Film-Tech Forum ARCHIVE: Color temp drift on aging Xenon bulbs?

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Posted by Larry Davis (Member # 609) on 02-16-2001, 07:42 PM:

I am wondering how much color temp drift should be expected in Xenon bulbs as they age. I mean as they approach 1000 hours. I have a color analyzer and used it (properly I hope) on a 3 chip DLP I have (Electrohome Vista Pro). At the factory preset temp of 6500K, I get color temps of approximately 5100K with 30 and 80 IRE gray fields. The bulb is supposed to be replaced every 1000 hours. I have about 1050 hours on it. I am awaiting delivery of a new bulb. I'm wondering if the bulb's age can explain the discrepancy between the color temp of the preset 6500K setting and the value I obtained?

Posted by Darryl Spicer (Member # 711) on 02-16-2001, 07:52 PM:

I'm a film man so I don't no much about those DLP things. I would say that the temperature could change because of the age of the bulb and if it is darkening any. 1000 hrs. That's not very long. Film can still look very good with bulbs that last 4 times that if they are maintained properly.

Posted by Gordon McLeod (Member # 33) on 02-16-2001, 08:00 PM:

You are not actually measureing the lamp but the output of the prism assembly so there will be colour diffences

Posted by Larry Davis (Member # 609) on 02-16-2001, 08:41 PM:

Believe me Darryl, I'm not a "good riddance to film" person. I love film and I am very opposed to DLP replacing 35mm projectors if that means Hollywood will foist garbage transfers on the public. Time will tell how that works out.

Gordon,
That's a pretty good point. I hadn't thought of that. But even so, since the light has to pass through 3 panels anyway, mustn't that have been factored into the 6500K preset? I'm getting my new bulb on Tuesday, but I'm curious if I should expect a higher color temp. The Vista Pro doesn't allow the end user to adjust the color temperature without the service codes and I don't have them.

Posted by Paul Mayer (Member # 355) on 02-16-2001, 10:45 PM:

I asked that question to some engineers at ORC a few years ago. Their answer was that color temperature does not vary over either the life of a xenon bulb nor over the operating current range. Bulbs do get darker as they age or if you reduce current, but they do not change color if they are operating normally (color temp could change if the bulb quartz itself becomes blackened for some reason).

Differences in color between various lamphouses are caused mostly by the reflectors, heat shields, or other components in the optical path. Dichroic coatings and the like certainly do age and can affect color temperature when they do.

Regarding your specific question, were you able to obtain a 6500K reading at any IRE level? If you were, but getting 5100K at 30 and 80, you have some gray-scale tracking issues to deal with. In a machine meant for professional use, there should be enough range in the RGB gain, gamma, and black level controls to achieve a consistent color temperature reading at any level from near 0 to 100 IRE. If your color temp "curve" was fairly smooth, adjustment of these controls may yield improved results. If your curve had some notches in it, that would indicate aging or dirt problems somewhere in the optical path. How smooth was your curve?

Posted by Jerry Chase (Member # 660) on 02-16-2001, 11:02 PM:

I've never noticed a change, nor would I expect any. The color of the output is primarily from the gas mix of the incandescent gas ball. If the gas remains the same, current remains the same, and sputter from the electrodes is redeposited, then any shift would be from the selective absorbsion of light by the deposited metal. AFAICT, that absorbsion seems to be pretty much neutral.

Flourescents change because the phosphorescent mix ages and different parts lose their ability to "glow" before others. Standard filament lamps boil off metal, and I think this causes a red shift, but I don't remember the mechanism involved.

Posted by Randy Stankey (Member # 64) on 02-16-2001, 11:15 PM:

Really silly question...

To what degree does the incandescent glow of the hot electrodes contribute to the C.T. of the light?

Probably little, if any but I was just wondering wheter one had to let the lamp get "hot" before making any CT measurements.

I know... I always think about silly things like that but I can't help it. I was born that way. (Like a birth defect?)

Posted by Jerry Chase (Member # 660) on 02-17-2001, 09:42 AM:

I don't think it would affect it much. Remember that a properly focused lamp spreads the light from the very small gas ball more or less evenly across the screen. Any light from the electrodes would be at off angles, and more likely to illuminate the interior of the lamphouse.

Also, if you've focused a lamphouse with a double reflector, the electrodes in a poorly focused lh look like shadows in comparison to the light from the gas ball. Still, that anode gets awfully hot.

Posted by Josh Jones (Member # 447) on 02-17-2001, 10:59 AM:

In the old days of carbon lighting, most of the light produced was from the anode crator, which glowed at some 5000 odd degrees F. Is this the same with xenon fired lamps or does the gas produce most of the light? Those electrodes have to be made of tungsten to stop from melting after all.
Josh

Posted by Gordon McLeod (Member # 33) on 02-17-2001, 12:08 PM:

The electrodes do ot contribute to the output.
The hole in the collector (if properly designed) is equal to the magnified size of the cathode and the fron t diameter is the limitiation of the anode shadow.
That is why certain reflectors work better with certain lamps

Posted by John Pytlak (Member # 331) on 02-17-2001, 12:59 PM:

Remember, that in a 3-chip DLP projector, the light goes through dichroic prisms to separate it into red, green and blue portions of the spectrum to be modulated by the DMD chips, and then recombined into "white" light. So the "color temperature" is not only a function of the lamp, but of all the optics involved.

As Paul Mayer notes, gray scale "tracking" is important for any electronic display, to avoid having a color shift as you go from highlights to shadows. The absolute color temperature is not as critical as good gray scale tracking, since the eye will quickly adapt to minor variations in overall color temperature.

FWIW, "daylight" is approximately 5500 degrees Kelvin, and is a mix of sunlight and bluer "skylight". I suspect human vision has evolved optimized for daylight. Xenon and carbon arc lamps provide light that closely approximates daylight.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
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Posted by Randy Stankey (Member # 64) on 02-17-2001, 04:27 PM:

I didn't really think the glow of the electrodes made any difference. I was just wondering, that's all.

Acually, that's one of the more "normal" things I wonder about late at night. I just figure if I didn't think about things like that I probably wouldn't be any good at this job. KnowaddImean?

Posted by Larry Davis (Member # 609) on 02-17-2001, 04:34 PM:

I want to thank everyone for their excellent replies. It occurred to me that my contrast settings may have skewed the results as they are not at the defaults. At certain levels, it gives the whites a pink look. That's even mentioned in the manual. I'll take readings next week with the new bulb.

Posted by Paul Mayer (Member # 355) on 02-20-2001, 10:51 PM:

Ah yes, absolute picture black and white levels should be set before evaluating or setting gray-scale color temps. That way you will have established the dynamic range that you will be working with. Some good PLUGE and 100 IRE test patterns are useful here.

As for the whites going pink at some drive levels, that sounds like the kind of problem many CRT-based projectors have. Data-grade CRT machine designs had to resort to playing with the gamma in the blue channel in order to get enough light output at the upper IRE levels. Which made it impossible to get true grays at all levels (try telling that to a client that's become convinced of your color blindness! ). I don't know the characteristic gamma of DLP or D-ILA imagers or their drive circuits, but I would think that they have fewer problems than CRTs in this respect. It'll be interesting to see the results you get after the new lamp is in place and you've had a chance to play with the various RGB settings.

And regarding John's point about 5500K daylight, one of these days I'm going to read up on how the SMPTE settled on having two different standards for the color of white in the picture business--i.e. film being at 5400K and video being at 6500K. It really bugs me everytime I walk out of a screening room and into an ajoining telecine suite! One of these days...

Posted by Larry Davis (Member # 609) on 02-20-2001, 11:00 PM:

Interesting points, Paul. Unfortunately when I replaced the bulb, the projector died.

I got an LED error message that warned of "DMD lamp failure". I removed and reseated the bulb several times. Same message. I replaced the original bulb. Same message. Christie Digital tech support says it sounds like I need a new lamp ballast. They told me that this is what happens when someone doesn't replace the bulbs in a timely fashion, but uses the bulbs way past what's recommended (I recently bought the projector "as is"). The new bulb uses more voltage and the "shock" to the lamp ballast causes the failure. I hope I haven't mis-stated his explanation. I'm not a happy camper, but at least it can be fixed. I would appreciate anyone's opinion, if they care to offer it.

Posted by John Pytlak (Member # 331) on 02-21-2001, 09:13 AM:

Paul Mayer asked: "And regarding John's point about 5500K daylight, one of these days I'm going to read up on how the SMPTE settled on having two different standards for the color of white in the picture business--i.e. film being at 5400K and video being at 6500K. It really bugs me everytime I walk out of a screening room and into a telecine suite! One of these days..."

AFAIK, it had to do with optimizing the efficiency of a video display --- you could get more light at the higher color temperature. Standard SMPTE 196M traces the film projection color temperature specification back to the days of high intensity carbon arc, which was very close to natural daylight. Xenon arc is very similar. There is some variation, mostly due to differences in dichroic coatings and optics, rather than the lamps themselves.

A recent technical paper on the subject was published in the SMPTE Journal: Berggren, Glenn, The Color of Light on the Screen -- New Measurements at Studios and Laboratories, 106:156, Mar. 1997.

There is ongoing discussion within the SMPTE DC28.8 Digital Projection Study Group regarding color temperature, with a push to specify a higher color temperature than 5400K for Digital Cinema.

Posted by John Pytlak (Member # 331) on 02-21-2001, 01:56 PM:

Which model Vista Pro do you have?

I recall that the Vista Pro used a CERMAX Xenon lamp module. What size lamp (watts) did yours take? What did the replacement module cost?

Surprised to hear the new xenon lamp blew the "ballast". (The term "ballast" implies a metal halide lamp). Isn't the power supply filtered DC with high voltage ignition, similar to a regular xenon lamphouse? Wasn't there a fuse or current limiter to protect the power supply from the "shock" of changing lamps?

Posted by John Pytlak (Member # 331) on 02-21-2001, 02:30 PM:

..

Posted by Larry Davis (Member # 609) on 02-21-2001, 03:18 PM:

Hi John,
It's an Electrohome Vista Pro. Just the plain old Vista Pro, not the Plus or 2000 model. It uses a 500 Watt CERMAX Xenon module. I paid $950. I don't know the answer to your other questions, but I am contacting Christie again. I have found a local service center, but I am hoping to find a few more, as I don't want to be gouged on the repair cost. Thanks for your thoughts.

Posted by Larry Davis (Member # 609) on 02-24-2001, 12:51 PM:

I received an email from a digital projection guru. He explained to me that my color analyzer was probably at fault, since it was designed to be used with phosphor CRT monitors. It's not the kind of high-tech analyzer Joe Kane would use. It was originally designed for direct view CRT's.

Posted by Steve Guttag (Member # 268) on 02-24-2001, 05:37 PM:

John,

I would think that the studio types would NOT want the 5400K increased based on my dealings with them in premier situations. In fact, they seem to prefer it on the lower side to keep flesh tones looking good. As the color temp increases to 6000K, people start to look dead.

Generally, when I have a screening room or a theatre that does more than it's share of studio screenings and where dailies and such might be run, the spec I use on the reflectors is 5400K +0/-200K but matched to each other (if 2-projector). It is within SMPTE spec and keeps the studio types happy.

I am working on a lab project now where the projector must put out either 5400K or 3200K!!! Oh what a pain! Since some of the 16mm film is destined for tungsten illumination, they want the 3200K reference as well.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Gordon McLeod (Member # 33) on 02-24-2001, 06:10 PM:

Use a wratten 85 filter they are available in professional optic glass from Tiffen and the likes

Posted by Steve Guttag (Member # 268) on 02-24-2001, 07:31 PM:

Gord:

Actually, this will be Kinoton's problem. There will actually be two filters, one for 5400K and one for 3200K. As I understand it, they are going to have the light coming out of the lamphouse higher than 5400K and filter it down to 5400K or 3200K as necessary. Since it is for a lab, they are fussy!

This puppy will do 16, Super-16 and 35mm, forward or reverse from 1 -30 fps projection, and up to 100fps shuttle (in either direction), all remoted with projection speed on a read out....Talk about your bells and whistles!

Steve

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"Old projectionists never die, they just changeover!"

Posted by John Pytlak (Member # 331) on 02-24-2001, 08:54 PM:

I've analyzed Glenn Berggren's data, which seems to show an average color temperature slightly higher than the current 5400K specification, but nowhere near the 6500K favored by the video folks. The labs and studio projectors Glenn surveyed also seem to be off the black body locus, and on the daylight locus.

My own preference would be to go with a D5500 specification, or the same color temperature as average "daylight". It would reflect the latest data such as Glenn's. But it would also be the same as the light most of us look at the outside world with--- DAYLIGHT.

Posted by John Pytlak (Member # 331) on 02-26-2001, 06:12 AM:

Larry: Typical 3-color color temperature meters may be "fooled" by the discontinuous spectral output of CRTs and dichroic prism devices such as a DLP projector. The best way to measure color temperature is to measure the full spectrum with a spectroradiometer, and calculate it based on the CIE color response functions.

As you know, the light output of a high intensity carbon arc or xenon arc film projector is very similar to natural daylight, whereas the "white" on a video projector is really a combination of red, green and blue light, with "gaps" in-between.

Posted by Larry Davis (Member # 609) on 02-26-2001, 01:07 PM:

Thanks for the explanation, John. You're one of the people that make this forum such a great resource.

Posted by Larry Davis (Member # 609) on 04-09-2001, 01:46 AM:

I obtained the service manual and service code for this projector. It turns out the default bias adjustments for the 6500K setting had been modified by someone else. That should explain the skewed color temp readings. I have set them back to the defaults.