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Legacy Lighting Technology

Learn About Legacy Lighting Tech

Lighting technology has progressed massively in the past decade, with the advent of high powered LED lamps and the electronics that power them. However, in order to fully appreciate how far lighting technology has come, it is important to understand how older legacy lighting tech worked and the performance benchmarks it set for the future. In this guide, we have detailed all of the major types of lighting technologies commonly used before the advent of LED lights.

Compact Arc Lamps

Known as short arch or compact arc lamps, these high pressure gas discharge lamps feature an arc length which is small considering the size of the electrodes they use. The actual arc length varies between a third of a millimeter to a centimeter depending on the rated wattage and the application it is intended for. These types of arc lamps feature the highest luminance and radiance of any constantly operating light, which makes them the closest type to a true point light source.

The envelope for these types of arc lamps is made from an optically clear quartz that has a spherical or ellipsoidal shape. The grade of quartz used in the envelope will determine the amount of ozone generated when the lamp is in operation. For electrodes, tungsten is by far the most widely used material.

Most compact arc lamps are intended for DC operation, due to the better arc stability and longer lifespan compared to AC operation. Consisting of an igniter and regulated power supply, DC systems use high voltage pulses of up to 50,000 volts to break down the gap present between electrodes, which ionizes the gas and heats the tip of the cathode to thermionic emitting temperatures, producing visible light.

Contrary to what one would expect, higher wattage compact arc lamps do not always result in increased light intensity. Instead, when a higher light output is desired for a certain application, the selection was instead made based on the light’s lumen rating and not wattage since the two were not always correlated.

Xenon Lamps

A specific type of compact arc lamp, Xenon lamps are filled with several atmospheres of xenon gas. A relatively slow starting lamp, they reach 80% of their final lumen output within 10 minutes of initial startup. The color of the arc is actually very close to daylight, in the 6000K range. The light spectrum is continuous in the range visible to the human eye, and even extends into the ultraviolet range. These lamps also exhibit strong lines in the near infrared range between 800 and 1000 nanometer (nm) range, with some weak lines in the blue portion of the spectrum as well.

These lamps are typically manufactured in wattage ratings from 70 all the way up to 30,000 watts. They are normally available in configurations that allow for either vertical or horizontal operation. The breakdown voltage between the electrodes on Xenon lamps runs between 10kv to 60kv depending on the lamp’s size. Larger lamps, such as those rated for 30kw or more, will have a higher breakdown voltage.

Xenon lamps are quite powerful, with a luminous efficiency of approximately 30 lumens per watt at 1,000 watts, and 45 lumens per watt at 5,000 watts. They are even capable of producing an outstanding 150 lumens per watt when run at 20kw.

Mercury-Xenon Lamps

A Mercury-Xenon lamp contains a specific amount of mercury and a small amount of xenon added at a pressure exceeding one atmosphere. The xenon is necessary to facilitate starting and to sustain the arc until the mercury is fully vaporized; it also reduces the warm-up period. Normal warm-up time is 10-15 minutes.

Mercury lamps are sensitive to cooling because the bulb temperature determines the vapor pressure. The lamp can be over-cooled to the point that full output in the mercury spectrum is never achieved. The cooling water should be ordinary tap water. Chilled water may decrease the operating voltage and interfere with the proper evaporation of mercury. In some cases, the mercury may not evaporate at all, causing unsuitable performance and shortened lamp life.

Typical steady state voltage of a Mercury-Xenon lamp is higher than that of a xenon lamp. The output in the visible range consists mainly of four mercury lines and some continuum, due to the high operating pressure. A properly warmed lamp will show no significant trace of the xenon gas spectrum.

Mercury-Xenon lamps are available in wattages from 200 to 7000 watts. The luminous efficacy is approximately 50 lumens per watt at 1000 watts and about 55 lumens per watt at 5000 watts.

Tungsten Lamps

Technical lamps consist of a coiled tungsten filament mounted in a precision glass envelope. The envelope may have a vacuum or, more commonly, be filled with an inert gas such as argon or krypton. Typical technical lamp operating parameters are 2.5 to 12 volts and .02 to 1 amp. Color temperature ranges from 2,200 to 3,000 degrees Kelvin; lamp life may be as high as 30,000 hours.

Tungsten-Halogen lamps feature a tungsten coil filament mounted in a quartz glass envelope that has been filled with an inert gas plus a trace of halogen (normally bromine). This gas creates the “halogen cycle”: tungsten that has evaporated from the filament combines with the halogen gas. Convection currents within the bulb carry this gas to the quartz wall where it is cooled and then returned to the proximity of the filament. The heat of the filament causes the tungsten and bromine to separate, and the tungsten is then deposited on the cold portion of the filament.

This regenerative process prolongs the life of the filament considerably, and also eliminates blackening of the bulb by preventing the evaporated tungsten from condensing on the envelope. The Halogen lamp color temperature runs from 2900 to 3400 deg. Kelvin and are available in wattages from 10 to 250 at operating voltages from 6 to 24; lamp life ranges from 10 to 2500 hours. Luminous efficiency is approximately 22 lumens per watt.

Tungsten-Halogen lamps must be operated at voltages that maintain an envelope temperature between 250 and 350 deg. C. Cooler temperatures will not allow the halogen cycle to take place, thus causing bulb blackening and shorter life; higher temperatures will cause oxidation of the conductors and lead to premature lamp failure.

Lamp Handling

Special storage cases are provided to eliminate possible hazards during shipping and handling. Safety goggles and soft cotton gloves should be worn when removing and installing lamps. Never touch the quartz envelope with bare hands; such handling may lead to deterioration and premature failure. If accidentally handled, clean the lamp surface with an alcohol swab to remove any residue.

Polarization

Some lamps can only be mounted one way in the PowerArc housing since the anode (+) and
cathode (-) have different diameters, thus making accidental polarization reversal nearly impossible. However, some lamps have the same diameter anode and cathode, allowing room for error. Refer to the lamp manufacturer’s data sheet for proper identification of the anode and cathode.

Note that reversed polarization will result in immediate and permanent damage to the lamp electrodes. A lamp that has been fired with reversed polarization will have obvious physical damage to the electrodes. A damaged lamp will fire, but it will exhibit unstable performance and a severely shortened operating life.

Lamp Stability

Short term stability is measured over seconds, while long term stability is measured over minutes, hours, or even days.

Short term stability is affected by arc “wander,” “flare” and “flutter.” Arc wander is the movement of the attachment point of the arc on the cathode surface. Typically the arc moves around the conical cathode tip in a circular fashion, taking several seconds to move a full circle. Arc flare refers to the momentary change in brightness as the arc moves to an area on the cathode having a preferential emissive quality over the previous attachment point. Arc flutter is the rapid side-to-side displacement of the arc column as it is buffeted by convection currents in the xenon gas which are caused as the gas is heated by the arc and
cooled by the envelope walls.

Arc wander and flare can be reduced by a slight decrease in the operating current. For example, a 75 watt xenon lamp rated at 5.4 amps may be operated at 4.5 amps for the first one or two minutes of operation, after which the current should be brought up to the specified normal operating level.

Lamp Life

The useful life of compact arc lamps is determined primarily by the decrease of luminous flux caused by the deposit of evaporated electrode material on the inner wall of the envelope. Frequent ignition accelerates electrode wear and hastens the blackening of the envelope. Average lamp life is based on approximately 20 minutes of operation for each ignition. The end of the lamp life is the point at which the UV output has decreased by approximately 25%, the arc instability has increased beyond 10%, or the lamp has ceased to operate under specified conditions. Lamps should be replaced when the average lamp life has been exceeded by 25%.

As the lamp ages, the operating voltage will increase. Lamp current should be decreased to maintain output until the minimum operating current is reached. At this time the lamp should be replaced.

Lamp life varies with different types. Check the manufacturer’s specifications for the rated lamp life.

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