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Topic: effect of lamp aging on D-cinema picture quality
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Monte L Fullmer
Film God
Posts: 8367
From: Nampa, Idaho, USA
Registered: Nov 2004
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posted 11-06-2011 01:24 PM
Basically, all mercury and xenon bulb lose about 20% of their light quality after the first 300 hrs, or so of bulb life and larger ones almost 50% quality at half warranty life.
Why large digital bulbs have to be pulled, not for warranty reasons, but the light output really drops off by then and display, esp with 3D, can be affected. The envelope can look clear and the contacts inside not looking to worn, but the light quality has gone downhill some. .. and no bulb rotation is ever done.
BARCO units has a switchable user setting in their system that will automatically ramp up current to meet a foot lambert setting, which this setting is called, "CLO" - constant light output mode. User can do a standard mode to maintain a constant current draw, with manual setting from "0" to "255" and let the bulb age normally, or go into CLO to allow automatic current adjustment to meet set foot lamberts for screen display.
On 2D releases in CLO mode, the bulb wattage can drop almost down to 50% of operation, whereas 3D releases, the bulb wattage ramps up to almost 100%, and sometimes slightly overwattage, of operation.
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Carsten Kurz
Film God
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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posted 11-06-2011 02:00 PM
D-Cinema lamps can be 'Standard Cinema lamps' or 'short gap digital lamps'. Most projectors eat both types of lamps. The optimized short gap lamps deliver more brightness in absolute and relative terms (more energy efficient), yet have shorter lifespan and are more prone to failures because of the increased operating stress. Together with the implied warranty intervals and high cost of digital lamphouse repairs, 'digital lamps' will usually need to be exchanged closer to their warranty runtimes than standard cinema lamps.
The light drop on Xenon lamps is there, but less decrease than in UHP beamer lamps. Yes, lamps in D-Cinema lamps can be turned over - some projectors like e.g Christie even have a warning/reminder function for bulb turn. Though, most people seem to think that it has little effect. When the sharper lamp exchange intervals for digital projectors mentioned above come into play, this neglection might make some sense compared to previous times when 35mm bulbs very often have been run until near extinction.
Regarding the aspects of aging lamps - I noticed other aspects of aging UHP lamps as well, or at least I think I noticed them. I did never do a full spectrum measurement on an old UHP lamp to compare it with a new one, so I don't know wether it's 'only' decreased brightness but also colour changes. Under real life conditions, ambient light, etc., reduced brightness will not just show a dimmer picture, but also different contrast/gamma, etc., and when aging lowers screen brightness into mesopic or scotopic vision ranges, naturally also different color perception will occur. So if this 'light quality change' is measurable in other aspects than brightness alone, I have no idea. The spectrum will definitly change over time, but to which extent, I don't know.
- Carsten
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 11-06-2011 03:18 PM
Color wise...xenon will hold its color over its life, pretty well...envelope darkening, aside.
In terms of lamp decay...it is a factor of how hard it is being run. If running within the lamp's "nominal" range...a digital lamp will only decrease about 20% over 300 hours and then start a very slow decay over the next 700+ hours. Conversely, "film" lamps will loose that 20% in the first 100 hours with the "high brightness lamps" typically loosing it faster.
All of the DLP projectors have a CLO feature, of sorts. Barco calls theirs CLO, Christie calls it Light Lock and NEC calls it Lamp feedback. Barco's works REALLY well and as others have noted, can be calibrated in fL for EACH FORMAT so it will read correctly for 2D/3D Scope and Flat (and any combination). Christie has been known to take inopportune times to readjust itself and has no problem with going over 100% rated current (Barco will not do that). NEC's is a bit primitive and I have less experience with using it to control brightness. However, it seems to be similar to Christie in the sense that one basically tries to match the number (representing nothing) that you determined was the correct light level. The number may be some thing like "1546"...if you say that is the correct number from the internal light sensor then that is the number it will raise/lower the current to achieve it. Barco is doing the same thing but turning the "number" into fL and even displaying that the Communicator if you want to see what your projector is putting out at the moment.
One thing to note...I encourage my customers to refocus the lamp every month or so...that will help one achieve a higher brightness throughput without having to turn the current up as much. Barco, on their auto-lamp-alignment feature even has a "focus only" button, if you get their motorized lamp mechanism.
As for 2D/3D...if you are using the same lamp for both...odds are overwhelming that you are not achieving the best light on both. As Monte noted...you are often at 50% (about as low as most systems will let you go) for 2D and about 100% for 3D. Not only are not hitting the necessary light for good performances, you also likely are shortening your lamp life by having it operate at opposite extremes. With our Barco customers, we STRONGLY encourage them to purchase another lamphouse for the projectors so they can keep 2D and 3D lamps/lamphouses. The lamphouse contains the lamp info so all the end user has to do is slide in the correct lamphouse.
-Steve
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