Film-Tech Forum ARCHIVE: effect of lamp aging on D-cinema picture quality

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Posted by Scott Norwood (Member # 30) on 11-06-2011, 01:08 PM:

Subject pretty much says it. The non-DCI 3-chip DLP video projectors that I am familiar with all use UHP or metal halide light sources. With most of these projectors, the picture looks significantly better in the first 100 (or so) hours of the bulb's life than it does after that time. Which is to say that the picture appears to be duller and flatter as the bulb ages. I don't know why this happens, but replacing the bulb on a standard video projector will almost invariably result in a signficant improvement in picture quality.

Does the same apply to the cinema units that use xenon light sources, or does the picture quality remain fairly constant throughout the lifetime of the bulb, as it does with a film projector? Does the lamp current need to be raised (as on a film lamphouse) over time to compensate for reduced light output? Do bulbs need to be rotated (is this even possible with DCI units)?

Is it correct to assume that the failure mode of the bulbs in a D-cinema projector is similar to that of a film projector (increased flicker and signficantly reduced brightness)?

Posted by Monte L Fullmer (Member # 2797) on 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.

Posted by Mark Gulbrandsen (Member # 72) on 11-06-2011, 01:33 PM:

I think if theaters would at least get them selves the new Harkness Digital Brightness meter you will see more consistant results on screen as far as brightness goes and when lamps are changed. It's inexpensive and although all projectors auto brightness function does work to some extent they still cannot signal a lamp that has "run out of brightness range" so to say. Some lamps are more prone to the brightness level falling faster than others more based on size than manufacturer...

Posted by Carsten Kurz (Member # 5396) on 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

Posted by Steve Guttag (Member # 268) on 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

Posted by Monte L Fullmer (Member # 2797) on 11-06-2011, 05:00 PM:

Nice thing on the Barco lamp modules and you're doing swapout between formats, it that the usage hours remains with the module.

Posted by Carsten Kurz (Member # 5396) on 11-07-2011, 06:28 AM:

Can anyone comment on the cost of an additional Barco lamphouse? Last time I checked it actually didn't look as if it could save too much money over the 50% for 2D/100% for 3D approach. It's extremely expensive. And on systems with a lot of mixed 2D/3D programming, I doubt that it will be changed too often. And if it is not changed EVERYTIME to different lamp types, your brightness setting macros will do the wrong thing.
Seems similiar to the Sony 3D lens changing issue. Nice idea, but will it happen?

- Carsten

Posted by Bruce McGee (Member # 131) on 11-07-2011, 11:02 AM:

2 of my machines make the lamps go almost black. The color on the screen has a reddish look on things that should be white. Things are OK again, after a lamp change.

Posted by Simon Wyss (Member # 6218) on 11-07-2011, 02:11 PM:

Carbon arc light never dulls (as long as you keep replacing rods).

Posted by Mike Blakesley (Member # 26) on 11-07-2011, 02:52 PM:

Life is full of tradeoffs.

Posted by Monte L Fullmer (Member # 2797) on 11-07-2011, 02:53 PM:

..just keep scraping the copper pits off the reflector, polish the reflector with "Bon Ami" paste, and have sufficient exhaust.

Posted by Mark Gulbrandsen (Member # 72) on 11-07-2011, 06:56 PM:

Monte... I have sufficient exhaust... especially after some good Corned Beef and Cabage!

Posted by Monte L Fullmer (Member # 2797) on 11-07-2011, 07:20 PM:

That's good stuff, you "methane manufacturer"..lol

Posted by Marco Giustini (Member # 4544) on 11-10-2011, 04:30 PM:

I think Scott is referring to the colour temperature of the UHP lamps, that changes massively while it ages.
On a digital "home" projector you will have to compensate the colorimetry every 100-150 hours to keep it good. I understand that those lamps tend to reduce the blue component with time, so you have to reduce mainly the red component on the projector as the lamp ages.
The dull effect is caused by wrong colorimetry but also by low brightness. Many "home" video projectors are rated a ridiculous amount of lumens, but if you calibrate the device the actual light on the screen is usually much much lower. If you have a 'just good' light level when the lamp is new, the light level will be absolutely inadequate when the lamp is towards the end of its life.

Talking about Xenon lamps, the light temperature should be caused by the Xenon gas and should be fairly constant so no, if the light level is adequate, the picture quality should be roughly the same throughout the life of the xenon lamp.