Cermax Lamp
`Engineering Guide
`
`399 West Java Drive
`Sunnyvale, CA 94089
`
`ASML 1123
`
`

`
`Copyright' 1998 by ILC Technology,Inc.
`
`This document may not be copied wholly or in part without prior written permis-
`sion from ILC Technology,Inc.
`
`Regardless of the content of this document, ILC Technology will not bear the
`responsibility for negative effects incurred as a result of the operation of this equip-
`ment.
`
`The contents of this document are subject to change without prior notification.
`
`Great care has been taken in the preparation of this document. However, if you
`should discover an error or an omission, please bring it to our attention.
`
`Cermax is a registered trademark of ILC Technology, Inc. Other brand or product
`names are trademarks or registered trademarks of their respective holders.
`
`ILC Technology,Inc.
`399 West Java Drive
`Sunnyvale, CA 94089
`
`Telephone: 408-745-7900
`Fax: 408-744-0829
`W eb: http://www.ilct.com
`
`Printed in U.S.A.
`
`

`
`Contents
`
`5.0 Lamp Operation and Hazards ...........................28
`5.1 Lamp Cooling.....................................................................28
`5.2 Electrical and Mechanical Connections........................29
`5.3 Lamp Safety........................................................................29
`5.3.1 Explosion hazard...........................................................29
`5.3.2 High-voltage hazard .....................................................30
`5.3.3 Ozone..............................................................................30
`5.3.4 High light levels ............................................................30
`5.3.5 Thermal hazards...........................................................30
`5.3.6 Lamp disposal ...............................................................30
`
`6.0 Lamp Lifetime.........................................................30
`6.1 Other Factors Affecting Cermax Lamp Lifetime ........31
`
`7.0 Applications .............................................................32
`7.1 Fiberoptic Illumination ....................................................32
`7.2 Video Projection ................................................................33
`7.3 UV Applications.................................................................35
`7.4 Other Applications ............................................................35
`
`References .......................................................................35
`
`Acknowledgments.........................................................36
`
`List of Figures.................................................................iv
`
`1.0 Introduction................................................................1
`1.1 Cermax Lamps .....................................................................1
`1.2 Major Lamp Characteristics..............................................1
`1.3 About This Guide ................................................................2
`
`2.0 Lamp Construction .................................................2
`2.1 Cermax Lamp Types...........................................................2
`2.2 Lamp Construction.............................................................4
`2.3 Mechanical Dimensions and Tolerances ........................6
`
`3.0 Optical Characteristics...........................................6
`3.1 Spectrum, Color, and Efficacy..........................................6
`3.1.1 Spectrum............................................................................6
`3.1.2 Color ..................................................................................7
`3.1.3 Efficacy..............................................................................9
`3.2 Arc Luminance ....................................................................9
`3.3 Illuminance.........................................................................11
`3.3.1 Elliptical lamps ..............................................................11
`3.3.2 Parabolic lamps .............................................................13
`3.4 Luminous Intensity...........................................................16
`3.5 Scaling Laws .......................................................................17
`3.5.1 Output versus input current........................................17
`3.5.2 Radiometric versus photometric quantities ............17
`3.5.3 Spectral quantities ........................................................18
`3.6 Other Optical Characteristics..........................................18
`3.6.1 Output variation with time ..........................................18
`3.6.2 Turn-on characteristics................................................18
`
`4.0 Electrical Characteristics ....................................19
`4.1 V-I Curves ...........................................................................19
`4.2 Lamp Ignition ....................................................................20
`4.2.1 Trigger.............................................................................20
`4.2.2 Boost ...............................................................................22
`4.2.3 Transition to DC operation ........................................24
`4.3 Lamp Modulation, Pulsing, and Flashing ....................24
`4.3.1 Modulation.....................................................................24
`4.3.2 Pulsing ............................................................................25
`4.3.3 Circuits for pulsing ......................................................26
`4.3.4 Cold flashing .................................................................26
`4.4 Lamp Power Supplies and Igniters ................................26
`4.5 Electromagnetic Interference (EMI)..............................27
`
`Contents
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`i i i
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`

`
`List of Figures
`
`Figure 1. Typical Cermax lamp and quartz xenon
`short-arc lamp.
`
`Figure 2. Typical Cermax lamps.
`
`Figure 3. Pictorial view and cross section of a
`low-wattage Cermax lamp.
`
`Figure 4. Cermax lamp subassemblies being removed
`from brazing fixtures.
`
`Figure 5. Cermax reflector ceramics being checked for
`reflector contour on a coordinate measuring machine.
`
`Figure 6. Cermax lamps ready for vacuum processing.
`
`Figure 7. Fill pressures for standard Cermax lamps.
`
`Figure 8. Reflector geometry and lamp body
`dimensions for a typical Cermax lamp (LX300F).
`
`Figure 9. Cooling ring dimensions for 1-inch and
`1-3/8-inch window Cermax lamps.
`
`Figure 10. Cermax lamp spectrum.
`
`Figure 11. Typical Cermax spectrum in the infrared.
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`Figure 12. (a)1931 CIE chromaticity diagram.
`(b) Chromaticity diagram showing isotemperature lines. 9
`
`Figure 13. Isobrightness contours of an LX300F lamp
`as a function of lamp age.
`
`Figure 14. Isobrightness contours of an LX1000CF
`lamp.
`
`Figure 15. Calibrated isobrightness plot.
`
`Figure 16. Relative arc brightness of the cathode hot
`spot as a function of lamp age for an LX300F lamp.
`
`Figure 17. Lumen output versus aperture size for
`low-power elliptical Cermax lamps.
`
`Figure 18. Continuation of Figure 17 to smaller
`aperture sizes.
`
`Figure 19. Lumen output versus aperture size for
`high-power elliptical Cermax lamps.
`
`Figure 20. Typical output distributions of lamps at 2
`hours and 24 hours.
`
`Figure 21. Typical elliptical Cermax beam shape
`compared to Gaussian distribution.
`
`Figure 22. EX300-10F spot diameter as a function of
`z-axis position and lamp age.
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`Figure 23. Spot illuminance as a function of lamp age
`and aperture size for an EX500-13F lamp at 500 watts.
`
`Figure 24. Lumen output versus aperture size for
`parabolic Cermax lamps with typical lenses.
`
`Figure 25. Typical pinhole scan of the focal spot of an
`LX500CF lamp with an f/1 lens.
`
`Figure 26. Graph for estimating the focused output of
`1-inch parabolic Cermax lamps.
`
`Figure 27. Typical farfield beam shapes of LX300F and
`LX1000CF Cermax lamps.
`
`Figure 28. Output of EX300-10F lamp as a function of
`time after ignition.
`
`Figure 29. A verage V-I curve for the LX300F lamp.
`
`Figure 30. Individual V-I curves for the EX300-10F
`lamp after 2 hours.
`
`Figure 31. Individual V-I curves for the EX900-10F
`lamp after 2 hours.
`
`Figure 32. Typical Cermax lamp trigger pulse.
`
`Figure 33. Typical lamp ignition circuit.
`
`Figure 34. Typical voltage and current waveforms
`during ignition of Cermax lamps.
`
`Figure 35. Typical Cermax lamp V-I and impedance
`curves for pulsing (LX300F).
`
`Figure 36. Typical complete Cermax lamp power
`supply (PS175SW-1).
`
`Figure 37. Typical measured temperatures on 300-watt
`Cermax lamps.
`
`Figure 38. Lifetime curves for standard Cermax lamps
`run at reduced current.
`
`Figure 39. Typical Cermax fiberoptic lightsource.
`
`Figure 40. Typical Cermax fiberoptic illumination
`systems.
`
`Figure 41. Typical lightguide numerical apertures and
`transmissions.
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`Figure 42. Typical Cermax video projector lightsource. 34
`
`i v
`
`List of Figures
`
`

`
`Cermax Lamp
`Engineering Guide
`
`1.0 Introduction
`
`1.1 Cermax Lamps
`ILC s Cermax high-intensity arc lamps are rugged and
`compact xenon short-arc lamps with fixed internal
`reflectors. Patented and trademarked by ILC Technology,
`Inc., their primary distinguishing characteristics are
`focused output, extremely high brightness, and safe
`operation. Cermax lamps also provide broadband and
`stable output spectra. Their high brightness makes them
`ideal for applications such as fiberoptic illumination,
`video projection systems, and analytic instruments.
`Except for some specialized low-wattage, high-pressure
`mercury lamps, Cermax lamps provide greater brightness
`levels than any other commercially available incoherent
`light source and in some cases replace lasers. The
`mechanical integrity of Cermax lamps far exceeds that of
`any other type of short-arc lamp.
`The purpose of this guide is to provide the system
`designer with the information needed to efficiently
`incorporate Cermax lamps into optical systems and
`achieve maximum performance. This guide describes the
`lamp construction details; the mechanical, optical, and
`electrical characteristics; operation details, including
`
`operating hazards and lamp lifetime; and specific
`applications.
`
`1.2 Major Lamp Characteristics
`Cermax lamps are similar in many ways to quartz xenon
`short-arc lamps, though they appear quite different (see
`Figure 1). The two types of lamps share spectral
`characteristics and often run from the same power
`supplies. Similarities also include stable color
`characteristics, excellent color rendition, instant-on with
`no color shift, and modulation capability. The
`fundamental efficacies of Cermax and quartz xenon
`lamps are close, about 20—30 lumens per watt below 1000
`watts. This compares to about 70—100 lumens per watt for
`typical metal halide lamps. However, Cermax and quartz
`xenon lamps are rarely used in situations where raw
`luminous flux is the only important characteristic.
`Because Cermax and quartz xenon lamps have small arc
`gaps and high arc brightness, their light can be focused
`more easily onto small targets. In the case of Cermax
`lamps, the reflector collects more of the light than the
`typical metal halide lamp reflector. Consequently, in
`many applications Cermax lamps focus more light on the
`target than similar- wattage metal halide lamps.
`
`Figure 1. Typical Cermax lamp (left) and quartz xenon short-arc lamp (right). (Photos not to scale.)
`
`Introduction
`
`1
`
`

`
`Cermax and quartz xenon lamps run from DC power
`supplies that are usually low-voltage (12—20 volts), high-
`current power supplies with trigger and boost circuits for
`lamp ignition. Lifetimes for Cermax and quartz xenon
`lamps usually range from a minimum of 500 hours to 5000
`or 10,000 hours, depending on the application.
`Although Cermax and quartz xenon lamps have
`many similarities, it s the differences that highlight the
`strengths of the Cermax lamp. Advantages of Cermax
`lamps over xenon lamps include compactness, small arc
`size, ruggedness, no devitrification of the lamp bulb, and
`the prealigned internal reflector.
`Because of the ceramic construction, a Cermax lamp
`(including reflector and cooling fins) is typically a fraction
`of the size of a comparable quartz xenon system.
`
`¥ A Cermax lamp s arc size is usually shorter and its
`current level higher than those of a quartz xenon lamp
`at the same power. This causes the greater brightness of
`Cermax lamps.
`
`¥ The ceramic construction makes the Cermax lamp
`very rugged and safe for user replacement. The
`ceramic-to-metal seals used in Cermax lamps achieve
`much higher strengths and are more consistent than
`the seals in quartz xenon lamps. Cermax lamps are the
`safest xenon arc lamps available.
`
`¥ The prealigned reflector eliminates the need for any
`field alignment of lamp to reflector. The sapphire
`window in a Cermax lamp allows for wide spectral
`output (UV to 5 microns), but by adding a filter coating
`(F type lamps), the UV can be kept inside the lamp.
`
`1.3 About This Guide
`The information in this guide is intended to cover the
`Cermax product family. Therefore, the data was chosen to
`represent typical performance characteristics. There are
`over 20 standard Cermax lamps and hundreds of
`nonstandard lamps that may differ in one or two
`specification items. For each standard Cermax lamp,
`there is a product specification sheet. Those sheets, along
`with this guide, should allow a designer to predict system
`performance in most cases.
`Occasionally, a reference is made to ILC engineering
`notes. These contain more detailed test data and are
`available from ILC Technology. Some of the data
`presented here is from those engineering notes. When the
`data source is not referenced, the data was generated in
`the test labs at ILC and is not available in published form.
`The information presented here is aimed at the
`system designer. In addition, there is a paper by
`Rovinskiy1 that is an appropriate introduction to how
`xenon short-arc lamps are designed and how the design
`parameters affect performance. There are also other
`published papers that address the details of performance
`
`2
`
`and electrode phenomena and may be helpful to system
`designers.2,3,4,5
`To fully optimize the illumination system that uses a
`Cermax lamp, raytracing with optical design software
`programs is often required. Standard lens design
`programs, such as Beam 46, Oslo7, and so on, are useful
`for rudimentary raytracing in optical systems that contain
`Cermax lamps. Nevertheless, to fully optimize the
`system, optical software programs (such as Solstis8) are
`required that can model the arc in the lamp and launch
`rays from many different points in the arc at many
`different angles. ILC Engineering Note 227 provides a
`numerical arc map of a 300-watt Cermax lamp. ILC
`engineering note 228 provides lens design parameters for
`some common commercially available aspheric
`condenser lenses.
`A recommended general reference on photometric
`and radiometric testing and lamps in general is the IES
`Lighting Handbook.9 A good reference on color is Color
`Science10 by Wyszecki and Stiles. Optical reflectors are
`covered in The Optical Design of Reflectors,11 by Elmer.
`
`2.0 Lamp Construction
`
`2.1 Cermax Lamp Types
`Figure 2 shows typical Cermax lamps. The various lamp
`models are most easily sorted by power level, reflector
`type, and spectral output.
`The first distinguishing characteristic is power level.
`The standard power levels are 125, 175, 300, 500, and 1000
`watts. Each of these power levels is actually a power
`range, with the nominal power level near the maximum.
`For instance, a 300-watt Cermax lamp will normally
`operate from 180 to 320 watts, a 175-watt from 150 to 200
`watts, and so on. The upper level of the power range is
`determined by the maximum temperature that the lamp
`can sustain and still meet its lifetime requirement. The
`minimum power level is determined by the requirement
`for long-term arc stability. The lamp will not be damaged
`if it is operated at very low powers for short periods of
`time. However, for example, operating a 300-watt lamp at
`120 watts for 100 hours may cause the arc to become
`unstable and the light output to flicker in intensity. Check
`the individual product data sheets for power range and
`other specifications.
`The second characteristic is reflector type. For most
`power levels, both elliptical and parabolic Cermax lamps
`are available. The elliptical lamps produce focused
`outputs and have slightly better collection efficiencies and
`slightly shorter arc gaps. The parabolic lamps produce
`collimated output beams and are usually used with
`focusing lenses. If an elliptical lamp is selected, the next
`choice is reflector f number. Elliptical lamps up to the 300-
`
`Lamp Construction
`
`

`
`Figure 2. Typical Cermax lamps. (a)) LX300F, (b) LX1000CF, (c) EX300-10F, (d) EX500-13F, (e) EX900C-10F, (f) EX900C-13F,
`(g) EX1000C-13F. (Rays are for illustration purposes and are not true raytraces.)
`
`watt power level have fnumbers of 1, 1.5, and 2. ILC
`defines fnumber as the on-axis length from the end of the
`reflector to the focal point, divided by twice the radial
`height of the highest marginal ray as it strikes the
`reflector. The 500- and 1000-watt elliptical lamps have f
`numbers of 1 and 1.3, respectively. It is important to
`distinguish between the theoretical f number and the
`effective fnumber. Because very few optical rays are
`reflected from the outermost edge of the reflector in
`Cermax lamps, the theoretical lamp fnumber that best
`matches a particular optical system may not be the same
`as the system fnumber. For example, an f/1.3 Cermax
`lamp may be the best match for an f/1.5 optical system.
`The third characteristic is spectral output. Cermax
`lamps are optimized for either ultraviolet (UV) emission
`
`or for visible use. The visible lamps have a filter coating
`on the lamp window to absorb and reflect unwanted UV
`back into the lamp. Therefore, Cermax lamps optimized
`for the visible have an F suffix, for filtered, in their model
`numbers, while the model numbers of UV-emitting
`lamps contain the letters UV.
`The model numbers for Cermax lamps contain
`information about the power and construction choices.
`Standard Cermax lamps have model numbers such as
`LX300F or EX500-13 U V. LX signifies a collimated
`output and parabolic-shaped reflector lamp; EX signifies
`a focused output and an elliptical-shaped reflector lamp.
`The next three or four digits give the nominal power (e.g.,
`300 watts or 500 watts). In the case of elliptical lamps, the
`-13 (or -10,-15, -20, etc.) represents the nominal fnumber.
`
`Lamp Construction
`
`3
`
`

`
`For example, -13signifies f/1.3. There are custom Cermax
`lamps for original equipment manufacurers whose model
`numbers begin with Y, such as Y1052. These numbers are
`assigned sequentially and do not contain information
`about construction. Such OEM lamps usually feature
`some characteristic, specification, or test parameter
`which differs from those of standard lamps. These Y-
`lamps are not generally available to customers other than
`those for whom the lamps were designed. It is very risky
`to relamp a fixture or lightsource with a Cermax lamp
`that does not have the exact model number of the original
`lamp. Cermax lamps that appear identical can have vastly
`different performance characteristics.
`
`2.2 Lamp Construction
`Figure 3 shows a pictorial view and a cross section of a
`low-wattage parabolic Cermax lamp. Most Cermax lamps
`are similar in construction, although individual parts
`may vary slightly. The lamp is constructed entirely of
`metal and ceramic. No organic (carbon-based) materials,
`mercury, rare-earth elements, or any other materials with
`disposal problems are used in the lamp construction. The
`fill gas, xenon, is inert and nontoxic. The lamp
`subassemblies are constructed with high-temperature
`brazes in fixtures that constrain the assemblies to tight
`dimensional tolerances. Figure 4 shows some of these
`lamps subassemblies and fixtures after brazing.
`
`There are three main subassemblies in the Cermax
`lamp: cathode, anode, and reflector. The cathode
`assembly (3a) contains the lamp cathode (3b), the struts
`holding the cathode to the window flange (3c), the
`window (3d), and the getters (3e). The lamp cathode is a
`small, pencil-shaped part made from thoriated tungsten.
`During operation, the cathode emits electrons that
`migrate across the lamp arc gap and strike the anode. The
`electrons are emitted thermionically from the cathode,
`meaning that the cathode tip must maintain a high-
`temperature and low-electron-emission work function.
`The cathode struts (3c) hold the cathode rigidly in
`place and conduct current to the cathode. The lamp
`window (3d) is ground and polished single-crystal
`sapphire (AlO2). Sapphire is chosen to allow the thermal
`expansion of the window to match the flange thermal
`expansion so that a hermetic seal is maintained over a
`wide operating temperature range. Another advantage of
`sapphire is its good thermal conductivity, which
`transports heat to the flange of the lamp and distributes
`the heat evenly to avoid cracking the window. Getters
`(3e) are wrapped around the cathode and placed on the
`struts. Their function is to absorb contaminant gases that
`evolve in the lamp during operation and to extend lamp
`life by preventing the contaminants from poisoning the
`cathode and transporting unwanted materials onto the
`reflector and window.
`The anode assembly (3f) is composed of the anode
`(3g), the base (3h), and tubulation (3i). The anode (3g) is
`
`Figure 3. Pictorial view and cross section of a low-wattage
`Cermax lamp.
`
`Figure 4. Cermax lamp subassemblies being removed from
`brazing fixtures.
`
`4
`
`Lamp Construction
`
`

`
`constructed from pure tungsten and is much more blunt
`in shape than the cathode. This shape is mostly the result
`of the discharge physics that causes the arc to spread at its
`positive electrical attachment point. The arc is actually
`somewhat conical in shape, with the point of the cone
`touching the cathode and the base of the cone resting on
`the anode. The anode is larger than the cathode, to
`conduct more heat. About 80% of the conducted waste
`heat in the lamp is conducted out through the anode, and
`20% is conducted through the cathode. Therefore, the
`anode has been designed to have a lower thermal
`resistance path to the lamp heatsinks. This explains why
`the lamp base (3h) is relatively massive. The base is
`constructed of iron or other thermally conductive
`material to conduct heat loads from the lamp anode. The
`tubulation (3i) is the port for evacuating the lamp and
`filling it with xenon gas. After filling, the tubulation is
`pinched or cold-welded with a hydraulic tool and the
`lamp is simultaneously sealed and cut off from the filling
`and processing station.
`The reflector assembly (3j) consists of the reflector
`(3k) and two sleeves (3l). The reflector is a nearly pure
`polycrystalline alumina body that is glazed with a high-
`temperature material to give the reflector a specular
`surface. Reflectors are batch-checked to ensure that the
`reflector figure will not degrade the lamp s optical
`performance (Figure 5). The reflector is then sealed to its
`sleeves (3l) and the reflective coating is applied to the
`glazed inner surface. For visible F lamps, the reflector is
`coated with a silver alloy. For UV lamps, the reflector
`receives an aluminum coating. An advantage of the
`sealed reflector construction of Cermax lamps, and of the
`
`inert xenon fill gas, is that the reflectors are quickly
`sealed into the final lamp assembly, eliminating the
`chance for oxidation to degrade the reflector s surface.
`The three lamp assemblies are finally sealed by
`tungsten-inert-gas (TIG) welding the cathode and anode
`sleeves (3l). Before sealing, the assemblies are checked to
`ensure that the arc gap is correct and is positioned
`accurately relative to the reflector. The lamps are leak
`checked, pressure tested to beyond operation pressure,
`and then evacuated and baked out on the pump and fill
`station to eliminate any remaining contaminants (Figure
`6). Cold xenon fill pressures for standard Cermax lamps
`are listed in Figure 7. After filling, the lamps are burned in
`for at least 2 hours to stabilize the cathode. All lamps are
`then tested for light output, either in the specific
`equipment where they will be operated or in generic test
`setups. The lamps also receive a test for triggerability and
`various dimensional and cosmetic checks.
`
`Figure 6. Cermax lamps ready for vacuum processing.
`
`Figure 5. Cermax reflector ceramics being checked for
`reflector contour on a coordinate measuring machine.
`
`Figure 7. Fill pressures for standard Cermax lamps.
`
`Lamp Construction
`
`5
`
`

`
`2.3 Mechanical Dimensions and Tolerances
`Figure 8 shows the reflector and lamp body dimensions for
`a typical Cermax lamp. Drawings such as Figure 8 are
`provided in individual lamp product specification sheets.
`The tolerances are important in designing heatsinks and
`mating mechanical parts, because good mechanical fit to
`the heatsinks is essential for extracting the heat from the
`lamp and maintaining low lamp operating temperatures.
`Because the tubulation is the most fragile part of the lamp,
`it should be carefully protected. A sharp blow to the
`tubulation may cause the cold weld seal to open, causing
`the gas to be vented and rendering the lamp useless. Such a
`failure is not explosive. In fact, squeezing the tubulation
`with a pair of pliers is the recommended way of relieving
`the internal pressure in worn-out lamps to render them
`totally harmless.
`The only additional mechanical information needed
`to construct heatsinks for Cermax lamps is the dimensions
`of the window cooling rings that attach to the window.
`Figure 9 lists the cooling ring dimensions for the two
`window diameters in elliptical Cermax lamps.
`The user often needs to know how accurately the light
`output direction and spot location are controlled relative
`to the lamp body.For parabolic Cermax lamps, the center
`of the output beam is within –2 degrees of the normal from
`the lamp base (i.e., the lamp surface that contains the
`tubulation). For elliptical EX300-10F Cermax lamps, the
`centers of the lamps focal spots lie within a circle 2 mm in
`diameter.
`
`3.0 Optical Characteristics
`
`3.1 Spectrum, Color, and Efficacy
`3.1.1 Spectrum
`One of the unique characteristics of Cermax lamps and of
`quartz xenon short-arc lamps in general is their
`remarkably stable spectrum. Figure 10 shows a Cermax
`
`spectrum in the UV, visible, and near IR. If the lamp has a
`coating to eliminate the UV, the 200- to 300-nm radiation
`will be missing from the lamp output. The radiation has
`several different components. The line radiation from
`800—1000 nm is the result of bound-bound transitions in
`the xenon atoms and ions. The continuum is made up
`primarily of recombination radiation from gas ions
`capturing electrons into bound states (free-bound
`transitions) and from Bremsstrahlung radiation (free-free
`transitions). As the lamp power changes over very wide
`power ranges (very much wider than those recommended
`for normal operation), the relative intensity of the line-
`versus-continuum radiation also changes. At extremely
`low powers, the line radiation dominates. As the power is
`increased, the continuum radiation becomes more
`dominant, until at extremely high powers the continuum
`radiation will almost drown out the line radiation. Such an
`extreme case is seen in xenon flashlamps at the peak of the
`lamp pulse.12 However, the power densities seen in
`normal Cermax lamps cover a minute range compared to
`these extremes. All Cermax lamps have the same
`spectrum in their specified power ranges.
`Another factor important in explaining the xenon
`spectrum is the plasma emissivity. Measurements have
`been made of the xenon plasma emissivity as a function of
`wavelength and peak current for flashlamps13 and of the
`transparency of high-pressure xenon arcs.14In general, the
`emissivity of xenon plasmas in Cermax lamps in the
`visible spectrum is less than 1. This means that the arc is
`partially transparent. The emissivity is higher in the
`infrared than in the visible and higher in the visible than in
`the UV. In the far infrared (1—5 microns,) the emissivity is
`close to 1. When the current is increased in a Cermax
`lamp, the arc expands slightly. The spectrum and the
`correlated color temperature (CCT) of the arc should
`change because of the increased power density.Infact,the
`spectrum in the visible hardly changes at all, because the
`arc expansion, the emissivity, and the blackbody radiation
`
`Figure 8. Reflector geometry and lamp body dimensions for a typical Cermax lamp (LX300F).
`
`6
`
`Optical Characteristics
`
`

`
`spectral power densities, from the emissivity and other
`plasma parameters.
`In the far infrared, Cermax lamps behave like
`blackbodies with high emissivities. Figure 11 shows a
`typical Cermax spectrum in the 1—5 micron range.
`Cermax lamps are occasionally used as infrared sources
`because their output can be temporally modulated.
`However, some of the infrared radiation results from
`incandescent radiation from the hot electrodes in the
`Cermax lamp. Also, xenon has a significant afterglow.
`So in the case of modulation in the lamp, even after the
`current pulse has gone to zero or to a very low value, the
`plasma can radiate and provide an infrared tail.
`One of the unique phenomena of xenon arc lamps,
`and of Cermax lamps in particular, is the existence of a
`cathode hot spot. Because of cathode emission processes,
`the arc is constricted at the cathode end of the arc gap and
`a hot spot appears in the gas detached from the cathode.
`The CCT of the gas is extremely high at the cathode spot,
`on the order of 20,000 K. The cathode hot spot in a
`Cermax lamp (see section 3.2) is at a higher CCT than the
`bulk of the arc. However, because of the extremely small
`size of the hot spot and because the Cermax reflector
`tends to blur the arc components, it is very difficult to
`make spectral measurements in the illuminated field of
`view of a Cermax lamp that shows different spectra.
`
`3.1.2 Color
`Figure 12 shows the 1931 CIE chromaticity diagram15that
`is usually used to describe the color characteristics of
`lamps. Other references explain the derivation and use of
`this diagram.10 The curved line near the center of Figure
`12a represents the locus of color coordinates and CC Ts
`for pure blackbodies. The numbers along the curved line
`(3500, 4800, 6500, etc.) represent the color temperatures.
`Figure 12b represents a magnified image of the curved
`line.
`Measurements of many Cermax lamps indicate that
`the average CCT is about 6150 K when the lamps are only
`a few hours old. There is a standard deviation of about 150
`K, indicating that the Cermax color temperature should
`never be specified closer than –450 K unless the
`application is extremely color-critical. (Near 6000 K, a
`variation of 150 degrees is almost an imperceptible color
`temperature difference. By contrast, a 150-degree color
`temperature difference near 3000 K would be noticeable.)
`As a Cermax lamp ages, the average CCT decreases by
`200—250 K in the first 400 hours and the same amount
`again in the next 600 hours.
`The Color Rendering Index (CRI) for Cermax as well
`as quartz xenon lamps is 95 to 99. A CRI of 100 would
`mean the lamp s ability to accurately render colors was
`equivalent to daylight.
`
`Figure 9. Cooling ring dimensions for 1-inch and 1-3/8-inch
`window Cermax lamps.
`
`changes tend to cancel each other out. The spectrum and
`CCT stay almost the same and the spectral intensity goes
`up uniformly.
`The temperature of the gas in the arc column of a
`Cermax lamp is much higher than the measured 6000 K
`CC T. However, because of the lower plasma emissivity at
`shorter wavelengths, the shape of the spectrum in the
`visible is flat and is a good approximation of sunlight. The
`xenon plasma emissivity is useful in explaining why the
`spectrum behaves in certain ways. However, because of
`the large number of variables involved, it is almost
`impossible to calculate the spectrum, or even selected
`
`Optical Characteristics
`
`7
`
`

`
`Figure 10. Cermax lamp spectrum. The typical spectral radiant flux for each lamp type is plotted versus wavelength. Output is the total light emitted from the lamp in all directions.
`
`

`
`Figure 11. Typical Cermax spectrum in the infrared.
`
`The average x,y color coordinates of Cermax lamps
`are 0.320 and 0.325. The standard deviations of these coor-
`dinates are 0.0026. By plotting the x and ycoordinates on
`Figure 12b, one can see how remarkably close a Cermax
`lamp color is to that of an ideal blackbody at 6150 K.
`
`3.1.3 Efficacy
`Efficacy is a term that describes a lamp s ability to produce
`efficient visible light. It is the total luminous flux emitted
`divided by the total lamp pow