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Xenon short-arc lamp: Difference between revisions
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[[File:Xenon diagram.jpg|700px]] | |||
A '''xenon short-arc lamp''' is a [[light sources|light source]] used for projection. Light is produced by electricity arcing between tungsten electrodes in a quartz envelope that contains highly compressed xenon gas. Most of the light is generated by a plasma ball that forms at the tip of the cathode. For maximum light output, the lamp is aligned so that this plasma ball is positioned as close as possible to the focal point of the [[reflector]]. This is one of the steps taken to calibrate overall [[optical alignment]]. | A '''xenon short-arc lamp''' is a [[light sources|light source]] used for projection. Light is produced by electricity arcing between tungsten electrodes in a quartz envelope that contains highly compressed xenon gas. Most of the light is generated by a plasma ball that forms at the tip of the cathode. For maximum light output, the lamp is aligned so that this plasma ball is positioned as close as possible to the focal point of the [[reflector]]. This is one of the steps taken to calibrate overall [[optical alignment]]. | ||
In the past, xenon lamphouses employed both vertical and horizontal lamp configurations. Vertical lamps had the benefit of a longer lifespan but were comparatively inefficient, and required an auxiliary mirror in addition to the main reflector to maximize light output. In all modern lamphouses, the lamp is installed horizontally, laying flat with the anode end pointing to the screen. Horizontal lamps have better light output and are easier to align, but they have a greater tendency to flicker because the flow of the heated xenon gas within the envelope draws the arc upward. To mitigate this, a magnet is installed below the lamp to stabilize the arc.<sup>1</sup> | In the past, xenon [[lamphouses]] employed both vertical and horizontal lamp configurations. Vertical lamps had the benefit of a longer lifespan but were comparatively inefficient, and required an auxiliary mirror in addition to the main reflector to maximize light output. In all modern lamphouses, the lamp is installed horizontally, laying flat with the anode end pointing to the screen. Horizontal lamps have better light output and are easier to align, but they have a greater tendency to flicker because the flow of the heated xenon gas within the envelope draws the arc upward. To mitigate this, a magnet is installed below the lamp to stabilize the arc.<sup>1</sup> | ||
The quality of light emitted from a xenon lamp closely resembles that of natural sunlight | The quality of light emitted from a xenon lamp closely resembles that of natural sunlight. | ||
==History== | ==History== | ||
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*Never run the lamp without ventilation, and run the blower for at least a 10min cooldown period after the lamp has been powered down. | *Never run the lamp without ventilation, and run the blower for at least a 10min cooldown period after the lamp has been powered down. | ||
*Always strike a lamp with the hand dowser closed. | *Always strike a lamp with the hand dowser closed. | ||
*Xenon lamps are highly pressurized and can explode. | *Xenon lamps are highly pressurized and can explode. Full safety gear and proper training is required to replace or handle xenon lamps in any way. | ||
*Lamp alignment takes place within the closed lamphouse and is safe to do without safety gear. Lamp alignment is one of the top neglected things in a projection booth and must be done whenever a lamp is replaced. | |||
===Lamp sizes=== | ===Lamp sizes=== | ||
Xenon lamps come in a variety of sizes, each designed to operate within a specific wattage range. The size of the lamp to be used in a given installation will depend on the illumination requirements and the electrical rating of the lamphouse. Adapters can be fitted to the anode and cathode ends of a lamp to allow it to fit different lamphouse models. As a lamp ages it requires more power to reach the same light output, so the power draw must be increased over the life of the lamp. When selecting the right lamp for a given installation, it must be able to maintain its brightness target for the duration of its lifespan. | Xenon lamps come in a variety of sizes, each designed to operate within a specific wattage range. The size of the lamp to be used in a given installation will depend on the illumination requirements and the electrical rating of the [[lamphouses|lamphouse]]. Adapters can be fitted to the anode and cathode ends of a lamp to allow it to fit different [[lamphouses|lamphouse]] models. As a lamp ages it requires more power to reach the same light output, so the power draw must be increased over the life of the lamp. When selecting the right lamp for a given installation, it must be able to maintain its brightness target for the duration of its lifespan. | ||
When using larger lamps, precautions must be taken to prevent [[film damage|heat damage]] to the film. The [[reflector]] should be a dichroic mirror, and a [[heat shield]] should be installed to filter out UV light. When running 35mm, the projector should be equipped with a water-cooled gate for lamps exceeding 3kW. Regulating heat is even more important when running 16mm, because the light is focused on a smaller surface area. Film can also be damaged by a lamp that is misaligned, creating a hotspot that blisters one portion of the frame. | When using larger lamps, precautions must be taken to prevent [[film damage|heat damage]] to the film. The [[reflector]] should be a dichroic mirror, and a [[heat shield]] should be installed to filter out UV light. When running 35mm, the projector should be equipped with a water-cooled gate for lamps exceeding 3kW. Regulating heat is even more important when running 16mm, because the light is focused on a smaller surface area. Film can also be damaged by a lamp that is misaligned, creating a hotspot that blisters (see "embossing" below) one portion of the frame. | ||
<gallery widths= | <gallery widths=200px heights=200px mode=packed>> | ||
File:Xenon lamp in shield.jpg|A 1.2kW Ushio xenon lamp in its shield. | File:Xenon lamp in shield.jpg|A 1.2kW Ushio xenon lamp in its shield. | ||
File:Xenon lamp close up.JPG|This lamp was removed at the end of its warranty. There is visible wear on the cathode. | File:Xenon lamp close up.JPG|This lamp was removed at the end of its warranty. There is visible wear on the cathode. | ||
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==Lamp Alignment== | ==Lamp Alignment== | ||
Whenever a new xenon lamp is installed it must be properly aligned on the X, Y and Z axis (horizontal, vertical, axial) to have even illumination and achieve the proper luminance on screen. Poor lamp alignment can result in hot spots of light on screen, damage the reflector, lens, and even the print. A lamp alignment should be performed when a new lamp is installed and periodically thereafter because the position may drift over time. | |||
Lamp alignment should be done whenever a new lamp is installed. Lamp alignment should also be checked if you see hot spots or shadows on screen or if you discover embossing or burns on prints that you have run. If you are switching between different [[film gauges]] on the same projector, the position of the reflector should be adjusted to achieve the correct [[working distance]] between the focal point of the reflector and the aperture. Most dual format lamphouses have a toggle switch to make this adjustment. | |||
Most lamphouses have a mechanical adjustment to shift the position of the cathode end, while the anode end is supported by a post or yoke with an adjustable height. The height of the anode can usually be set during the initial installation, with only the cathode end being adjusted at every lamp change. If you center the cathode end, it will be apparent if the anode position is incorrect because the lamp will not be straight. | Most lamphouses have a mechanical adjustment to shift the position of the cathode end, while the anode end is supported by a post or yoke with an adjustable height. The height of the anode can usually be set during the initial installation, with only the cathode end being adjusted at every lamp change. If you center the cathode end, it will be apparent if the anode position is incorrect because the lamp will not be straight. | ||
[[File:anode-shadow.jpg|right|thumb|450px|Shadow of the anode assembly seen on screen during lamp alignment, once the lens has been removed. ''Photo credit: Katherine Greenleaf'']] | |||
During alignment you will see the following on screen: The shadow of the anode assembly at the end of the lamp is the dark circle, immediately ringed by visible plasma (the electron arc). Surrounding it is the circle of light collected by the [[reflector]], and reflected upon the screen. This area comprises most of what we see, and should appear bright white and essentially even in luminance upon completion of the alignment. | |||
===Lamp Alignment Steps=== | |||
1. Turn on exhaust, make sure lamphouse dowser is closed. | |||
Always have lamphouse dowser closed before striking a lamp. Striking a lamp with the motor off and the lamphouse dowser open can melt the [[shutters|shutter]] blades and changeover dowser, and can crack the lens. | |||
2. Remove lens ([[Kinoton]] users should remove the skate so that it isn't damaged by the heat). Turn on lamp. Start motor and open dowser. | |||
After making | 3. Make the axial adjustment (the “focus” on the Z axis) til you see a circle with a little bit of plasma around the edges of the anode shadow (people call this “the eye”). | ||
4. Make your horizontal and vertical adjustments until you get as close to a symmetrical circle as possible. | |||
5. Re-adjust the axial alignment to bring the plasma back into focus, until you have a tight symmetrical circle free of light (plasma) fluttering around the anode shadow. | |||
6. Put in a lens ([[Aspect ratios|CinemaScope]] is usually best as you want the largest aperture). After you put the lens back in look for even illumination across the screen. With the lens in, the shadows will be gone and there will be a hotspot in the center of the screen. Focus the lamp (making fine X and Y axis adjustments as necessary) until no shadows are visible at the corners and the illumination is even across the screen. You want it bright, flat and centered with no hot spots! Changeover from projector 1 to projector 2. You want them to look as similar as possible. | |||
7. You may want to fine-tune the lamp focus with the lens in place until you have even illumination, then take the lens out again to check the symmetry of the anode shadow. | |||
'''WARNING:''' The heat from the lamp can crack the glass of the lens when there’s no film in the gate, so the dowser can only be opened for brief periods during the alignment. For a 2kW lamp, the dowser should be opened for no more than 30 seconds at a time, with breaks between for the lens to cool down. For large lamps, use quick bursts of light and a long cooldown period. Consult your service technician before making these adjustments! | '''WARNING:''' The heat from the lamp can crack the glass of the lens when there’s no film in the gate, so the dowser can only be opened for brief periods during the alignment. For a 2kW lamp, the dowser should be opened for no more than 30 seconds at a time, with breaks between for the lens to cool down. For large lamps, use quick bursts of light and a long cooldown period. Consult your service technician before making these adjustments! | ||
8. Finally, take a light reading with your luminance meter. | |||
===Measuring Luminance=== | |||
The [[Society of Motion Picture and Television Engineers|SMPTE]] specification is 16fL ([https://en.wikipedia.org/wiki/Foot-lambert foot lamberts]) with uniform illumination across the projected area without film in the projector (open gate). Because the 1.85:1 "Flat" [[aspect ratio]] uses the smallest aperture it is the least light efficient, while 1.33:1 silent or 2.39:1 CinemaScope apertures are the most light efficient. You can split the difference by using 15fL for [[Aspect ratios|1.85:1]] and 17fL for [[Aspect ratios|Cinemascope]] to achieve a balance within SMPTE specifications. | |||
In most circumstances a fixed-height screen will evenly balance the light output between the smallest aperture (1.85:1) and the largest aperture (typically 2.39:1) because the greater light output from the CinemaScope frame is used to illuminate a larger screen area. However, this is complicated by movable vertical [[masking]]. When the height of the screen can be increased for narrower formats, the light requirements can vary dramatically. Stopdown rings can also be used to reduce the light output of formats with a larger aperture (1.33 or 1.37) to achieve the necessary balance of light output between aspect ratios. | |||
If you’re checking luminance through the port glass with your meter, subtract 1~2fL from the reading. It’s always best to take your final readings in the auditorium itself. | |||
===Potential Issues=== | |||
* If you can’t evenly distribute the plasma with lamp alignment (you’ll notice it will appear asymmetrical even as you adjust the focus), the arc stabilization magnet may need to be adjusted by a trained technician. Excessive flicker may result if the plasma isn’t evenly distributed. | |||
* If dark spots appear in the area within the circle of light with the lens removed, the reflector should be inspected for pitting, explosion damage, or any other damage to the reflective coating. | |||
* Damaged lamp | |||
* Cannot focus lamp because of physical lamphouse alignment in relation to projector (the "working distance" between the focal point of the reflector and the aperture is incorrect) | |||
===Damage=== | |||
Improper lamp alignment or lamps that are run too hot can result in permanent damage to the film print. | |||
[[Film damage|Embossing]] is a permanent deformation that occurs when prints are projected with high intensity lamps and without proper heat absorbers. The excessive heat expands the picture area, and the frame stands out in relief. | |||
Always have lamphouse dowser closed before striking a lamp -- striking a lamp when the motor is off and the dowser open can result in the following: melted shutter blades, changeover plates, cracked heat filter glass (if installed), melted film, or cracked lenses. Some projectors have a fire dowser that only opens when the motor starts, but some do not. | |||
==Aging== | ==Aging== | ||
[[File:Xenon lamp-blackening envelope.jpg|right|thumb|400px|An aging lamp with burnt off tungsten blackening the envelope.]] | |||
Xenon lamps have a fixed arc gap, but over time the tungsten of the cathode gets burnt off by the plasma, increasing the distance between the electrodes and reducing the light emitted at a given power draw. Burnt off tungsten can also deposit on the envelope, creating black spots and accelerating the effects of aging. | Xenon lamps have a fixed arc gap, but over time the tungsten of the cathode gets burnt off by the plasma, increasing the distance between the electrodes and reducing the light emitted at a given power draw. Burnt off tungsten can also deposit on the envelope, creating black spots and accelerating the effects of aging. | ||
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==Lamp Safety== | ==Lamp Safety== | ||
[[File:Safety gear.jpg|right|thumb|200px|Example of safety gear worn during a lamp change]] | |||
An exploding lamp will expel shrapnel in all directions, so a face shield and full-body protective clothing must be worn when handling xenon lamps. Some lamp manufacturers sell safety gear, but protective clothing designed for welding is more readily available and usually cheaper. | An exploding lamp will expel shrapnel in all directions, so a face shield and full-body protective clothing must be worn when handling xenon lamps. Some lamp manufacturers sell safety gear, but protective clothing designed for welding is more readily available and usually cheaper. | ||
Each theater should stock safety gear in multiple sizes. This is especially important for gloves, because ill-fitting gloves will limit dexterity and make handling the lamps more dangerous. | Each theater should stock safety gear in multiple sizes. This is especially important for gloves, because ill-fitting gloves will limit dexterity and make handling the lamps more dangerous. | ||
Lamp changes should only be done by a service technician or someone who has been properly trained by a service technician. | |||
Safety gear should include: | Safety gear should include: | ||
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Never open the lamphouse or handle the lamp when it’s hot. A hot lamp is under much greater pressure than a cool lamp. Let the lamp cool for at least 10min before handling it. | Never open the lamphouse or handle the lamp when it’s hot. A hot lamp is under much greater pressure than a cool lamp. Let the lamp cool for at least 10min before handling it. | ||
Before performing a lamp change, prepare your workspace. Place the empty shield or other protective enclosure on a clean, clutter free surface with adequate room to work. It should be open and ready to receive the used lamp. Make sure you have a clear path to the lamphouse with no obstacles or tripping hazards. | |||
===Safety Gear Manufacturers=== | ===Safety Gear Manufacturers=== | ||
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*Ushio lamps ship in a ballistic shield, with the anode and cathode ends supported by cardboard blocks. To dispose of used lamps, Ushio recommends returning the lamp to its original shield and box and dropping it from a height if three feet.<sup>[4]</sup> | *Ushio lamps ship in a ballistic shield, with the anode and cathode ends supported by cardboard blocks. To dispose of used lamps, Ushio recommends returning the lamp to its original shield and box and dropping it from a height if three feet.<sup>[4]</sup> | ||
In practice, their packaging is secure enough that it can be very difficult to destroy the lamp in this fashion. | *In practice, their packaging is secure enough that it can be very difficult to destroy the lamp in this fashion. | ||
==See Also== | |||
*[[Lamphouses]] | |||
*[[Carbon arc lamp]] | |||
*[[Light sources]] | |||
*[[Reflector]] | |||
==References== | ==References== | ||