Choosing the Right Replacement Lamp

In your hand is a blown lamp, the likes of which you've never seen before. This wouldn't be so bad if your lamp supplier hadn't revealed that whilst they can order a direct replacement from a warehouse in Darmstadt, Germany, or their branch office in Valley Falls, Oregon, the only chance you've got of seeing light out of that luminaire this week is to take one of several "close equivalent" lamps which are available ex-stock. Let's look at the difference between an exact replacement and a close equivalent and whether or not an equivalent may actually do a better job than the replacement.

There are many parameters which specify a particular lamp; some of them critical for correct operation in a given luminaire and others that offer a range of possibilities for different applications. The list of parameters includes: voltage, wattage, current, colour temperature, base type, rated life, burning position, ignition voltage, light centre length (LCL), maximum overall length (MOL), envelope type, beam angle/s, CIE chromaticity coordinates, envelope finish, arc length, gas filling, filament type, luminous efficacy, Colour Rendering Index (CRI) and output lumens. Whatever else we do, a replacement lamp must at least fit the luminaire, requiring a match of base type, burning angle and either LCL for single-ended lamps or MOL for double-ended.

Base types are generally identified by a letter indicating the base family and a number indicating the size, although there are exceptions such as the EMEP (Extended Mogul End Prong) bases found on PAR 64s and the MEP (Mogul End Prong) found on PAR 56s. Common base families are: bayonet (B), Edison screw (E), prefocus (P), bi-pin (G), uneven bi-pin (GY), medium bi-pin (GX) and recessed contact (R). The size indicates the diameter in the case of a round base and the pin centre spacing for bi-pin bases: the common domestic Australasian BC lamp has a B22 base (bayonet -22mm in diameter), most modern 1kW luminaires have a GX9.5 base (medium bi-pins at 9.5mm centres) and most cyc floods have R7s bases (Recessed base - 7mm diameter, single contact).

The Light Centre Length, the distance between the top of the lamp base and the centre-point of the filament, defines where the light-centre of the lamp is located in relation to the optics of a luminaire. Lamps of different wattages, and therefore filament sizes, can have identical LCLs, whilst lamps of identical base type and wattage can have different LCLs. Installing a lamp with the wrong LCL will produce a distorted blob rather than the normal beam shape of a luminaire. The MOL of a double-ended lamp will govern whether or not it will physically fit between the mounting points in the luminaire.

A lamp's burning angle indicates the range of angles over which the lamp was designed to operate. Typically figures are quoted as an orientation and range of angles such as VBD45 (vertical, base-down ± 45deg) or H15 (Horizontal ± 15deg). A lamp will almost certainly work outside its specified range of burning angles, but it may not work for long.

Wattage and voltage are obviously important to match, but in desperate times, even though a 220volt lamp may not last too long, it may get you through the day or two until the 240volt version is available. Similarly, if the other parameters match, a change of lamp wattage will at least get light coming out of the luminaire; even if you have to tweak a few states. Most older 500Watt luminaires will cope perfectly well in the long term with 650W or 750W lamps, and for a couple of shows with a 1kW lamp.

For such applications as skin-tone or product imaging for film and video, colour rendering is critical, requiring very close tolerances of colour temperature (for incandescent sources) or CRI and CIE Chromaticity coordinates (for discharge sources). In most situations where colour filters are used or where the performance is not for the camera, almost any shade of white is acceptable and frequently undetectable. In view of the very close relationship between colour temperature and rated life, it may be worth looking to see if there is a longer-life version of the lamps you use, even though you may lose a hundred or two Kelvins in the process. (The mysteries of colour temperature are examined elsewhere.)

If that lamp in your hand is an incandescent (filament) lamp without a halogen gas filling (say a T/1,) you will almost certainly benefit in luminous efficacy, rated life and lumen maintenance (how rapidly light output declines during the life of the lamp) by replacing them with their Quartz-Halogen equivalents. If however it's a discharge lamp, you had better make sure that the alternative lamp you are considering matches all of the critical criteria; if in doubt check with the technical support (not sales) staff at your luminaire supplier or, heaven forbid, RTFM.

by Andy Ciddor
© Copyright The Kilowatt Company