Application Notes - Subminiature Lamps

The information presented here is intended to provide basic information to assist you in specifying the lamp that best suits your application. Note that all performance data for subminiature lamps is based on design voltage, and that these characteristics can be altered significantly by rerating (see definition below).

If you need a custom lamp design, please call 908/273-8998 or fax 908/273-0853 or e-mail us with your requirements.

Lamp terminology:

Design Volts: The voltage for which the lamp was originally designed. All other ratings (amperes, brightness, and life) are measured at this voltage, and can be changed by rerating the voltage.

Design Amperes: The approximate current the lamp will draw at design voltage (stated amperage ratings are plus or minus 10 percent). Wattage can be calculated by multiplying design volts by design amperage.

Design Life: The rated average laboratory life when lamps are operated under stable environmental and electrical conditions. Actual life will vary with the particular application.

Where lamp replacement is difficult, light output can be sacrificed for longer life. A reduction in the applied voltage (see lamp rerating below) will also increase lamp life significantly. Lamp life varies inversely to the 12th power of the ratio of the applied voltage versus the design voltage.

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The three operating factors that most contribute to shortened lamp life are:

Keep-alive current: Because the resistance of tungsten filaments is far greater when the filament is cold, supplying a small current (as little as one percent of operating voltage) to the lamp when switched off will keep the filament warm and buffer it against inrush current when switched on. This greatly extends lamp life.

Lamp Rerating:

Rerating a standard lamp (adjusting the voltage above or below the stated design figure) can frequently produce the desired relationship of current, life, and brightness, avoiding the expense of a custom design. Changing the voltage affects other characteristics according to the formulas and the chart below.

Rerating formulas:

Candlepower is directly proportional to the 3.5 power of the ratio of the applied voltage, and can be increased at the expense of lamp life.

Current consumption is approximately proportional to the 0.55 power of the ratio of applied voltage versus the rated voltage.

Other rerating formulas in our print catalog can be used to calculate the effects of lamp rerating. The interrelationship of voltage, current, MSCP, and life for a particular lamp can be found on the charts below. Simply multiply the rated figures by the factors to find the effects of rerating at specific percentages of design volt.

Example: Lumitron P/N L-715-AS15 is a 5-volt lamp with .150 MSCP, 115mA current draw, and 40,000 hour life. Operating the lamp at 6 volts, or 120 percent of design volts, yields the following characteristics:

MSCP: Multiplying the 5-volt MSCP figure of .150 by 1.893 (from the chart) yields an MSCP of .284 when the lamp is operated at 6 volts.

mA: Multiplying the 5-volt mA figure of .115 by 1.105 (from the chart) yields a current draw of .127 mA when the lamp is operated at 6 volts.

Life: Multiplying the 5-volt life figure of 40,000 hours by .112 (from the chart) yields a life expectancy of 4,480 hours when the lamp is operated at 6 volts.

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MSCP (Brightness measurement):

Lamp brightness is expressed as MSCP (Mean Spherical Candlepower). MSCP is an industry-wide standard of measurement that effectively integrates and averages the brightness of a lamp in all axes.

To convert MSCP to lumens, multiply by 12.57 (4 pi).

MSCP of a lamp is derived by centering the lamp within an integrating sphere (see below) with an interior diameter of one foot (30,5 cm). The interior of the sphere has a bright-white, matte coating that disperses and reflects the light evenly.

A photocell in the base of the sphere converts the light energy to electrical current, and sends the current to specialized instrumentation for measurement.

A baffle mounted below the lamp shields the photocell from direct illumination by the lamp, ensuring measurement of a true average of the light produced by the lamp.

Aged & Selected (AS): AS lamps are aged a minimum of 16 hours and selected for MSCP. A lamp designated AS15 is selected to have an MSCP tolerance of +/- 15 percent.

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Lamp Construction:

All Lumitron lamps are constructed with tungsten filaments. Because voltage capacity is directly related to filament length, the tungsten filament wire is either coiled or double-coiled in order to fit the greatest possible length of filament within the lamp envelope. Molybdenum support wires are added to lamps with very long filaments to prevent filament sagging when the lamp is on.

All Lumitron lamps are constructed for high reliability using the "bead-sealed" method. Dumet (copper-clad alloy) wires are fused within a glass bead, and the filament ends are swaged to the wires. This assembly is then placed in a glass tube, the air is evacuated from the tube, then the tube is sealed to the glass bead to preserve the vacuum within the lamp. The glass bead provides mechanical strength at the base of the lamp where the wires pass through, and ensures a reliable hermetic seal.

The lead wires are tin or gold plated, and the completed lamp is assembled into a base if required.

Lamp sizes are designated by the letter T (indicating tubular construction), followed by a number expressing the approximate glass diameter in eight-inch increments. For example, a T-1 lamp is a tubular lamp with 1/8 inch nominal (.118 +/-.007 inch, 3mm +/-0,2 mm actual) diameter.

Quality Control:

The quality control system in effect at Lumitron is documented by procedures approved to MIL-I-45208. The calibration system is designed to meet MIL-STD-45662, and test and measurement equipment is traceable to the National Institute of Standards and Technology (N.I.S.T.). Based on lot size, an acceptable quality level (AQL) as called out in MIL-STD-105, and statistical process control type sampling (SPC) applies to all shipments.

 

Inspection includes electrical parameters, mechanical dimensions, and photometric inspection reports with performance data and control charts. Other testing can be performed to meet special customer requirements. Inspection data and test reports are retained on file and available for review. Certificate on Conformance available upon request.

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Lead and Base Types:

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Lens Ends:

A lens formed in the end of the lamp envelope significantly increases the amount and intensity of light projected from the front of the lamp. Lens ends add approximately .060" (1,5 mm) to the lamp length.

Other Options:

Color coating: Our durable, scratch-resistant hard coatings can provide the exact color for your requirements, and allow maximum light transmission and fade resistance. Colors coatings can be provided to meet all MIL spec requirements or C.I.E. chromaticity coordinates.

Silicon boots: All lamps can be provided with silicon boots over the glass, which diffuse and color the light evenly while offering the interchangeability of color options.

Helium-Retardant lamps: Pioneered by Lumitron, these lamp types feature thick-wall glass envelopes to retard outgassing and increase lamp life when used in helium-atmosphere instrumentation.

MIL-spec lamps: We can supply lamps to a wide variety of MIL specifications, including Aged & Selected types.

Custom designs: If one of our standard lamps does not fit your needs, we can provide a custom filament, glass, or base design for your application.

908/273-8998      FAX 908/273-0853    E-mail: sales@lumitroncorp.com