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How is the "power" of color filtering gels affected by the intensity of lights?


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#1 Kelvin Xu

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Posted 28 August 2015 - 03:55 AM

Hi Cinematography, 

 

When one applies color filtering gels on a light source, is the filtering capability affected by the intensity 

of the light it's intended to filter out? Let's say when a filter claims 40% reduction in a 500~550nm wave length, and the light at that region is 100lux then the result would be 60 lux. When I pump up the intensity by 10, will it reduce from 1000lux to 600lux? 

 

Will it saturate? 

 

Thank you. 

Kelvin

 

 


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#2 Albion Hockney

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Posted 28 August 2015 - 08:06 AM

No, the color of a filter is not effected at all by the intensity.


Edited by Albion Hockney, 28 August 2015 - 08:07 AM.

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#3 Stuart Brereton

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Posted 28 August 2015 - 09:12 AM

The filter's transmission characteristics will stay the same, regardless of how bright the lamp is.. However, the intensity of the lamp, relative to your exposure on camera, will affect the color saturation. An overexposed source will appear much less saturated than an underexposed source.


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#4 John E Clark

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Posted 28 August 2015 - 03:56 PM

Hi Cinematography, 

 

When one applies color filtering gels on a light source, is the filtering capability affected by the intensity 

of the light it's intended to filter out? Let's say when a filter claims 40% reduction in a 500~550nm wave length, and the light at that region is 100lux then the result would be 60 lux. When I pump up the intensity by 10, will it reduce from 1000lux to 600lux? 

 

Will it saturate? 

 

Thank you. 

Kelvin

 

 

 

The general concept here is called 'linear' response, that is the amount of attenuation percentage wise does not change with the intensity of the light.

 

A 'non-linear' filter would be something like the glasses that change from 'sunglasses' to clear depending on the amount of light. Some people buy these, rather than have to take off the sunglasses or change glasses when changing lighting situations.

 

Most photographic filters are linear in this regard.


Edited by John E Clark, 28 August 2015 - 03:57 PM.

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#5 David Mullen ASC

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Posted 28 August 2015 - 05:58 PM

What Stuart said is important enough to bear repeating -- the saturation will be affected by the exposure.

 

Ignoring any gel fade from putting the gel in front of a more powerful lamp (and a smaller light on full spot can also increase the speed of fading), if you light a large room for moonlight, let's say, and the windows in the far background are lit from outside with 10K's with Full CTB (blue) on them, and you light a foreground face with a much closer 650w Tweenie with Full CTB, and all your lights big or little give you an f/2.8 at the distance the subject will be from the light, then the color would be the same whether the person was standing under the light of the 10K or the Tweenie.

 

But if the 10K with Full Blue gave you an f/5.6 and the Tweenie with Full Blue gave you an f/2.8 and you shot at, let's say, f/4 -- then the overexposed light from the gelled 10K would give you a paler color effect compared to the slightly underexposed light from the gelled Tweenie.  Color temperature-wise, both are giving you the same amount of blue but the effect on film will be that the brighter light looks less saturated.


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#6 Kelvin Xu

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Posted 31 August 2015 - 08:20 PM

Thank you all for the replies. I now understand that the linearity of the filters. 

 

Will this idea work in order to save me some funds? 

 

Assume when I purchase two LED lights with bundled softbox and color gels to be attached to the front of the softbox.

Instead of buying two sets of the gels, I could just buy one set, cut the gels into pieces and clip to the front of the lights, 

the surface area of which is much smaller. (I can live with lessened softness from the arrangement)

 

http://www.videoligh...ls-filters.html


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#7 David Mullen ASC

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Posted 31 August 2015 - 09:14 PM

Sure, you can color gel the smaller LED light instead of the larger softbox.  If it were a tungsten lamp, you'd want enough distance between the gel and the lamp so that it didn't melt, but with an LED, you can put a gel close to the light.

 

I wouldn't do this with diffusion gel though because the softness of a light is determined by the size of the source, so if the diffusion gel is the same size as the front of the LED unit, it hardly would do anything in terms of softening the light other than spread it a little and blend the multiple bulbs better.


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#8 Guy Holt

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Posted 02 September 2015 - 04:11 PM

 

Kelvin,

 

You should be aware that the gels listed at the link you gave above will give unexpected, and most likely unsatisfactory results, when used on your LED lights. A major problem with low CRI sources like your LEDs is that gels (party, effects or color correction) do not produce the same repeatable result that you’d expect from gelling a high CRI continuous source like tungsten.

 

LED_3200K_Remote_Phosphor.jpg

 

Take for example the discontinuous spectrum of even high CRI remote Phosphor LEDs like the Cineo Trucolor above.  As you can see by the spectral distribution graph for the Cineo with 3200K panel above, they have a spike in the blue range that does not exist in the continuous spectrum of a tungsten light source (green line.) The greater proportion of blue and green/cyan in the Cineo Trucolor 3200 will result in an unexpected and undesirable result if a color correction  (CTO, CTS, CTB), or color effect gel (Congo Blue, Bastard Amber, etc.) calibrated for the continuous spectrum of tungsten light is used on this discontinuous spectrum of a 3200K remote phosphor LED.

 

LED_5600K_Remote_Phosphor.jpg

 

The same is true of the Cineo Trucolor with the 5600K panel above. The quite prominent blue spike (black line) will likewise result in an unexpected and undesirable result if a color correction, or color effect, gel calibrated for the continuous spectrum of a continuous daylight source is used on the discontinuous spectrum of a 5500K remote phosphor LED.

 

Color correction and color effect gels are a part of a finely calibrated imaging system that involves a highly specific light receptor (film emulsion or video sensor), light sources, and color correction or effects gels calibrated for both. Where that exists between film emulsions/video sensors and tungsten and/or daylight sources it is possible to mix dyes in a gelatin materials to create desired effects  (it has taken decades to hone this system.) To use the available color correction and effects gels on the discontinuous spectrum of LEDs is a misapplication of a finely tuned system designed for continuous spectrum light sources only.  Use this link for examples of what happens when you put a color effects gel designed for tungsten source on the discontinuous color spectrum of LEDs.

 

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


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#9 Kelvin Xu

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Posted 02 September 2015 - 07:47 PM

Thank you for the detailed explanation. It's very useful. 


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