# Calculating gels

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### #1 Phil Rhodes

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Posted 30 April 2015 - 07:59 AM

Hi folks,

I have some CDM discharge lights (along the same lines as Cool Lights' CDM150 fresnel) which are sort of roughly tungsten, and some others that are sort of roughly daylight. The cooler ones are about 4200K (datasheet) and the warm ones are about 3000K (datasheet). They have reasonably good CRI in the high 80s for the warm and a very good mid-nineties for the cool. For comparison, Osram's HMI bulbs are usually specified as 90 CRI. The warm CDM produce an attractive, golden light that's very nice on people's skin. It's an efficient way to warm light and I like them a lot.

It'd be nice, however, to be able to match them with reasonable accuracy to conventional 3200K tungsten or 5500K daylight. A bit of mathematics for the warm ones:

(1000000/3200) - (1000000/3000) = 312.5 - 333.3333 = -20.833

The mired shift value of eighth CT blue is -18, which should put us within 25K of correct. This is almost three mireds off, which is likely to be visible as a warm cast, but I'm not aware of a better way to do it.

Regarding the 4200K types:

(1000000/5500) - (1000000/4200) = 181.82 - 238.094 = -56.274

Again, the nearest option is not quite perfect - a layer of quarter blue, and a layer of eighth blue has a total shift of -53 which puts us within 3.2 mireds of correct.

If anyone has experience of using this sort of mathematics, I'd be grateful for any input.  The accuracy with which any of this works is also likely to be modulated by the imperfect CRI of the lights, especially the warm ones which have a fairly pronounced green spike:

The 4200K types are better:

...but it seems worth a try.

Phil

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### #2 Peter Daffarn

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Posted 30 April 2015 - 08:10 AM

Hi Phil,

Have you tried asking the good folks at Lee Filters?

They have always been happy to help me when I have asked such things.

Regards

Pete

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### #3 Phil Rhodes

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Posted 30 April 2015 - 08:55 AM

I find that Lee's lighting calculator pretty much duplicates my results, so I guess I'd better get in some quarter and eighth blue.

Have you ever considered doing any CDM stuff? I guess it seems a bit bargain-basement, but it is very effective, very affordable, and you can even get blue and green emitting types for chromakey work.

Phil

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### #4 Peter Daffarn

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Posted 30 April 2015 - 09:00 AM

We did have a complete range of CDM that we were working on......

Then Osram and GE sent out a message saying they would be discontinuing the CDM lamps.

So we cut that development short and moved onto Plasma.

Here we are many years later and CDM is still available...Grrrr

Maybe we should dig out those prototypes and look at getting them into production.

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### #5 Phil Rhodes

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Posted 30 April 2015 - 09:02 AM

I can't imagine they're going to stop doing it any time soon. It's used endlessly in retail.

P

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### #6 JD Hartman

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Posted 30 April 2015 - 11:11 AM

Might one ask why you have chosen Ceramic Discharge Metal Halide fixtures to shoot  (your short?) with and not just gone with simple Tungsten Halogen fixtures instead?

Didn't ARRI come out with a few fresnels using those globes, maybe 6 or 7 years ago?

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### #7 Mark Dunn

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Posted 30 April 2015 - 11:23 AM

I don't think he chose them as such. The question is- they've turned up, how useful are they, and can I put them into service for a lot less than the ideal items?

Edited by Mark Dunn, 30 April 2015 - 11:26 AM.

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### #8 Phil Rhodes

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Posted 30 April 2015 - 11:26 AM

It's stuff I own (and have owned for years). Unfortunately there's only so many spare organs available to fund HMI purchases.

But in all seriousness, as to why, well, same as HMI or LED. It's four times more efficient than tungsten. It's HMI performance (lacking hot restrike) for about 10% the price. Well. 5% the price.

P

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### #9 JD Hartman

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Posted 30 April 2015 - 11:40 AM

Okay then.  Follows the same reasoning of shooting with MLBs (Major League Baseball lights), they were borrowed and HMIs weren't in the budget.   So what kind of light output can you expect after correction?  Compared to an HMI, same a a 800?, 1.2k?, 2.5k??

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### #10 Phil Rhodes

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Posted 30 April 2015 - 12:01 PM

It's probably equivalent watt for watt to HMI, so the 150-watt lights are equivalent to whatever a 150-watt HMI would be if such a thing existed (actually, such things do exist, but they're not very common). Probably about equivalent to 650W tungsten fresnel. I don't think a one-eighth CTB is going to cost too much.

There are larger options. 250W versions of more or less equivalent spectral output exist (should've gone for those, really, but - parts on hand, etc). Osram's PowerStar HQI-T lamp is available at 400W, so a bit less powerful than a 575W HMI, with 5500K output and an encouraging 92 CRI. The problem with those is that they're very large, over 280mm long. There 2000W HQI types available, but I've not yet found one with sufficiently good numbers for film and TV work.

P

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### #11 John E Clark

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Posted 30 April 2015 - 02:22 PM

It's probably equivalent watt for watt to HMI, so the 150-watt lights are equivalent to whatever a 150-watt HMI would be if such a thing existed (actually, such things do exist, but they're not very common). Probably about equivalent to 650W tungsten fresnel. I don't think a one-eighth CTB is going to cost too much.

There are larger options. 250W versions of more or less equivalent spectral output exist (should've gone for those, really, but - parts on hand, etc). Osram's PowerStar HQI-T lamp is available at 400W, so a bit less powerful than a 575W HMI, with 5500K output and an encouraging 92 CRI. The problem with those is that they're very large, over 280mm long. There 2000W HQI types available, but I've not yet found one with sufficiently good numbers for film and TV work.

P

You should see of there's someone in the UK that can 'lend' you a Sekonic C-700 to create graphs of the various spectra, and also the 'color bins' which their respective CRI values.

The nit I have with the C-700, was that apparently one can not get the spectra out as a say an 'excel' or even a CSV, spreadsheet. But the 'PDF's generated do have the more pictorial representation

of the spectral info.

Another poster here, Stuart Allman, who also has a blog on tech topics, mentioned it a few weeks ago in another thread, and I visited the Sekonic 'table' at NAB... I may or may not be able to get a demo unit in the future... I've known the rep for a number of years, but the device is going to be popular...

Edited by John E Clark, 30 April 2015 - 02:23 PM.

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### #12 Stuart Allman

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Posted 30 April 2015 - 05:55 PM

I had one on loan through NAB.  We did our blog video with it, as per the agreement I had with Sekonic and then the meter had to go back.  So there's at least one back in the demo pool.

I expressed the same desire to have a table of spectral data output to the Sekonic product manager.  Maybe they'll add that in the future.  The Sekonic meter is really meant to match and correct lights on set.  It does that function very well.

I had the UPRTek MK350 on loan from a friend a while back.  I vaguely remember that tool allows you to download a csv file with the spectral data.  So if you're looking to do more scientific measurements there are deals on those devices on Ebay.  I looked a few weeks ago just out of curiosity.  However it doesn't do a lot of handy functions of the C-700.  I don't remember it providing color rendering numbers on the individual CRI patches, but maybe it does by now.  It certainly didn't have a database of Lee filters with recommendations on how to correct a light.

Stuart Allman

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### #13 Stuart Allman

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Posted 30 April 2015 - 06:01 PM

...Oh, and the C-700 was quite a good lie detector when we went around the NAB booths looking at LED lights.  I'll just say that Fiilex and Cineo scored very well with regards to color rendering.  Some other common brands didn't score nearly as well as they would have you think.  That's not to say that I measured everyone's lights and those were the only good performers.

For the record, I have no association with any vendor of LED lights.  Wish I did.  I want some free lights.

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