Film-Tech Forum ARCHIVE: Changing the lamp's color temperature

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Posted by John Walsh (Member # 168) on 02-19-2002, 10:51 PM:

OK, suppose I buy a 3-color temperature meter. I analyze the light on screen and I find it's too blue or whatever.

Now what? How would you change it?

Posted by Mark Gulbrandsen (Member # 72) on 02-19-2002, 11:32 PM:

John,
I've been through this several times on location dailies. I've found that 90% of the cinematographers don't care, and probably never notice. They're usually more interested in density and light levels, plus if the focus puller did his job or not than color balance. They're always one light dailies anyway. I guess they're going to take care of color balance at the lab when the color timming is done.
I've had a few persnikity DP's that HAD to have the lamps matched and what we did was to measure them both with a color temp meter(Minolta) and then put Kodak Color Correction Gel filters over the lenses to match em up. I was running ORC lamps with out heat filters in them and needless to say no Dichroic reflectors so they were already quite close. Several DP's I worked with didn't like the colder dichroic light output of lamps with heat filters in or dichroic reflectors so I pretty much stuck with the ORC's on these jobs.
Once you find the correct balance it may be possible to aquire a glass filter, but the gels worked out fine and there was no perceptable loss of definition with them either. Have fun!
Mark @ Home

Posted by Phil Hill (Member # 371) on 02-20-2002, 04:18 AM:

The only time I am concerned about color temperature is when I do a 3D theater. It is *very* important that both "eyes" have the same color temperature as dictated by the lamphouse reflector. Every reflector (at least Strong lamphouses, which I use exclusively,except for 1570) has a 3-digit letter color code that designates it's color temperature. It is imperative that these be matched for a 3D venue. Even though the xenon lamp has a "white" light, the lamphouse reflector will "color" the light.

>>> Phil

Mr. 3D!

Posted by John Pytlak (Member # 331) on 02-20-2002, 06:13 AM:

I agree, the dichroic coating on the reflector is probably the biggest variable in matching xenon lamphouses for color. If you are trying to match two projectors, start out with reflectors from the same batch. Port glass, lenses, lamp focus, and screen condition (as screens age, they generally get yellower) can also affect color. Contrary to popular belief, lamp current has only a minor effect.

When we try to perfectly match two projectors at Kodak (for critical side-by-side comparisons of new films), we start with reflectors from the same batch, and xenon lamps of the same batch and age (hours). Even lenses of the same brand may have a slight color difference, so we try to simply switch the lenses between the two projectors to get the best color match. The final "tweek" is to use a SINGLE gelatin filter (usually a Wratten color correcting filter) in a filter holder in front of each lens. Note, using a filter to get "perfect" color match will only work on a smaller screen, as a really large lamp will fade or even warp the filter. Needless to say, the filter should be free of scratches or dirt, and monitored for any fading.

Standard SMPTE 196M specifies the color temperature for theatre projection. For theatres, the light reflected from the screen should be 5400 Kelvin, with a tolerance of -200 and +600. In other words, between 5200K and 6000K. Natural daylight has a color temperature of about 5500K.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 585-477-5325 Cell: 716-781-4036 Fax: 585-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Jerry Chase (Member # 660) on 02-20-2002, 11:12 AM:

"It is *very* important that both "eyes" have the same color temperature as dictated by the lamphouse reflector." (for 3-D dual strip projection)

I'll have to disagree on this. The different color IS apparent at first, but within a few minutes the brain compensates and the cones balance. I work with full color anaglyphs all the time, which have about as strong a variation in color as is possible. Since my eyes have become trained, normalizing occurs within five minutes or less. After a session, for the first few minutes after I remove the red/blue glasses, one eye sees objects with a pinkish tint, the other blue.

Color temperature tweaking can almost be called a subjective science, since about 9% of the population has color deficiency and a large portion of the population has other vision defects, such as night blindness, which can affect color perception at lower light levels.

Jerry (member of NSA, and certified 3-D nut since age 6.)

While color matching for 3-D is a nicety, and provides a good initial impression, many other factors are more important.

Posted by Pat Moore (Member # 380) on 02-20-2002, 01:30 PM:

Interesting topic that seems to be getting a little more attention recently. Couple observations:

Lets clarify "color temperature" and "color matching", where color matching is getting two (or more) projection sources to have the same color temperature on screen within a desired range. SMPTE 196M that John mentions specifies a color match in terms of "chromaticity", the x- & y-coordinates of the color spectrum. I don't have 196M handy to reference, but I don't remember a +/- range in Kelvin for matching in the latest document. Not all that long ago, I think it was +/- 200K, but I wouldn't bet the house on it. (John, do you remember or have a copy handy?)

There are a lot of factors involved in getting the desired color temperature on screen, from reflector (to heat filter) to lamp to lens to port glass and yes, even the screen itself. It's amazing the people that do color temperature measurements without considering ambient light from things like exit signs. A few years ago I had a guy that wanted to trade out all his reflectors -- until we determined he was doing his measurements with the auditorium cleaning lights on. Oi.

In my experience, the xenon lamp is fairly constant in color temperature, the main change (if any) due to age rather than variations between manufacturers, operating amperage, etc. The reflector does play perhaps the biggest part, since it's generally dichroic coated, but they are pretty consistent within batches. Some big differences come from port glas and the screen, something that's very oftem overlooked in items that affect color temperature.

Pat

Posted by John Pytlak (Member # 331) on 02-20-2002, 01:46 PM:

Pat: SMPTE 196M-1995 specifies 5400K -200K/+600K for theatres.

D5500 has x=0.332 and y=0.347

Posted by John Walsh (Member # 168) on 02-20-2002, 05:35 PM:

The reason for my asking is: a person who rents our theater (single projector/platter) wanted to know if the color was correct. They weren't saying it was wrong, but they wanted some proof it was right. So, my orginal thought is not how to match two projectors, but making *any* projector conform to SMPTE standards.

Not that I'm running out to get a 3-color temperature meter, anyway. I checked the price (admittedly, this was 10 years ago) for a thing made by HP, and it was about $20,000.

So, it may have to remain a mystery!

Posted by Steve Kraus (Member # 476) on 02-20-2002, 06:51 PM:

Mark, in my experience the dailies have vitually always been timed, not one-light. Hence the timing reports that the labs send over. But I agree that it is rare that anyone questions the color at our end. In one instance at the post production center a young AC checked our color temp with a fancy 3-color meter he had purchased, determined we were too blue and insisted we project with a pale yellow gel. The guy looked like a fool when it was realized that he had read the color of the incident light beam, not that reflected off the screen which was ever so slightly warmer and was quite correct.

There were two things that surprised me somewhat when I got in this business mumblemumble years ago. One was how much footage that you would think would be MOS (since the shot contained no dialog) in fact was actually shot sync sound. And secondly, that dailies were timed, not one light. When you consider that the dailies will become the workprint and get shown to the big shots and to test audiences this makes good sense.

I would also like to mention that one should not get too hung up on the concept of color temperature. Technically, color temperature is only valid for a hypothetically perfect "black body" radiator. The glowing filament of an incandescent bulb comes close to this ideal--producing a range of colors. What we call color temperature is the temperature (using the Kelvin scale which is Celsius plus 273.16) the radiator would need to be to produce this range of light intensities. (Blue hot is hotter than red hot!) A discharge lamp like a xenon bulb is nothing at all like a black body radiator. The most we can say is that the light resembles that of a black body radiator at a given temperature. Since this is an imperfect comparison do not be surprised if two light sources that read the same on a 3-color meter do not in fact look exactly the same. You'd have to examine the entire spectrum to see how things really compare.

Posted by Gordon McLeod (Member # 33) on 02-20-2002, 10:13 PM:

Most timers I know insist on the hot Rhodium reflectors and matched heat filter to get the best colour match and want about 5400

Posted by Paul Mayer (Member # 355) on 02-20-2002, 10:38 PM:

I ran into this situation on my one-and-only dailies job here in Lost Wages (Casino)--the rental machines didn't match and both the DP (Robert Richardson) and director (Martin Scorsese) wanted them to. So Universal Pictures paid to have Technicolor fly up and read my screen with their fancy Minolta spot luminance/color temp meter (model CS-110 I think). Turned out one machine was close enough at 5500K but the other was at 6200K using the little ORC RPS-2500 lamps at 1600W on a 5x12' screen. BTW this was Super 35 with custom-cut plates to give us the 2.39 extraction without anamorphics. We used Schneider lenses.

I worked out the MIRED shift values using the tables listed in the ASC Cinematographers Handbook and ended up using a 1/4 CTO gel on the bluer machine. I just taped it to the back of the shutter housing instead of projecting through it. After that I eyeballed the difference between the machines during setup each day and ended up changing the gel about once a week or so--we were cranking about six hours of dailies a day on that production, plus other special-request stuff on top of that. Lots-o-hours for four months of principal photography!

Last time I checked, that Minolta CS-110 listed for around $5800 US. As is, it reads out in x,y chromaticity coordinates and foot Lamberts, and comes with a chart for converting x,y to correlated color temp. For another $1700, Minolta supplies a little processor that connects to the meter and provides the correlated color temp directly plus printer capability. Kinda pricey!

Paul
SMPTE Hollywood Section
Unemployed mercenary film/video projectionist/engineer
"Otaku wa tsurai yo" <-- Yeah I know, bad pun. Sue me.
It's tough being a fan!
-28

Posted by Steve Kraus (Member # 476) on 02-21-2002, 09:19 PM:

I'm sure there's a way to take three readings through quality color filters (Kodak Wratten 25, 58 and 47B) using a conventional meter and turning that data into a color temperature. The problem would be working out the calibration/conversion.

Likewise a digital camera could do it, with auto white balance disabled and set to expose the white screen as a middle gray. Then just read the RGB value of a pixel in Photoshop. Once again, calibrating is the problem.

BTW, I hope you enjoyed your Super 35 screenings. Native S35 generally looks great. Until they blow it up.

Posted by John Pytlak (Member # 331) on 02-24-2002, 08:14 PM:

Another possible way would be to use daylight balance KODAK Ektachrome slide film to compare the color of the illuminated screen to that of natural daylight (D5500). Photograph an "18% gray card" illuminated by D5500 Kelvin natural daylight (sunlight + skylight) during midday. On the same roll, photograph the "open gate" illuminated screen. After processing, compare the color of the gray card to the color of the screen.

Note, an 18% gray card illuminated by sunlight on a clear blue sky midday will have a luminance of about 1400 footlamberts. Your screen should have 16 footlamberts. So you will need to adjust camera exposure by about 6-1/2 camera stops. But the color temperature should be nearly identical.