effect on the subject with softer shadows than a true hard source. Manufacturers may offer different sizes of Fresnel, smaller for portable and larger for studio use. Larger units give more light output for a given wattage and a more pleasing quality of light because the longer focal length of the lens makes the light source effectively further away and therefore more like the natural light from the sun which we are used to. In some instances, a smaller lamp can be used if the large lens is required but with less power - for example a 'Studio' 300W can be achieved by putting a 300W lamp in a 650W lamphead.

So why use other instruments besides the Fresnel? In the theatre, its use is restricted to short throws, typically above the stage since the lens produces a certain amount of unwanted spill light which would be distracting if used above the audience. Here a theatrical lighting designer would use a more controllable instrument called a 'profile' spot which produces a narrower beam of light, typically 5-45 degrees, which can be cut off precisely with shutters or an iris inside the lamphead and produces virtually no spill. This type of fitting can also be used to project an image, which could be a simple metal cut-out of say, a window frame, or a colour picture on a high-temperature glass slide. In the theatre, the term 'gobo' inevitably refers to this type of cut-out, where in film and TV it can also mean a large pattern which is used some distance in front of a light source. Again, a properly designed unit will give an even field though, unlike a Fresnel, the edge of the beam should be sharp. By adjusting the lens, the shutters may be defocused to allow a soft edge to be produced.

In the film industry a less controllable but very much more efficient instrument is used for narrow angle applications. This is a type of beamlight and as such depends on the reflector to produce near-parallel rays of light with perhaps only 5-10 degree beam angle. It is commonly referred to as a PAR and typical examples are the ARRISun and Pocket Par ranges, which use metal halide discharge lamps.

In the 1970's the standard instrument for Rock Concert lighting was the infamous 'PAR Can'. These instruments are simple metal tubes containing a sealed beam lamp, similar to a car headlight, which includes its own optical system. These were called Parabolic Aluminised Reflector lamps, parabolic being a description of the curve of the reflector. The lens was moulded into the front of the unit and the rear was coated with aluminium to form the reflector.

These PAR lamps came in various sizes and ultimately metal halide discharge versions were also produced. Because of the higher cost of the lamps, unlike the tungsten equivalents, commonly one type with a narrow beam angle was made, wider angles being achieved by using one of a choice of separate lenses.

With the advent of the single-ended discharge lamps such as MSRs, it was more economical to mount a standard lamp through a hole in a separate reflector rather than using the sealed beam 'PAR' lamps. Using a larger reflector improved efficiency and also allowed the lamp to be moved, although not every manufacturer offers this. This principle is well known from the 'Maglite' torch and serves two purposes. Without a lens, the peakiness of the field can be controlled, even to the point of producing a centre dark hole, which can be useful to eliminate a hot spot where a softbox or diffuser is being used. When lenses are used, the lamp adjustment gives a useful variation of the beam angle.

This type of source can be highly efficient, particularly at narrow angles, but has some disadvantages. Since the entire reflector acts as a light source, the quality of light can be somewhat unnatural giving strong but ill-defined shadows. Secondly, the source can be difficult to control - barn doors on the lamphead are only useful in reducing glare. These can be overcome if the instrument is used for longer throws and controlled by means of flags placed some distance in front of the head. Comparing the photos shows that in the shot illuminated by a Fresnel (on the left), as well as a cleaner, more natural shadow, the barndoors achieve a smooth cut-off on the left of the frame.

Although the increased sensitivity of film and cameras means that less efficient but more controllable Fresnels are now becoming more favoured, these focussing floodlights can still be useful, particularly with softboxes or bounced of reflectors because of their high efficiency.

The next article will continue to look at some of the different types of available lampheads.

By Andy Barnett, ARRI GB.

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