TECHNICAL INFORMATION

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What is a Miniature Lamp? The term miniature lamp applies to units ranging in size from the so-called "grain of wheat" lamps to automotive headlights. It is not as much indicative of physical size as of low operating voltage - rarely more than 28 volts and often as little as 1.5 or 2.5 volts.

Bulb Designations
Bulbs are identified by a letter or letters followed by a number. This letter indicates the shape of the bulb and the number indicates the diameter of the bulb in eighths of an inch. Various bulb shapes are as follows:

For example, "S-8" indicates a S, or straight-sided, bulb with a diameter of 8/8ths or approximately 1 inch.
Following are examples of various miniature bulb types:

Light Center Length (LCL)
LCL is measured from the geometric center of the lighted filament to that part of the base listed below:

Maximum Overall Length (MOL)
This dimension represents the overall length of the lamp from the top of the glass to the bottom of the base.

Filament Configurations
Filament configurations are identified by letters and a suffix number (or number and letter) indicating the shape of the filament and the number of any supports. See Filaments and Bases for more information and illustrations o f filament configurations.

Bases
See Filaments and Bases for information and illustrations of the different types of miniature lamp bases.

Design Volts
The socket voltage for which the lamp will draw at design volts.

Amperes
The approximate current which the lamp will draw at design volts.

Watts
The approximate power the lamps will consume at design volts. Watts = Amps X Volts.

Candlepower
Candlepower is the accepted measure of luminous intensity in a miniature lamp. The rating shown is the mean spherical candlepower produced at design voltage. Where the critical factor for normal lamp use is the current drawn, then ampere or watt rating is shown.

Candlepower is increased rapidly with an increase of applied voltage and conversely is reduced rapidly with a reduction of applied voltage. Should a lamp be operated at low voltage to take advantage of increased life, there will be an inevitable loss of candlepower.

Life
Rated life is not guaranteed life and does not necessarily represent service life expectancy. Rated life, along with rated amperes at design volts, is a factor establishing the laboratory efficiency of a lamp. It is useful only as an index of comparative longevity of various lamps.

The life of an incandescent filament is voltage sensitive. A 10% increase in applied voltage will reduce the filament life to only 30% of its design value, while a 10% decrease in applied voltage will multiply the design life by a factor of approximately three. It is often feasible to "derate" an incandescent lamp for the sake of increased life.

Resistance
The resistance of tungsten increases with its temperature, and as its applied voltage is increased a given tungsten filament will become hotter. Therefore, at elevated voltages, the rate of current change is less than at reduced voltages. This characteristic sometimes makes tungsten filament lamps useful as circuit ballasts.

Vacuum
The source of light in all miniature lamps is an incandescent tungsten filament which must be protected from exposure to the atmosphere. In vacuum type lamps, the filament is protected by processing the lamp so that a vacuum exists inside the glass envelope. The level of vacuum prior to operation of the lamp is on the order of 10-2 mm Hg or better, and during operation lamps will generally improve to about 10-4 mm Hg because of gettering of residual gasses by tungsten evaporation.

Tungsten evaporation rates in vacuum lamps are generally much higher than in gas filled or halogen lamp types. As a consequence, normal bulb blackening causes an almost constant decrease in light output during the life of the lamp.

Gas Filled
Gas filled lamps use an inert gas to protect the filament during operation. In most automotive lamps, the gas most used is a mixture of 88% argon and 12% nitrogen. The pressure of the fill gas at room temperature is about 3/4 of an atmosphere.

The use of a fill gas reduces the net rate of tungsten evaporation by several orders of magnitude, so that little blackening of the bulb occurs during the life of the lamp. Since the net evaporation rate in gas is less than in a vacuum, the filament can be operated at a higher temperature for the same life. For this reason , gas filled lamps are more efficient than vacuum lamps.

The blackening that does occur is usually confined to the top of the bulb because of convection currents produced during operation. To avoid "water cycle" and premature blackening, "getters" are usually used to chemically or physically bind contaminants and assure a "clean" fill gas. Purity of the gas fill initially and during operation is critical in gas filled lamps.

Halogen
Halogen lamps, in addition to being gas filled, have a gaseous halogen compound added to the gas fill. The halogen compound constitutes a small portion of the fill gas and must be accurately controlled. The quantity used is on the order of 100 parts per million of fill gas and is usually a bromine compound. Its purpose is to prevent any blackening of the bulb by returning evaporated tungsten back to the filament through a "halogen cycle". Too little halogen permits blackening of the bulb to occur. Too much halogen destroys the tungsten filament.

Since with proper operation of the "halogen cycle" no blackening occurs, the bulb can be smaller than what is normally used in regular gas filled lamps. With smaller envelopes, higher molecular weight inert gasses and higher fill pressures are practical. Krypton is generally used in automotive halogen lamps and the fill pressure at room temperature is approximately 5 atmospheres.

The higher molecular weight gas and higher fill pressure permits operation of the filament at even higher temperatures than regular gas filled lamps, so that for the same life, halogen lamps have the highest efficiency of all miniature lamps.

LAMP IDENTIFICATION CHECKLIST

We offer a lamp I.D. service which includes finding de-listed, hard to find and obsolete lamps. Often we can identify a lamp over the telephone. We can also work from a picture which you can fax or e-mail to us. You can also send us a sample of the lamp. Any lamps sent to us for identification will be returned upon request. If the lamp you need is not available, we may be able to cross reference another lamp or it may also be possible for us to have the lamp custom made to your specifications.

MINIATURE FILAMENTS & BASES

FILAMENTS
Filaments for miniature and subminiature lamps may be a straight wire (S), a coil (C), or a coiled coil (CC). Coiling the filament wire effectively shortens the filament length so that smaller bulbs can be used. In addition, in gas-filled lamps, coiling the wire reduces thermal losses and increases efficiency.

Tungsten is almost universally used as lamp filament material because of its high melting point at incandescence. The number following the coil identification letter(s) denotes the arrangement of the filament on the supports.

BASES
Bases provide electrical contact to the lamp and, in most cases, also support the lamp in the fixture. For miniature and subminiature lamps, bayonet or wedge base types are generally preferred over screw types when vibration is present. In addition, wedge bases reduce socket size and complexity. Flanged or collared types are usually associated with requirements for filament location.