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Do older flash-tubes become warmer?
Old 12-20-2006, 08:44 PM   #1 (permalink)
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i received an email from a reader of my blog wherein he was telling me he wasn't getting the warm skin-tones he sees in many of the images i post. he had recently purchased a mola beauty dish and he was using the same little trick i use on mine, that is, affixing a small piece of bastard amber onto the dish's opalescent glass baffle to warm things up a bit. i spelled out for him exactly what i do when shooting. he also asked if i adjust the skin tones much in post and i told him i do not, other than a bit of saturation or even de-saturation. i also mentioned that luma adjustments in the mid-range will effect color. I should mention that I do not warm the images by white balance adjustments. i simply use the strobe setting.

but then i started thinking and i mentioned to him some factors that i'm now theorizing might be responsible for the warmer images coming out of my camera. Here's what I wrote to him regarding other factors:

"The flash tubes on my lights are kind of old(er) so they might be a bit warmer. When they're replaced, i might find my images are less warm. I'm also often shooting with my novatron monolights at lower power output settings so this could mean the novatrons are drifting, color temp wise, to the warm side. i don't like to think either of these are what's making my images seem warmer but it certainly might be the case."

in the recent and infamous " alien bee ring-light" thread, there was some discussion about cheaper lights holding color temp. my novatron monolights aren't cheap but they aren't expensive. they're in the mid-range. But now I'm wondering if its the monolight itself that might cause a color temp drift or is that in the domain of the quality of the flash tube? Like batteries and other stuff, there are after-market manufacturers of flash-tubes out there. I wonder if after-market flash-tubes might be inferior in terms of color temp? Let's say, as an example, you have broncolor monolights and ther are some Hong Kong manufacturers who also produce flash-tubes for them. Will the high-priced broncolor monolight then perform as it's supposed to? And what if some lighting manufacturers are out-sourcing their flash-tube needs? Are different vendors supplying flash-tubes of different quality?

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Re: Do older flash-tubes become warmer?
Old 12-20-2006, 08:55 PM   #2 (permalink)
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If I understand how phototubes work, they are filled with inert gas that should always keep the same temp when fired. So, unless the tube starts to leak (letting gas in or out) they should stay the same temperature for their entire life. If they do start to leak, I would guess they would stop working soon afterwards.

A suggestion for your blog reader, do a color balance without the bastard amber first and then add the gel, that would give maximum impact with quite a bit of control.
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Re: Do older flash-tubes become warmer?
Old 12-20-2006, 08:59 PM   #3 (permalink)
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Quote:
Originally Posted by Mark_Oehler View Post
A suggestion for your blog reader, do a color balance without the bastard amber first and then add the gel, that would give maximum impact with quite a bit of control.
thanks. that assumes he's doing custom white balancing, but i'll ask.
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Re: Do older flash-tubes become warmer?
Old 12-20-2006, 10:39 PM   #4 (permalink)
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FWIW, I've done some lab work with xenon flash tubes, and can say that the color temperature DID change with time - even without "leakage." Some of the deterioration was due to build-up of a residue on the exterior (aka "crap") which could be removed by washing with acetone. [Note that this work was done in a lab with a fairly high level of organics in the atmosphere, and pretty poor ventilation...but it could happen in a studio, I suppose.] However, some of the deterioration was not reversible - it may have been due to impurities outgassing from the metal electrodes, or small leakage through the seals...never really tried to figure it out. Leaks of air into the tube was obvious in the appearance of significant amounts of light from nitrogen, at least, in the case of hydrogen discharge tubes. Can't claim to have seen that with xenon flash lamps.
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Re: Do older flash-tubes become warmer?
Old 12-20-2006, 11:54 PM   #5 (permalink)
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I would think that the temperature of the bulb on the outside of the might rareify the gases or other airbourne debris that might deposit on the tube causing a discoloring of the glass as finger oils will do?
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Re: Do older flash-tubes become warmer?
Old 12-21-2006, 12:51 AM   #6 (permalink)
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Xenon flash tubes do get a little warmer as they age, but nothing drastic. Still, if one tube died on my lights, I'd replace all of them to ensure a color match. But much of my work depends on color accuracy. For the other 98% of the people out there, I wouldn't sweat it.

Regarding color drift: color temperature rises as power output decreases, so the color of your light gets colder, not warmer, as you reduce power. This is due to the nature of the current pulse. At the beginning of the discharge you get a tall spike well into the blue portion of the color spectrum. As the discharge abates, it gradually falls into the orange and red parts of the spectrum. At full power you get optimized spectrum distribution. That is, you get blue and red illumination in an more or less equal amounts and the light matches daylight.

When you lower your output power, you essentially cut off the discharge. You end up with a spike well into the blue spectrum, but no gradual falloff into the red spectrum. If the manufacturers don't correct for this, you end up with light that is several hundred degrees cooler. You also get a color-balance nightmare.

Correcting this phenomenon can get expensive, from a manufacturing standpoint. The basic method of correction is to lower the discharge voltage. With lower voltage, the spike doesn't travel as far into the blue portion of the spectrum. It falls into the red portion sooner and gives you better color balance. How effectively, consistently, and accurately this is done goes a long way to explain why some strobes cost $300 each and why others cost 3, 5, or 10 times as much. Another reason is discharge consistency. Many of the cheaper strobes are horrible below 1/4 power and can vary by as much as 3/10th's of a stop from shot to shot.

Regarding flash-tubes: the amount of gas in the tube and how much pressure it is under go a long way in establishing the characteristics of the light you'll get out of it, what kind of flash duration you'll get out of it, and how consistently you can get it. High-pressure tubes can give you much shorter, more consistent output. But higher pressure means thicker and/or stronger glass. Combine high pressure with more stringent quality control and you get more expensive flash tubes. Again, there's a significant difference between a $22 tube and a $360 tube. If you buy a tube that comes in a manufacturer's box, you can bet it meets the manufacturer's quality control specs and will perform as the manufacturer intended regardless of who's actually making the tube. I'd bet you that someone using Broncolor equipment isn't going to put a cheap flash tube of unknown origin in their heads. That'd be kind of like putting Ford Escort tires on a Ferrari and expecting the same performance out of the car.

Regarding white balance: if you're using auto white balance in the studio then you are most assuredly getting a white balance based on the color temperature of the modelling lights and not the flash tubes. The light output of the tube is way too short for any of today's digital cameras to balance anything accurately. You'll get color balance variability from shot to shot and it will drive you crazy. Measuring short flash durations accurately is the primary reason that color-meters cost so much. To eliminate frustration, set your white-balance manually.

Now that you've read far more than you probably ever wanted to know about flash-tube physics, I'll add that there's a very good book out there that explains all of this stuff in even more excruciating detail: Professional lighting technique by Jost J. Marchesi. ISBN# 3-7231-0059-7. Note: The book and author are affiliated with Broncolor - I think Broncolor is responsible for publishing the book. It's not a book that markets Broncolor products, but after reading it you'll understand why Broncolor equipment costs as much as it does.

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Re: Do older flash-tubes become warmer?
Old 12-21-2006, 01:14 AM   #7 (permalink)
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Quote:
Regarding white balance: if you're using auto white balance in the studio then you are most assuredly getting a white balance based on the color temperature of the modelling lights and not the flash tubes. The light output of the tube is way too short for any of today's digital cameras to balance anything accurately. You'll get color balance variability from shot to shot and it will drive you crazy. Measuring short flash durations accurately is the primary reason that color-meters cost so much. To eliminate frustration, set your white-balance manually.
I'm not sure why anyone would use Auto White Balance when shooting with studio strobes. In fact, I 'm not quite sure why one would ever use Auto White Balance since it usually means the shots vary continously in the color you see in the end results and they all have to be tweaked.

I usually use either CWB or just dial in a Kelvin setting for the warmth I want. I shoot RAW, by the way. I generally use the K setting when I know I'm going to have to produce instant proofs for someone, so the shots will be a littler warmer than what CWB will give. But if I have the time, I then like to custom tweak WB in the RAW converter. Now with the new features in CS3's RAW converter, there's even more power in this conversion process.

I have actually tested Custom White Balance scenarios with my strobes (Novatron power pack and 4 heads). I tried doing the custom white balance reference shot with all the lights in the room out and no modeling light, and then with modeling lights on, and then finally with modeling lights on and ambient lights from windows and overhead tungsten. In all three cases the reference shot was identical and the resulting test shots of a subject using that CWB all showed identical color results. Then tried a similar sequence at 1/2 power. Seemed to get no difference that I could see between the two power settings in WB.

Cheers,
rfs
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Re: Do older flash-tubes become warmer?
Old 12-21-2006, 01:30 AM   #8 (permalink)
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Jimmy

I don't know what camera he uses vs yours. Perhaps it's the camera.

It's been my experience that Canon, Nikon, Fuji etc will shoot different regardless of custom white balance or same color temperture.

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Re: Do older flash-tubes become warmer?
Old 12-21-2006, 02:40 AM   #9 (permalink)
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Quote:
Originally Posted by R_Fredrick_Smith View Post
I'm not sure why anyone would use Auto White Balance when shooting with studio strobes.
i never said i use AWB. I said i set the WB to the"flash" icon. That is not AWB. That sets and locks the WB to 6000K.
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Do we become warmer as we get older? (grin)
Old 12-21-2006, 02:48 AM   #10 (permalink)
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jimmy, you must have read my mind, I've been working on this article since yesterday, but here is hopefully some great info I wrote for our articles here: http://www.glamour1.com/forums/artic...tml#post239557

On a side note, setting the WB at 6000K is appropriate. I noticed some (not you) referenced AWB, or automatic white-balance for their strobes--you can't do this logically as no camera today is quick enough to see the flash and calculate the white-balance based on that flash. Most folks wind up with an incorrect WB based on that technique, usually a white-balance to the modeling bulb, hence colder looking images when the flash goes off. Ideal white-balance for flash, set the camera to "flash" mode, or 6000K for warmer images or 5500K for cooler images.

Thanks again for another great post--crap, I'm starting to like you now , have a great Holiday Season and thanks for being a big part of the G1 family and community, rg sends! (BTW, I just got back from shooting the Spurs and we did a white-balance check on the lighting, found it at 3700K, thought that was odd, later, rg.)

(Image shot with Hensel Porty Premium powerpack with a Hensel Ringflash with the Hensel Sunhaze adapter. A Rosco #02 bastard amber gel was placed inside the reflector, model approximatel 15 to 20 feet from the flash unit, aperture F/8 at ISO 100--image posted is from the scanned book cover image, not actual image)
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Do we become warmer as we get older? (grin) 
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