specsheets

At some time or another we've all sat down to get some valuable information from a luminaire specification sheet, brochure, web page or CD ROM, only to discover that either the information was totally unintelligible or it was deliberately obscure to prevent you from meaningfully comparing products. Every manufacturer has their own style of spec sheet designed to emphasise those points that the marketing department consider important. Let's look at what sort of information is provided and what it actually tells us.

On most spec sheets Beam Angle and Field Angle figures are provided, although Cut-Off Angle is sometimes also quoted. The Beam Angle, or to use its official CIE[1] title, the Half-Peak Angle of a luminaire is defined as that part of the beam where the light level varies between its peak brightness and half the peak level. Just as our ears can only detect sound pressure level differences greater than 3dB (ie when they double or halve), the smallest difference in light level that our eyes can detect is a doubling or a halving of intensity (a change of one f-stop). This means that the area defined as the beam angle is the part of the beam that appears uniformly bright to our eyes.

The CIE's term for the Field Angle is the One-Tenth-Peak angle, which covers the part of the beam where light varies between its peak brightness and one tenth of the peak. To our eyes, this covers most of the bright part of the beam. Cut-Off Angle is generally taken to mean the angle where the light has stopped completely, which can be useful if you are concerned about the amount of spill. Intriguingly enough, no one seems to give figures for the angle or intensity of the great wash of light that leaks out of the top, rear or sides of their luminaires, even though in some cases it would appear to exceed what comes out of the front.

The terms Peaky Field and Flat Field are widely used in spec sheets but appear to be whatever the marketing people chose them to mean. Peaky Field is usually accepted as describing the situation where the lamp has been tweaked to give the brightest possible central hot-spot and Flat Field is its opposite; where the lamp has been adjusted to give the most even possible beam. The peaky field figures are useful for judging a luminaire's performance as a tight special, whilst flat field figures give an indication of its usefulness in wash or area lighting. It's not exactly unknown for the marketing department to use the peaky-field Beam Angle and the flat-field Field Angle in order to claim that their zoom profile's range is 11 to 44 degrees despite the fact that even a two-to-one zoom range is a decent optical achievement for a profile spot. Another point to watch for is that while many luminaires are named for their Beam Angle (eg. Selecon and Strand), others such as ETC's Source Four are named for their Field Angle.

One of the most useful pieces of information you can get from a spec sheet is data on beamspread and light levels, but again you have to be careful about what figures have actually been used. Beamspread is the width of the area covered by the Beam (Half-Peak) Angle at a given throw distance, but it's not unknown for the beamspread figure to be based on the Field (Tenth-Peak) Angle instead, to make the coverage look more generous. Always check the fine print tucked away down the bottom, on the back of the spec sheet

Light levels can be a thorny problem for several reasons; firstly, the figures quoted are for the luminaire with a high-output film and television lamp installed, even though that's almost certainly not the lamp we'll all be using; next because the levels quoted are for peaky-field output, whilst the beamspread figures are those for the flat-field beam; and most importantly, because the figures quoted are difficult to comprehend due to the units chosen.

The most meaningful light intensity unit for the working lighting person is lux, the unit of illuminance, though lumen, the unit of luminous flux, and Candela the unit of luminance, are also used, even though they aren't particularly suitable for our needs. Luminance tells us how bright a light source is in comparison to an internationally agreed standard "candle". Luminous Flux is essentially a measure of how much light there is in the beam - how many photons or wave-packets it contains. Illuminance is an indication of how much light (lumens) is hitting each square metre of the stage.

Rather than the more useful lux, some spec sheets give figures for total beam lumens, which is fine if we also have the beamspread, as lux are lumens per square metre, so it's straight forward to calculate the average lux for the beam. The only thing to watch here is that the measurements usually quoted are for peak lux. Once again, the use of a peaky-field adjustment can be misleading.

A quick and simple test of the reliability of beamspread and intensity figures is to check them with the inverse square law, which tells us that if we double the throw distance, we should see a doubling of beamspread and a quartering of the incident light levels. If the spec sheet shows a two metre beamspread at 1,000 lux for a throw distance of five metres, then we would expect a beamspread of four metres and 250 lux for a throw distance of ten metres; anything else leads us to wonder how the spec sheet compiler goes about breaking the laws of physics.

The final point to consider is that the specification sheet is seen by the luminaire manufacturer as a marketing tool rather than an altruistic fact sheet. Don't even believe the distributor's telephone number until you've checked it in the White Pages.

[1] CIE - Commission Internationale de l'Éclairage or International Commission on Illumination, the international body which sets lighting standards.