All theatrical lamps, also known as Stage & Studio lamps, have a unique code to help identify them for replacement. The most common is a 3-letter ANSI (American National Standards Institute) code such as BTL, FEL, or HPL, however, the lamp could also be identified by a 4 or 5-digit number.

If you know the 3-letter code, or the 4 or 5 digit number, those are unique to that lamp type, and that is typically all the information you need to order replacement lamps.

If you have any incandescent fixtures manufactured by ETC (Electronic Theatre Controls), the lamp type is HPL, although there are a wide variety of HLP lamps that will fit in your fixture. For HPLs, the subsequent information following ‘HPL’ is also important.

The letters HPL are followed by Wattage and voltage, and sometimes a letter or two. 

For example: HPL575/115 is a 575 Watt lamp, which is designed to operate optimally at 115 volts.

If the voltage is followed by an ‘X’ or ‘LL’, that stands for eXtended or Long Life. Please note that eXtended or Long Life lamps do last approximately 5 times longer than standard life lamps (300 hours), albeit at a 25% reduction of lumen output.

Voltage is also an important consideration when replacing lamps. 115 volt lamps are designed to be used on a dimmer system, which has some inefficiencies that prevent full input voltage to reach the lamps. By utilizing 115 volt lamps on a dimmer system, you can achieve fill output from your lamp and fixture. If you are plugging a fixture straight into the wall receptacle, go for the 120 volt version to achieve the rated life. If you use a 120v on a dimmer, chances are that you will never achieve full output. HPLs are also available in 77 volts to be utilized with ETC Dimmer Doubling.

The Wattage rating is about energy in and lumens out. HPLs are available in 375, 575 and 750 Watts. 750 Watt HPLs work great in ETC PARs, ParNels, Multi PARs and ETC Fresnels, however, I do not recommend them for ERS (Elliptical Reflector Spotlights) aka ellipsoidals. To achieve the higher 750 Watt lamp, the manufacturer lengthened the filament and in doing so, exceeded the area from which filament output could be efficiently captured by the elliptical reflector. In side be side comparisons, most humans can not distinguish between a 575 and 750 in a Source 4 ERS.

Most professional theatres prefer the HPL575/115, while many schools and houses of worship go for the HPL575/115X to achieve extended lamp life, even with the sacrifice in output.

G9 Base, aka Bi-Pin is the most common base for most non-ETC Elliptical Reflector Spotlights or ellipsoidals.

These are available on a variety of Wattages and voltages. It is important to check your fixture for the maximum acceptable Wattage when replacing the lamp.

Many fixtures will accommodate the 1000 Watt FEL, however, the elliptical reflector cannot capture all the output so much of the output is lost inside the fixture, and goes to waste as heat.

As with the FEL, other old style Medium Bi-Pin lamps such as EHD (500 Watt), and EHG (750 Watt), all 120 volt, have a more efficient version available which utilizes a more compact filament and operates at 115 volt for maximum performance.

The single coil filament FLK (575 Watts), originally sold as the HX-600, is a great replacement for the FEL, EHG and EHD. There is also has a long life version, FLK/LL originally sold as the HX-601. 

With a slightly different filament design, four smaller coils clustered together instead of one large coil, the GLC (575 Watt) originally HX-604, and LL version GLA originally HX-605 are also great replacements, but are the specified lamp for many newer ERS fixtures.

For higher output, there is the GLD originally HX-754 and LL version GLE originally HX-755.

If the reflector is faceted, the multi-coil GL? lamps work best, or if the reflector is smooth or evenly pebbled, using the single coil FLK is more advantageous.

Medium Pre-Focus is a typical bace found in many Fresnels and other fixtures utilizing the P-28 Socket.

These Fresnel lamps typically often begin with the ANSI Code BT?, such as BTL (500 Watt), BTN (750 Watt) or BTR (1000 Watt)

There is an older style of ellipsoidal which utilizes the P-28 Medium Pre-Focus Base. These begin EG?, such as EGE (500 Watt), EGG (750 Watt) and EGM (1000 Watt) or EGJ (also 1000 Watt) but with more lumen output and higher CCT (3200K), albeit shorter lamp life (300 hrs). There is a higher performance version of this lamp, the BTH (575 Watt) which is 115 volts, so you can achieve full output on a dimmer system.

The BT? and EG? are not interchangeable, as the LCL (Lamp Center Length) is different, and would place the filament in an unusable location.

A larger version of this design, the P-40 socket, aka Mogul Pre-Focus, is common in incandescent Follow Spots. These lamps would be the BVT or BVV (1000 Watt), the later outputting higher lumen and CCT at a shorter lamp life, DTA (1500 Watt) and BVW (2000 Watt). Some of these are becoming difficult to source as fewer manufacturers are producing them.

Double-Ended Linear lamps utilize a pair of RSC Bases or Recessed Single Contact sockets, which work in opposing pairs and are typical of Cyc Light fixtures and many incandescent work lights.

These sockets are spring loaded, so you must gently push the lamp into one socket to clear the opposing socket in order to remove and reinstall the lamp.

These double ended lamps come in a variety of lengths, so it is important to select the proper lamp to make good contact.

This style of lamp is available in a variety of Wattages, and many are available frosted.

500 Watt varieties include FCL, FDN and FDF, EJG for 750 and FCM and FFT at 1000 Watts.

Ever wonder what that letter or two on a lamp means?

Hint, they have nothing to do with the ANSI Codes found on most Stage & Studio Lamps.

115 volt lamps are optimized for theatrical dimmers.

Always disconnect fixture from power when re-lamping.

Take care when re-lamping fixtures not to get smudges or oils from fingers on the lamp envelope. Any foreign material could burn onto the lamp envelope and create a light blockage which will overheat and cause a weak spot in the envelope. If this has occurred, use isopropyl alcohol and a lint free cloth to remove fingerprints prior to energizing the fixture.

Get the most out of your light fixtures, clean reflectors and lenses at least once a year. Many current lighting fixtures utilize coated optics for better light transmission. Always use a soft cloth and cleaner formulated for this application. Paper products can be abrasive and some cleaners can permanently damage lenses and reflectors. Never use ammonia based cleaners on your theatrical optics.

Check your lamp socket every time you change lamps. Burned, corroded or damaged sockets can severely reduce lamp life through excessive heat at the lamp base. Have you ever removed a non-functional lamp to discover the filament still intact? This is likely the result of a bad socket creating resistance heat which literally un-solders the inside of the lamp.

For those who really want to nerd-out on lamps.

Did you know lamps are measured in eights of inches?

The lamp size is determined by the maximum overall diameter (MOD) of the bulb’s shell. It is represented in eighths of an inch (1/8″).

The lamp’s length (or height) that is defined as the maximum overall length (MOL) expressed in inches.

The annual average amounts of coal, natural gas, or petroleum fuels used to generate a kilowatthour (kWh) of electricity, the amount required to operate a single 1000 Watts for 1 hour are:

• Coal–1.12 pounds/kWh
• Natural gas–7.36 cubic feet/kWh
• Petroleum liquids–0.08 gallons/kWh
• Petroleum coke–0.82 pounds/kWh

96.5% of the Wattage (energy consumed by an incandescent lamp) is turned into heat, only 3.5% actually creates light.