Lamps will have design ratings based on their operational voltage. They can be operated at higher or lower voltages if the other effects resulting from the voltage variation can be tolerated. Figure III (Nomograph) can be used as a guide to show the changes in lamp properties as a result of the voltage change.

The Nomogrpah should be used only as a reference and should not be considered as an absolute indication of what will happen to a lamp that has been re-rated. The greater difference between application voltage and rated voltage, the greater the percentage of error. The Nomograph is based on the following formulas that can be used to determine the application life, amperage and MSCd:

Amperage
Amperage is the amount of current following through a lamp filament when operated at a given voltage. Current is determined by the resistance of the tungsten; the higher amperage lamps use a larger diameter tungsten and will have a lower resistance. The lower current lamps use a small diameter tungsten and will have a higher resistance. The standard tolerance for amperage shown is plus or minus 10%.

Inrush Current
When a subminiature lamp is energized, there is an inrush of current because of the low resistance of the cold filament. The filament color temperature (Kelvin) at which the lamp is designed to operate determines the inrush current (Figure IV).

The inrush current will be 8-12 times the rated current and the “rise time”, or the amount of time it takes the current to stabilize at its rated value, will be approximately 30-100 milliseconds.

Inrush current can be determined by measuring the cold resistance of the lamp and dividing it into the rated voltage.

The inrush current can be reduced by using a “keep alive” voltage which should keep the filament warm but below the level of incandescence (below 1000 degrees Kelvin). The keep alive voltage can reduce in the inrush current by as much as 50%.

Mean Spherical Candelas (MSCd)
Mean spherical candelas or MSCd is the unit of measurement used to indicate the intensity of a subminiature lamp. MSCd is the total quantity of flux (lumens) emitted from a lamp when it is measure in the center of an integrating sphere. The sphere is attached to a photometer and then calibrated with an NBS (National Bureau of Standards) traceable candle power standard which is similar in size and candlepower to the lamps that are being measured.

MSCd ratings can be converted into lumens by multiplying the rated MSCd times 4 π (12.57).

Our standard MSCd tolerance is plus or minus 25%. Custom or tighter tolerances of plus or minus 15%, or 10%, or lamps selected to just the plus or minus side of the nominal, are also available.

End Foot Candles (EFC)
End foot candles (EFC) is the illuminance measurement off of the end of the lamp and is used for lamps with a flat end or a lens end.

Aging
Aging is a very important process to stabilize quality after production. Aging is used to eliminate the early failures due to broken or shorted filaments, vacuum leads and to “clean up” the filament and offer MSCd stability. Vacuum clean up is evident by a decrease in current and an increase in brightness. In general, the filament goes through changes within the first 16 hours and then stabilizes. All of our lamps are aged at rated voltage for up to 16 hours and are designed to meet specifications after aging. Additional aging of up to 100 hours may b required for special applications where abnormally tight MSCd tolerances are specified.

Efficacy and Kelvin
Efficacy is the conversion of watts of electrical energy to visible radiant energy (MSCd/watt). When comparing lamps with similar voltages, the lower the efficacy, the longer the life.

Kelvin is the color temperature of the filament. The chromaticites of a filament are very close to the chromaticities of a blackbody radiator.

The typical Kelvins of the standard subminiature vacuum lamps are between 1800 Kelvin to 2400 Kelvin. A filament must be heated to a temperature of at least 1000 – 1200 Kelvin to product visible light. (Zero degrees Celsius equals 273 degrees Kelvin.)
The relationship between efficacy and Kelvin is illustrated in Figure V.

It is possible to have lamps with the same voltage and amperage and have different intensity value and life. This is done by making small changes in the filament length and diameter. An example of this can be illustrated by comparing the following two lamps.

OL 330BP
14V 80mA .50MSCd 1,500 hrs.
.446 efficacy 2300 Kelvin

OL 382BP
14V 80mA .30MSCd 15,000 hrs
.268 efficacy 2100 Kelvin

The lower MSCd of the OL 382BP is achieved by increasing the tungsten diameter and length, or changing the loaded density (watts per unit area) of the filament. The results prove to be a lower efficacy and Kelvin and an increase in life.

Life
Lamp life is inversely proportional to the 12th power of the applied voltage and is the most difficult characteristic to give absolute information about.
Average lamp life is based on when 50% of a large group of lamps, operating at rated voltage burn out, or the MSCd has dropped by 20%. In some cases, rated life tests are not practical, so life is based on a theoretical calculation of the burn off rate of the tungsten when operating at the rated Kelvin.

Life may vary due to the individual manufacturer’s materials and processes, DC voltage, shock and vibration, voltage variations, temperatures exceeding 100 degrees Celsius, series operations, flashing or switch operations or a lack of controls on the electronics used to drive the lamps.

It is recommended to operate a lamp at a derated voltage to increase life. This will reduce the operating Kelvin and evaporation rate.

Application and Design Considerations

DC Notching and the Soret Effect
DC voltage can reduce life by approximately 50%-70% due to the “sawtooth” or notching effect. Nothcing, also know as electromigration, is a change in the molecular structure of tungsten due to the DC voltage. The results are hot spots that accelerate the thermal conditions, the evaporation rate and embrittlement.

Notching can also be caused by the “soret effect” which results from a temperature gradiant. Notching, due to the soret effect, takes place next to the mounting posts and support wires and occurs in both AC and DC voltage applications.

Water Cycle
As a general rule, when lamps are subjected to temperatures in excess of 100 degrees Celsius for a period of time, life may be reduced due to the outgassing of the glass envelope. The water vapor molecules will start to bread down and the oxygen combines with the tungsten to form tungsten oxide that is transmitted to the glass walls of the bulb. The tungsten is left on the glass walls and the oxygen then combines with the bydrogen again to form water molecules which return to the filament to start a new cycle. This is called the “water cycle” and is recognizable by darkening of the glass and a reduction in intensity.

Recently, we developed a line of lamps for use with Night Vision Goggle Filter that will operate effectively in temperatures above 100 degrees Celsius. Our development in this area is ongoing, and we encourage you to contact us for further details if you have such an application.

Series Operation
Life of a lamp can be severely reduced by using a series application because of the different resistances of the lamps used in the series. The applied voltage is not distributed evenly over all of the lamps in the series, so each lamp is operating at a slightly different voltage. This means embrittlement and the evaporation rate will not be equal, so some lamps could fail prematurely. It is recommended that lamps be selected for amperage and operated at a derated voltage for maximum reliability in series operations.

Shock and Vibration
The best filament design for use in a shock and vibration environment is a low voltage, high amperage filament. This represents a short, thick filament. The higher you go in voltage, the longer the filament, which then necessitates the use of hangers. The hangers will increase the higher voltage filament reliability, but the effects of notching due to the soret effect are increased.

Resonant frequencies of each lamp must also be taken into consideration because this is one of the most detrimental types of vibration to a lamp. Lamp environments should be designed to eliminate the lamp from dwelling for any length of time at one of its resonant frequencies.

Orientation of the filament in relationship to the axis of vibration will also affect the lamp performance in a shock and vibration environment.

The most critical consideration in protecting a lamp against the effect of shock and vibration is the packaging by the user. Filament damage, due to shock and vibration, can be reduced or amplified by the way the lamp has been assembled into the users product.

Multiplexing
DC pulsing of lamps is possible without affecting DC life. It is important to control the frequency, duty cycle and peak voltage to maintain life. The Kelvin and efficacy of the lamp must be equivalent to what the lamp would be when operated at rated voltage.
It is recommended to use a frequency of 400 Hz plus to prevent thermal cycling that would be detrimental to life and strength. The rated pulse voltage is determined through the following formula:

Color Filtering
Lamps can be filtered to almost any color because of their wide spectral distribution. Coloring can be obtained by coating, booting or by using external filters. The Kelvin temperature of a lamp is a critical factor to consider when color filtering. In order to achieve, as an end result, the proper color coordinates, the spectral distribution of the Kelvin must be measure with the transmission data of the filtering media. Emissive colors can be specified in X Y chromaticity coordinates taken from the 1931 CIE chromaticity diagram or in U’ V’ coordinates taken from the 1976 UCS chromaticity diagram.

Lens End Lamps
The lens causes a gathering of light that will offer a 3 to 10 times increase in light coming from the end of the lamp. There is not specific controlled spot pattern.

Helium Retardant Lamps
These lamps are manufactured for use in a helium atmosphere. They are made with an increased wall thickness to retard the penetration of the glass envelope wall by the helium molecules. The penetration of the helium molecules causes a cooling of the filament which results in a loss of intensity and reduced life.

Acid Frosting
Acid frosting can be used to diffuse the light or to prevent the filament from being recognizable to certain types of photo sensors or in some backlighting applications.

Application Recommendations
• Consult us in the early stages of design for technical assistance and recommendations as to the best lamp to use for a specific application. We can offer a variety of customized lamps which include lamps that have longer or shorter leads, bent or formed leads, and/or electrical characteristics that are tailored to your individual requirements.
• Use low voltage, high amperage lamps for shock and vibration environments.
• Provide good heat sinking and airflow to keep the ambient temperature below 100 degrees Celsius.
• Derate the lamps, whenever possible, to reduce tungsten evaporation and increase life.
• Use AC voltage over DC voltage, when possible.
• Use the lowest efficacy lamps for the longest life. Consider reflective coatings, contrast filters, or lenses for increased luminance or enhancement.
• Use a “keep alive” voltage for maximum reliability.
• Design your system to insulate the lamps as much as possible from the effects of shock and vibration.
• Take care in handling – avoid excessive shock and vibration during packaging, shipping and production. Avoid lead bending or stress at the seal area to prevent vacuum leaks.