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Old 21st of June 2007 (Thu)   #1
TMR Design
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Default Question About Color Temperature and Using Gels to Correct Temperature

I use Alien Bees strobes, which have a specified operating temperature of 5600 degrees K.

I have my hair light behind a custom diffuser that seems to warm up the color of that strobe about 500 degrees and it's too warm for skin tones. I don't want that one light to affect the overall color temperature of my shot but I love the diffusion so I'm wondering how I can adjust the color temperature of that one strobe.

Is the answer to use a gel in front of that light that lowers the color temperature by 500 degrees? Are gels available to accomplish this? Are there other methods used when mixing light sources of different color temperatures?
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Old 21st of June 2007 (Thu)   #2
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

You might try one of the Rosco filters.

http://rosco.com/us/filters/roscolux.asp#Colors

Roscolux #3216: Eighth Blue
Boosts 3200K to 3300K

Roscolux #3208: Quarter Blue
Boosts 3200K to 3500K

Roscolux #3206: Third Blue
Boosts 3200K to 3800K

Maybe one of these?
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Old 21st of June 2007 (Thu)   #3
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Thanks.

Ok so here is the question. The ones you are suggesting are all boosting temperature. How does that help with the one strobe I have that is 'hotter' than the others?

I see one that's close to what I need. It's actually the first one listed and it converts 5500 degree to 4900 degrees. I need a change of about 500 degrees and that is 600 but it's the closest thing.

How precise is all of this and how precise do we need to be? Will that gel do the job I need?
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Old 21st of June 2007 (Thu)   #4
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

A clarification of terminology is in order.
Higher color temperature light creates a "cool" (blue) appearance.
Lower color temperature light creates a "warm" (yellow-orange) appearance.

The blue gels mentioned by Breal will increase the color temp and make the image cooler (more blue).

CTO (color temperature orange) gels will decrease the color temp and make the image warmer (more orange).

Confused yet?
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Old 21st of June 2007 (Thu)   #5
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

If your hair light is too warm you need to convert cooler (bluer). I may have misunderstood but it seems you are saying the hair light is 500 degrees warmer than you want.
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Old 21st of June 2007 (Thu)   #6
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Thanks guys,

Ok thanks for that clarification.
I shoot and set white balance for 5600 K with the un-diffused light. If I shoot the same thing with the diffused light it is more orange. If I adjust the camera's color temp down to 5100 and shoot with the diffused light it matches the original with no diffusion.
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Old 21st of June 2007 (Thu)   #7
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Quote:
Originally Posted by TMR Design View Post
If I shoot the same thing with the diffused light it is more orange.
Sounds like one of the blue filters will take care of that.
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Old 21st of June 2007 (Thu)   #8
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Thanks Curtis and breal101.

I think I may try one of those blue gels.
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Old 17th of December 2007 (Mon)   #9
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Did that blue gel work out? Are some blue and orange gels temperature rated for +/- K?
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Old 17th of December 2007 (Mon)   #10
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Can you explain the components of your diffuser? I'd like to pinpoint what is warming it up.
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Old 18th of December 2007 (Tue)   #11
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Quote:
Originally Posted by TMR Design View Post
Thanks.

Ok so here is the question. The ones you are suggesting are all boosting temperature. How does that help with the one strobe I have that is 'hotter' than the others?

I see one that's close to what I need. It's actually the first one listed and it converts 5500 degree to 4900 degrees. I need a change of about 500 degrees and that is 600 but it's the closest thing.

How precise is all of this and how precise do we need to be? Will that gel do the job I need?
Robert...

The problem is that the color shift is not linear. It is different based on the color temperature you are starting from. The factor is expressed as the "Mired shift".

Mathematically, this is defined as:

Mired Shift = 1000 * (1000/T2 - 1000/T1)

where T1 is the color temperature you have and T2 is the color temperature you desire

The Rosco and Lee sites do not show the mired shift values for all "color conversion filters". However, the samples do supply the mired values.

For Roscoe CTB cinegels, they are:

# filter mired
3202 ctb -131
3203 3/4 ctb -100
3204 1/2 ctb -68
3206 1/3 ctb -49
3208 1/4 ctb -30
3216 1/8 ctb -12
3220 2x ctb -260

Hope this helps...

Ooops....just read that you may need to go in the other direction. Same rules apply, just need to use CTO filters. The Rosco site http://www.rosco.com/us/filters/cinegel.asp has mired values for the different CTOs.
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Last edited by jrsforums : 18th of December 2007 (Tue) at 09:35.
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Old 18th of December 2007 (Tue)   #12
TMR Design
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Quote:
Originally Posted by Heatseeker99 View Post
Can you explain the components of your diffuser? I'd like to pinpoint what is warming it up.
Hi A. J.

Been a while since this thread was active. My diffuser is not any different from small softboxes and enclosed light boxes. I've got 4 sides that are reflective white with a front diffusion panel made of white nylon ripstop. That's it.
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Old 18th of December 2007 (Tue)   #13
TMR Design
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Default Re: Question About Color Temperature and Using Gels to Correct Temperature

Quote:
Originally Posted by jrsforums View Post
Robert...

The problem is that the color shift is not linear. It is different based on the color temperature you are starting from. The factor is expressed as the "Mired shift".

Mathematically, this is defined as:

Mired Shift = 1000 * (1000/T2 - 1000/T1)

where T1 is the color temperature you have and T2 is the color temperature you desire

The Rosco and Lee sites do not show the mired shift values for all "color conversion filters". However, the samples do supply the mired values.

For Roscoe CTB cinegels, they are:

# filter mired
3202 ctb -131
3203 3/4 ctb -100
3204 1/2 ctb -68
3206 1/3 ctb -49
3208 1/4 ctb -30
3216 1/8 ctb -12
3220 2x ctb -260

Hope this helps...

Ooops....just read that you may need to go in the other direction. Same rules apply, just need to use CTO filters. The Rosco site http://www.rosco.com/us/filters/cinegel.asp has mired values for the different CTOs.
Thank you John. VERY good information. I'll actually be getting outside and with my portable lighting kit once the weather begins to to turn and we have warmer and drier days. Then I'm sure I'll be working with these gels. As far as working in the studio, I have yet to try to use gels for color temperature correction but I know in time I'll be getting to that too.
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