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Hirokazu Taka

Gel, Transmission, and Colour Temperature

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Hi,

 

I've been conducting a little research about gel and I wasn't sure the relationship of transmission and stops.

 

Say, a gel has transmission of 30%. Does that mean that the gel reduces the luminance by 70%? If so, is it in lux or lumen or stop? (I don't have a firm understanding of these units so I might be saying random things) Also, is it helpful to know the transmission vs. wavelength chart?

 

I'd also like to know how the colour temperature of an original source affects the end result through gels. Is there a certain colour temperature that's most appropriate for gels? Or is it totally up to users? If you have your way of working with gels that'd be amazing to know as well.

 

Thanks!

 

 

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It's a complicated subject. 30% probably means that 30% of photons of some kind of white light might make through the filter, so you would expect a loss of more than a stop (a one-stop loss would be 50%.) The thing is, this is likely to be greatly complicated by the fact that you might then using very deeply-coloured light, given it's a filter having only 30% transmission. Most cameras have a response to very saturated colours that isn't very well-characterised by the conventional assumptions about exposure, and as a result you may simply have to test in order to see what sort of response you end up with.

 

P

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... I'd also like to know how the colour temperature of an original source affects the end result through gels. Is there a certain colour temperature that's most appropriate for gels? Or is it totally up to users? ....

 

Most color correction and color affect gels are designed for the spectral distribution of tungsten bulbs. Besides, the now well established deficiencies in the color output of LEDs, they also have the problem that, where you know what to expect when you put a color correction gel, say ¾ CTO, on HMI or Tungsten lights, you don’t know what you will get putting that same gel on an LED light. The reason is that because of their discontinuous spectrum, the use of CC gels on LEDs have unintended and undesirable consequences.

LED_CC_34CTO_Day_Conv.jpg3/4 CTO gel passes only certain wavelengths (represented by the spectral transmission curve (center) of daylight (left)

to create the color spectrum approximating that of a 3200K tungsten light (right.)

 

LED_CC_34CTO_LED_Conv.jpg

The same 3/4 CTO gel applied to a daylight LED (left) passes the same wavelengths (represented by the spectral transmission curve (center)) to create an unknown color spectrum that does not approximate a 3200K tungsten light.

 

A good example of this is what happens when you try to convert the 5500K out-put of Phosphor White LEDs to 2900k with Full CTO gel. Where you can do it with some success with HMIs because there are long wavelengths in it’s continuous spectrum to pass disproportionately to the blue part of the spectrum to achieve a nominal 2900K, since LEDs don’t put out much beyond 625nm, there is not much for a filter to pass to rebalance the light output to 2900K, so the “corrected” light is too cool. Another undesirable consequence comes from the fact that Full CTO is designed to pass extra green (there is a bump in the spectral transmission curve of Full CTO in the green portion of the spectrum) and so it creates, given the amount of green inherent in Daylight LEDs to begin with, a disproportionate amount of green (creating an overall green bias) to the "corrected" light when used on Phosphor White LEDs ( link to test results demonstrating this with a Lightpanel 1x1 Daylight Spot.)

 

The gel pack that eventually made the Lightpanel 1x1 Daylight Spot in the test marginally similar to a tungsten light, was only able to do so at the expense of two stops – so much for the greater efficiency of LEDs.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

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Macks, thanks for the link. This is actually really helpful!

 

-------------

... The thing is, this is likely to be greatly complicated by the fact that you might then using very deeply-coloured light, given it's a filter having only 30% transmission. Most cameras have a response to very saturated colours that isn't very well-characterised by the conventional assumptions about exposure, and as a result you may simply have to test in order to see what sort of response you end up with.

I see. I suppose I should examine the whole workflow with gels including colorgrading. Thank you!

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Most color correction and color affect gels are designed for the spectral distribution of tungsten bulbs. Besides, the now well established deficiencies in the color output of LEDs, they also have the problem that, where you know what to expect when you put a color correction gel, say ¾ CTO, on HMI or Tungsten lights, you don’t know what you will get putting that same gel on an LED light. The reason is that because of their discontinuous spectrum, the use of CC gels on LEDs have unintended and undesirable consequences ... since LEDs don’t put out much beyond 625nm, there is not much for a filter to pass to rebalance the light output to 2900K, so the “corrected” light is too cool. Another undesirable consequence comes from the fact that Full CTO is designed to pass extra green (there is a bump in the spectral transmission curve of Full CTO in the green portion of the spectrum) and so it creates, given the amount of green inherent in Daylight LEDs to begin with, a disproportionate amount of green (creating an overall green bias) to the "corrected" light when used on Phosphor White LEDs ( link to test results demonstrating this with a Lightpanel 1x1 Daylight Spot.)

 

This is very interesting. It seems like something similar would happen with fluorescent lights, looking at its spectrum chart. Thank you very much.

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