Lamp Characteristics & The Halogen Cycle
Most fiber optic light sources use an MR16 projector lamp, designed for use in slide projectors. The lamp is made with a coiled tungsten filament and quartz glass envelope. A combination of inert and halogen gas (Bromine) are injected into the envelope to create the performance characteristics described below.
The reflector of this lamp is usually elliptical, and may be faceted, depending on lamp manufacturer. Most reflectors have a dichroic coating to allow the IR portion of the output to pass through the reflector, rather than be focused on the input of the fiber optic product. Only 20% of the lamp’s output is given off in the visible (400-780nm) region of light; .3% in the UV region, and the balance, about 80%, is given off above 780nm.
In spite of this limitation, in comparison to other types of lamps, the Quartz Halogen lamp offers the best combination of intensity, uniformity and life. Other lamps, such as the LED (Light Emitting Diode) and HID (High Intensity Discharge) have differing strengths, which are performance advantages in some applications.
For fiber optic applications using Quartz Halogen, three lamp types are commonly used: DDL,EKE, and EJA.
The Quartz Halogen tungsten filament lamp marketed by FTI and other major manufacturers are made within the following parameters:
- Intensity +/- 10% (Batch dependent)
- Voltage 20-21 volts (full rated voltage)
- Color Temperature 3100-3400°K
- Mean Life – 40-6000 Hours
- Uniformity – +/- 10% center to edge of the output cone at the focal length. (A function of the lamp and reflector combination)
As you may have noticed above, lamp output can vary as much as 20% from lamp to lamp. In addition, all lamps degrade continually over their life. A properly ventilated, shock and vibration isolated lamp, running continuously, will lose about 15% of initial output by the end of its rated life. Contributing factors may expedite and increase the loss. This phenomena is present in all types of lamps, including LED and HID, although the rate and degree of deterioration vary depending on lamp type.
Maintaining Intensity: Light Feedback
Because output can vary as much as 20% from lamp to lamp, and the lamp itself degrades about 15% over its life, sensitive applications should employ the use of a stabilization loop (light feedback) to maintain consistency over time. As long as the required output value is less than 100% (when using a lamp of average output), light feedback maintains a pre-selected optical value chosen by the user, over some period of time. As lamp output degrades, the feedback circuit senses the drop in intensity, providing more voltage to the lamp to maintain output. Because the voltage is changing (increasing) to maintain output, a reduction in overall lamp life results. The tradeoff of lamp life for stable output is almost always an acceptable tradeoff.
A note about light feedback and intensity: Some manufacturers create “headroom” in their design, to allow feedback management at “maximum” output. In reality, maximum output of these light sources is less than models without headroom, and less than the lamp manufacturer’s rating. Therefore, the same intensity value/feedback management can be attained by reducing the output of light sources without “headroom”. To learn if a feedback design uses “headroom”, ask your supplier to provide information on the maximum voltage supplied to a specific lamp. Compare the value with the manufacturer’s full voltage rating. If “headroom” is built in, the light source manufacturer’s maximum value will be less than the lamp manufacturer’s rating. (see below for some common voltage ratings)
The three lamp types used in most fiber optic applications have the following intensity values, expressed in lumens, at full rated voltage:
- DDL – 35
- EKE – 80
- EJA – 354.
When lamps are operated at less than full rated voltage, intensity decreases, color temperature decreases, but lamp life increases. If your application can stand it, run the light source lamp voltage as low as possible to achieve good lamp life and stable performance. To learn what the expected increase in life could be, consult our Lamp Life Calculator.
The three lamp types used in most fiber optic applications have the following full rated voltage:
- DDL – 20V
- EKE – 21V
- EJA – 21V
Voltage And The Halogen Cycle
Under normal conditions, tungsten evaporates from the filament and contacts the glass wall, at which point it reacts with the halogen gas to form tungsten bromide. This compound is then freed from the glass, and migrates back to the filament where the tungsten is re-deposited on the filament. The halogen gas is freed from the compound to repeat the process.
When lamps are run at less than 80% of full rated voltage, the quartz envelope may become too cold to create tungsten bromine and maintain the halogen cycle. Tungsten, evaporated from the filament, deposits and remains on the cooler glass wall, obstructing output.
To maintain long life and consistent output, use a light feedback loop. As the lamp envelope darkens and restricts output, the sensor will react by increasing voltage, thus increasing intensity (and temperature). The resulting increase in temperature heats the quartz envelope and starts the halogen cycle again, restoring clarity. The increase in output is picked up by the sensor, which reduces voltage to the lamp and keeps the system in balance.
Voltage affects color temperature in an almost linear ratio. A 20% reduction in voltage (to 80%), reduces color temperature by about 7%. Conversely, a 20% increase in voltage (to 120%) increase the temperature a little more than 6%. Actually, it’s not the voltage, but the change in filament temperature resulting from voltage input, which effects color temperature. As you might imagine, managing the color temperature by manipulating voltage has its limit. A more effective way to manage color temperature is through the use of filters. Use the Google temperature conversion calculator, and identify the right filter to achieve a specific color temperature, depending on the initial color temperature of the lamp you select.
Most machine vision applications use black and white CCD cameras, with peak sensitivity in near IR.(800-900 nanometers). Coincidentally, the peak output of a Quartz-Halogen lamp is about 850nm. To get the most power from your lamp for B&W applications, (if your application can stand it) consider removing the IR filter from the light source (which blocks output above 700nm), and use a lamp without the dichroic reflector (Substitute one with an aluminum or gold reflector, for example).
You can try this without damage to the fiber optic component for short periods of time. If you achieve a good result, talk to us, or your current supplier, to be sure the input can tolerate the added IR energy without melting the epoxy at the input. Of course, if you are running a color application, the best color temperature is around 5600°K, which can be accomplished with color correcting filters. Be sure the filter is dichroic (reflective) and not absorptive to insure long life and consistent performance.
The three lamp types used in most fiber optic applications have the following color temperatures at full rated voltage:
- DDL – 3150°K
- EKE – 3200°K
- EJA – 3350°K.
The life of a lamp is based on a statistical interpolation of results derived from testing a sample population. Also known as MTBF (Mean Time Between Failure), rated life is determined when 50% of the batch, set up to run under ideal conditions, fail. Lamp manufacturers use this information to create a design point slightly higher than the statistical 50%. Therefore, the published rated life is the amount of time a lamp should operate before it may fail. The life you can expect from your lamps is dependent on the lamp type, environment, application, and the manufacturing process.
For practical purposes, lamp manufacturers strive to work with the following guidelines: With the exception of manufacturer defect, all lamps will work for at least 70% of the expected lamp life. The remaining lamps will suffer premature failure due to defect. The AQL value (Accepted Quality Level (DIN 40080)) for low voltage lamps is 6.5. Therefore, 6.5% of all lamps manufactured could fail before reaching the minimum (70%) stated life. As an example, an EKE lamp with rated life of 200hrs, manufactured without defect can be expected to operate for at least 140 hrs (70% of 200 hrs). For every 100 lamps purchased, as many as 7 lamps will not meet this performance criteria.
The biggest factor in lamp failure is over voltage, either from line voltage fluctuations, or excessive cycling (an in-rush current 14 times greater than operating current “hits” the lamp every time it’s energized).
The three lamp types used in most fiber optic applications have the following rated life at full voltage:
- DDL – 500hrs
- EKE – 200hrs
- EJA – 40hrs
The consistency of the filament, the glass envelope, gravity, and voltage all play a part in uniformity. From all lamps tested, Quartz Halogen lamps offer the best uniformity/intensity/life proposition. But sometimes, even these lamps are not uniform enough for the application. To maximize uniformity, consider operating the lamp so the filament is always in the same orientation. As tungsten heats up, it sags, changing the location of the brightest spot. Don’t wait until the lamp fails. As the halogen cycle re-deposits tungsten on the filament, it does not re-deposit in the originating site, so the filament gets thinner (and brighter) or thicker (and less bright) in some locations.
Use randomized fiber accessories. Randomization distributes hot and cold spots in the lamp among the entire output area helping to “mix” the light.
Refocus the lamp. Moving the lamp forward and back along its optical axis will change uniformity at the input (As well as intensity). Experiment first by moving the lamp back.
A Word About LED And HID
Today’s LED offering can be as much as 4x brighter than Quartz Halogen lighting. Additionally, LEDs exhibit an order of magnitude longer operating life. Red LEDs have an MTBF of 100K hrs. White LEDs have the shortest useful life (around 50K hrs). These electronic devices are heat sensitive, fluctuating output 15-20% from a cold start-up to an operating state. Once the device reaches an operating temperature, output stabilizes, unless the device has poor heat management design. If heat is not adequately dissipated, the device will start a self-destructive loop, continuing to produce more heat, and less light. If the condition is left unchecked, the LED output will continue to decline and fail.
Both LED and HID sources have “gaps” in their transmission wavelengths. White LEDs have three distinct peaks (Red, Green and Blue). This characteristic does not model color as well as the halogen lamp, which has no real wavelength peak. Sometimes, the consistency of lamp wavelength performance is required to render colors (and materials) accurately, as is the case in Spectral analysis.
When LED light sources are combined with a fiber optic light guide, the resulting lighting package is optimal for industrial and continuous-use applications. The output has no IR or UV component, and the input has active heat management, generous heat sink, and coupling optics to optimize performance.
The same lamp type, made by different manufacturers, will have different performance characteristics. Stick with one supplier’s lamp to minimize performance variations.