"Making" white with gels?

F

FatBird19

Well-known member
If you put a red, green, and blue gel in front of a white light, would it come out as white again(albeit a much dimmer version)?
 
Even if they were pure Red, Green and Blue, I have to believe that there would be some color tint.
 
Well, esssentially it would come out as ND gel, so long as they were complementary (I think that's the right word) colors. For example, CTO and CTB (engineered to do exactly the opposite of each other) together looks just like a heavy ND to my eye at least. I've never bothered to strap the two on a light together and check with the camera, but there's no reason to believe that it wouldn't be true, at least with CTB and CTO, Full Plus Green and Full Minus Green or any of those combinations. Don't know about the Red, Green & Blue situation though. Why not give it a shot and let us know what you find. Take it easy guys.
Peace,

-Harry
 
I believe this phenomenon only works within the visibile light spectrum; Red, Orange, Yellow, Green, Blue, etc, etc

If you took the colors red, green and blue and painted them together you'd get black

I would imagine these 3 gels combined would give you something close to a ND like dudeguy illustrated
 
in the real world, gels don't stay the same color for very long when you put strap them onto a Baby for hours on end!
Question is would they fade at the same pace?
 
thisiswells said:
in the real world, gels don't stay the same color for very long when you put strap them onto a Baby for hours on end!
Click to expand...

So the answer to the original question is:

Eventually ! :cheesy:
 
The reason I was wondering is because I want to know if it's possible to do color offsets to produce new colors. Let's say I'm doing a greenscreen and there's a slight green cast on the actors. If I shone a light on them with a slightly magenta gel(I think magenta is the opposite of green in light), would the green cast go away?
 
The use of a magenta gell on kickers, back lights and such is an accepted practice for green screen work. The beauty of American marketing is the amount of color and the gels that support it is readily available. Contact the major gell makers and follow their guidlines.
 
You need to light the green screen and the actors separate. Light the green and then put in all the light on your actors but keep it off the back you can't use the same lights for both. Well you could but you will run into not lighting the green righ
 
I would like to see more discussion on gels and their use. For instance if I'm using my quartz work lights and theres sunlight coming in the window and a lamp with a hundred watt bulb in the corner, which light and which gels would I use? I have Jackmans book on lighting and it addresses some color correction but I'd like to hear from some dvx users on how they deal with mixed lighting. I'm about to spring for some gels and would like to slant my purchase towards the lighting that I expect to be correcting the most. Any thoughts?
 
I would clip a CTB (blue) to the work light, swap out the 100w for a screw in daylight fluo, or keep both and cover the window with a big sheet of CTO (orange)
 
Probably the same as a tungsten, although I've never had the need to try one. The only time I would use one is as a practical. A Fresnell w/ Gel and Diffusion has so much more control....
 
FatBird19 said:
The reason I was wondering is because I want to know if it's possible to do color offsets to produce new colors. Let's say I'm doing a greenscreen and there's a slight green cast on the actors. If I shone a light on them with a slightly magenta gel(I think magenta is the opposite of green in light), would the green cast go away?
Click to expand...
Yes, in theory. I depends on the technique used. when you say slightly magenta, how slightly is it? It might be too weak. Where did you place your backlight with the magenta gel? And what is the distance from the greenscreen to your subject?
 
FatBird19 said:
If you put a red, green, and blue gel in front of a white light, would it come out as white again(albeit a much dimmer version)?
Click to expand...

You are describing subtractive filtration. Assuming you have a single light source and "pure" color gels, then you have in effect a ND filter. The red is reducing cyan, the green is reducing magenta, and the blue is reducing yellow light.

If you have 3 light sources and have one filter on each light, where the 3 colors fall they will add and produce white light. This is an additive filtration.
 
You shouldn't have to add any gelling to get a good matte. The reason being is that if you have adequate separation between subject and greenscreen, and the screen is evenly lit, shooting at the color temp of your subject lights is all that's needed (usually 3200 kelvin) The key is separation, and just having a backlight that is properly scrimmed to provide just enough but not too much light. Turn off of the greenscreen light and light the subject separately, then turn on the greenscreen lights. Those lights always fool the eye, so light the subject with them off. Ideally you will have a laptop on set with some keying software, and you will digitize a few seconds of footage and do a quick key with render before filming the scene. It takes a few minutes, but then you know exactly what you have. It's like a polaroid of a film set in the film world.
 
Yes you can get white by combining red, blue, green gels... and yes, technically you just made an ND gel... because the intensity of the passing light will be reduced by each of the three gels' bandpass of light.

If you have a good place for a greenscreen with plenty of room between the actors and screen then you're one of the lucky ones... if you have a greenscreen in a garage like I do... and you want a cleaner key... then it is not uncommon to use a magenta kicker from behind and over the head of the actors... to create a slightly magenta rim light which will clean up the key. I don't know too many people who can get a truly clean key as it is (in these limited room situations)... all the magenta kicker does is sharpen the line between actor and key, which must normally be blurred via a slider in your NLE.

Using a magenta gel doesn't mean your doing your greenscreen wrong... it just means you have limited space between the actors and the screen... or you need to sharpen up the seperation. (you can see this by fewer "jaggies" when you zoom in)
 
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