Anyone good at calculating color temps?

[AlanG]

This thread got me to do a little more research.  Check out this page that show the spectral response curves for various GE lights:

First click on the daylight and tungsten links to see what a normal smooth curve from those look like. Then click on the others to see how different they may be. An extreme example is Low Pressure Sodium which basically puts out a narrow band of orange. Once you filter that out, there's nothing left.  So you basically couldn't shoot color under it.

This should give everyone some idea why color correcting can be so difficult.

http://www.gelighting.com/na/business_ligh...tion_curves.htm

[dwdallam]

This thread got me to do a little more research.  Check out this page that show the spectral response curves for various GE lights:

First click on the daylight and tungsten links to see what a normal smooth curve from those look like. Then click on the others to see how different they may be. An extreme example is Low Pressure Sodium which basically puts out a narrow band of orange. Once you filter that out, there's nothing left.  So you basically couldn't shoot color under it.

This should give everyone some idea why color correcting can be so difficult.

http://www.gelighting.com/na/business_ligh...tion_curves.htm
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Which is why you find low pressure sodium lamps mixed with other sources, like fluorescent. Also, many cities are converting from HPS street lamps to LPS lamps because the overall cost and energy use is lower. Our city in Arcata CA uses them. So for outdoor applications, expect to see more of them as energy strains continue to increase worldwide.

[AlanG]

Last night it crossed my mind that there is a simpler solution to matching these sources.

You already have a test shot of a grey card shot under existing light. Bring that up in your raw conversion software and set it to do no correction - just the camera setting for strobe or daylight.

Then at any convenient location,  shoot the same grey card with your strobe and try the guessed at correction filters over it.  Bring that image up in the same software and at the same settings.  If your filter choices are correct, they should match without using any raw adjustments.  If they don't match, you can shoot again with different filters.

Then click grey balance on the strobed shot and copy the same settings to the existing light shot. If they match and both have a clean grey, you are set.

The advantage to this approach is that you only need one set of filters for one light. And you can test the filter pack at your convenience once you have the location test image.

[Dansk]

Then at any convenient location, shoot the same grey card with your strobe and try the guessed at correction filters over it. Bring that image up in the same software and at the same settings. If your filter choices are correct, they should match without using any raw adjustments. If they don't match, you can shoot again with different filters.

  Actually I did exactly that but there is no need to shoot the second shot I already have one balanced. I set the preview to 2up portrait of those two shots and held up various filter swatches and the best match I came up with is a combo of 1/4 plusgreen and 1/2 CTO which looks very very close to the off color shot so I'm going to start there and see how it goes. I have some other levels of plusgreen and CTO if need be I can make pretty much any combo of the two but I hope the test pairing works okay.

The shoot is next week I'll update with how it went afterwards. Thanks again for the help

[AlanG]

I think you'll be fine. It seems to me that if your over all exposure is balanced to the existing light then those exiting lights will be contributing most of the light and color to your primary subject.  So your strobes will mostly serve as accent lights and fill unless you let the background go darker.

[Dansk]

3/4 plusgreen and 3/4 CTO did the trick. Not perfect I still have some tweaks to do to the color but the match between house lights and strobes was very close. Overall I'm glad that shoot is over it was a marathon

[Jonathan Wienke]

I have some very odd ambient lighting that I have to balance some strobes up with and I'm scratching my head a little over it as I have never quite seen this temp before. I run Phase systems and profoto strobes with all sorts of modifiers and my default wb is 5400K with +4 magenta. I did a site survey at the shoot location a week back and its a huge warehouse full of the equipment and people I have to shoot so I have to soak up some ambient just due to the sheer size of this place I'd be adding ten zillion lights to cover this place if I wanted to use all my own lighting.

  Anyways I did take a few grabs of the place with a gray card on the survey and ran them through Phase just now and the temp comes up at 2900K with +42 magenta. I would add a full CTO and call it done but this +42 magenta is confusing me so what gel/s would you recommend to start with here? Main problems I have are 1. I will need to buy more gels than I currently have and would rather avoid buying 200 gels that i will never use again and 2. I dont have the time on set to play around too much with this so I'd like to be as prepped as I can be on the day.

There's a simple way to make your own gels if you have a reasonably well-profiled printer that can print on transparency stock.

1. Shoot a WB reference (Color Checker, WhiBal, etc) in RAW mode. under the ambient light you want to match, and your strobes.

2. Do a click-WB on the strobe white reference RAW, and make a note of the color temp/tint settings.

3. Convert the ambient white reference RAW with the strobe WB settings.

4. Sample the color of the WB reference in Photoshop. Without altering the A and B values, set the L value to 90 or so.

5. Fill a page with this color. Print it on the transparency stock.

The transparencies are now custom gels for your strobes. As long as the printer profile is reasonably accurate, the color match will be very good. You can match any strobe to any light source this way.

[sniper]

Dare I suggest an alternative suggestion?  As you need to ballance front lighting, why not just get some light holders the right size and use use the tubes they already have, you could easily rig up some holder on stands, as your then using the same tubes the colour problem goes away.  Just a bit of lateral thinking for another option.  Wayne

[Dansk]

I wanted to use a bunch of different modifiers and diffusion which I did and the gels worked out fine. Plus I used the strobes to freeze the action and only soaked up enough background as needed. At least the place had all the same lighting throughout it could have been worse.