CMY Mixing

[jmaudio]

Hey

 

I've had a look round the web for an answer to this question, including on the Blueroom Wiki, however I have not found a satisfactory answer.

 

Basically, how does CMY mixing actually work? I understand the principle that if you subtract cyan, magenta and yellow from white light you get black, and therefore different quantities of each give you a very fine control of colour output. But it seems to me, this is a tricky thing to achieve mechanically in an intelligent fixture.

 

From what I have read so far, its done with 3 glass plates in the optical path, one for each of the 3 subtractive colours. These glass plates can be moved around altering the colour output of the light.

 

All well and good so far (unless I am totally wrong!), except for one thing. How do the plates affect the whole light beam equally? For example, you have one of these glass plates, which I am assuming (possibly incorrectly) have a gradient of 1% cyan at the bottom, and 100% cyan at the top, then, depending on the DMX input, these plates moves up and down giving you 0% to 100% subtraction. EXCEPT, this arrangement would mean, that the light output would not be consistent throughout the beam, because the top of the beam would pass though 100% cyan, and the bottom would pass though (lets say) 90% cyan.

 

Now, I know this rambles on slightly, I just can't get it clear in my mind how it works, and the above is my best stab at trying to think of something that would work, based on what I know!

 

So, if someone can come along and explain it to me, that would be fantasic :D And, if anyone has a intelligent light with CMY filters and the hood off next to them, and a camera to hand, then some photos of the mechanism would be wicked!

 

Cheers

 

Jamie.

[niclights]

In many fixtures the mechanisms are quite crude and the colour is indeed not evenly distributed. However, the problem can minimised depending on the position of the mixing plates in the optical path and wash fixtures obviously suffer much less than a tightly focussed profile.

 

There are exceptions. While many use a colour 'flag', Varilite 2500/3500 series use discs, with graduation occurring around a whole 360 degree plate. This means all three elements are affected in the same way (rather than flags which often come from different directions) and over a much larger area of glass. This principle is also used for dimming for the same reasons. These are the first components in the optical path and the beam passes through a 'homogeniser' before moving onto other elements. This effectively gives full field mixing and dimming.

[paradoxism]

Different fixtures use more or less crude methods but most rely on a disc or slide for each colour with many lines that start thin and get thicker as they advance into the beam - like these for example.

 

Because they are not in the focal area of the lantern the coloured and white light blends together in the beam.

[dbuckley]

The really important thing about CMY mixing is that you only ever use two flags. Any desk that understands subtractive colour mixing should do this automatically. Download Visual Color Picker and have a play, in particular, slide the hue across whilst watching the C/M/Y channels

[tom_the_LD]

Just as a side note to this topc; I assume it matter which order the CMY discs or whatever are in?

 

For example, if the Cyan is closest to the lamp and you put it in, you are going to be removing all the colours in the spectrum which come before it meaning even if you put the magenta disc in you would not gt purple becuase there is no magenta left in the spectrum.

 

Basically - does it need to be the magenta cloest to the lamp or does it not matter?

[niclights]

Just as a side note to this topc; I assume it matter which order the CMY discs or whatever are in?

 

For example, if the Cyan is closest to the lamp and you put it in, you are going to be removing all the colours in the spectrum which come before it meaning even if you put the magenta disc in you would not gt purple becuase there is no magenta left in the spectrum.

 

Basically - does it need to be the magenta cloest to the lamp or does it not matter?

It matters not. You might also want to consider that the magenta filter removes everything other than magenta (using your example)! :)

[Wol]

In the same way that if you're putting two gels into a lantern, it doesn't matter which one ends up closest to the light - the colour doesn't change.... thankfully!

[mac.calder]

In the same way that if you're putting two gels into a lantern, it doesn't matter which one ends up closest to the light - the colour doesn't change.... thankfully!

 

That said, there are a number of other factors that manufacturers consider, things like how hot the gels get (no real difference when sandwhiched in a gel frame, but there is a difference with dichroics in a colour changing module)