LEDs, Colour Mixing and Crossfades

Monday, August 1st, 2011 - LED Stage Lighting - by:

A question from On Stage Lighting reader Michael prompts an article looking at a common consideration in modern stage lighting – successful transitions with colour mixing lighting fixtures.

***Note: Interesting product update at the bottom of this article.

Colour mixing facilities is rapidly becoming the norm on many types of show across a range of budgets. Lighting fixtures in all markets are now sporting technology that allows for a range of mixed colours, either in the form of CMY subtractive mixing or additive LED based mixing using RGB, RGB+W and more.

Colour Mixing

At the cheap end of the market, RGB LED PARs and floods are finding their way into every DJ rig, while at the other end of the scale pro kit such at the ETC Selador range and Vari*Lite VLX colour mixing engine are providing high intensity, consistent colours for shows all over the world. For the moment, this looks like the future even though there is still a place for good ol’ white light sources and gel filters.

While these technologies vary in output quality across price ranges, all are rapidly improving but they present the lighting programmer with a specific programming problem. It’s possible to control simple colour mixing equipment with the simplest of desks.  But whatever control you are using, how do you create acceptable colour transitions with colour mixing lighting fixtures?

Colour mixing technology might be relatively new, but the question isn’t and is only too familiar to anyone that used colour washes as the basis for their concert and theatre lighting or lit a cyc by mixing multi coloured battens. Getting from one “state” to another in full view of the audience without accidentally visiting some unwanted colours en route.

What is the problem?

In lighting, we commonly use crossfades to provide a smooth and seamless transition between lighting looks on stage. Cross fading of light intensity isn’t too complicated, some fixtures get brighter, some less bright or go out altogether.

The only possible issue is that intensities that are remaining roughly the same in each scene may fade down a little with the outgoing state, before being pushed back up again by the incoming cue – causing an unwanted dip. We have this mostly covered by dipless crossfade features within the lighting desk or the judicious dexterity when pushing faders by hand. We can also use split fade times or even channel specific fades or waits to overcome this and achieve visually pleasing crossfades.

The issue with intensity dips is a function of the transition from look to look in a fairly linear format – Scene 1 fades down, Scene 2 Fades up at the same time and in the middle somewhere, we have the potential for unwanted results while the intensity levels float to their new values.

The problem with colour transitions is the same, only this time the unwanted results aren’t a dip in intensity but a journey via colours X,Y and Z during the crossfade from A to B. For example, using a simple RGB fixture, a simple crossfade from Red to Blue starts at:

Red @ 100%
Blue @ 0%

and fades to

Red @ 0%
Blue @ 100%

With a simple linear fade, the mid point of this cue has values of Red @ 50% and Blue @ 50%, creating a Deep Pink / Magenta colour and a range of mixes at other points in the fade.

Now, a fade from Red to Blue via Pink might be fine for your needs or even look quite good. But what if it isn’t what you wanted? Another RGB example which might not be so forgiving is a Yellow to Blue. First up, let’s say we have Yellow values of:

Red @ 100%
Green @ 100%
Blue @ 0%

Again, with a straight crossfade between the two, there is a point where all RGB values are at 50%, creating a dirty white colour not to mention the hundreds of other colours that result from all other steps in the fade.

LED Cross Fades Red To Blue

What’s more, now we have a situation where we were using R and G LEDs at Full, then just Blue, theoretically halving the intensity of the second look. But that is for another day, we are concerned with colours during the fade. What can we do to “pretty up” our transitions?

Getting Better Colour Mixing Transitions

The first step to improving live colour fades is to understand what is happening, in particular how additive and subtractive colour mixing actually achieves the colours. That way, you can make informed changes to your cues with the tools at your fingertips. How do you get Amber from an RGB LED, what makes up RED in a CMY moving spot? Read up on light colour mixing theory and experiment with your fixtures.

Blackouts

The simplest and most effective way to manage your colour fades is never to allow the unwanted mixing in the first place. This involves fading the outgoing colour to black before bringing in the new one. No piling on of colours that take you to whacky disco land between beautiful scenes. But blackouts are a statement in themselves, causing an obvious dip.

Multi Part Fade Cues

Either using automated playback or good ol’ finger pushing technology, you can create multi part cues to avoid colours you don’t want. That doesn’t mean the physics can be changed to allow for an “in view” fade without going via another colour, but it does let you choose the route. How about going from Yellow to Blue in a more orderly fashion?

Q1 Yellow = Red @100%, Green@ 100%, Blue @ 0%

Q2a Red = Green to 0%
Q2b Pink = Blue to 100%
Q2c Blue = Red to 0%

While this goes through a few colours, it does avoid a whole bunch of dirty white and pastelly ones on the way. You could equally take this multi part cue through Green and Cyan, although I’m not sure how many people choose green as a stopping off point in many shows. These part cues all run smoothly together, creating a single transition, but via colours you choose within the confines of colour mixing physics.

The key to this approach is to take something away first (in the example, Green), then add (Blue). Take away, then Add. I’ve used this trick by hand on many live shows, where I am performing long, long cross fades on colour mixing kit during the action.

Cross Fade Behaviour

Another way to tidy up your colour mixing transitions is to change the way the fade happens from it’s standard linear format. Professional lighting consoles often have powerful fade curve controls, but the principle can be applied using multi part cues, split fade times, follow ons or just finger technology during a live show.

Taking our Red to Blue example, we might not like the 50/50 Pink that a linear fade takes us through but instead try spend more time of the fade on shades of Lavender.

In this case, we might retard the fade in of the Blue, while taking some of the Red element out before quickly ramping up the Blue quickly past the more pinkish shades and then easing the fade to travel through some pleasant Lavs and slowing removing the last of the reds to rest at Blue.

It sounds more complicated than it is, especially if you experiment using your eyes and controlling a fade by hand, before diving down into cue fade curves if you have the facility. Alternatively, you could set up a few individual cues or part cues and then experiment with fade and wait times to get the cross fade you are after.

LED Cross Fades Complex

Hasn’t this always been an issue?

The effect of colour mixing in lighting cues has always played it’s part in cross fade decisions, well before RGB LED kit came along. Fading out a Straw scene into a Deep Blue wash, while less obviously travelling through other colours, can suffer from unwanted results.

But the use of single light sources with colour mixing, and the fashion for high intensity, saturated colours in all kinds of show lighting, has made live colour changes even more obvious and needs our careful attention.

The future?

In the future, lighting desks should continue to develop user friendly ways of taking control of what happens to your stage picture between A and B for all attributes including colour. This could be the ability to visually pick a path through the available colours between the cues, based on a calculation of the possible permutations, so the user could avoid some colour combinations. The software could indicate reasonable accurate predictions of the more complex mixes which would mean less mental calculation and guesswork by the programmer, or even better align the transitions between a set of RGB additive and CMY subtractive colour mixing fixtures in a single cue.

If colour mixing really is our future, lighting controls will need to continue to reflect those particular needs when programming cues and not settle for a simple graphical colour picker and leave it at that. Good stage lighting is as much about transitions as destinations and we need control over both.

In the meantime, it’s up to us to use the tools we have and to understand the basics of colour theory to continue to have command over new technologies to create great art.

What’s your strategy when it comes to wrangling those colour mixed cross fades? Leave a comment in the box as usual. Cheers.

 

UPDATE:

Bleep ……..March 2012…..Interesting product news…….Bleep……!

Since we had a poke at the (very) prototype last year, it’s worth mentioning the new Lumonic ilumo Zoom Spot LED PAR-like and it’s advertised Colour Crossfade Engine.  According to Lumonic, not only does the fixture profile include the “standard” DMX channels of control for the LED colours, but also a calibrated CIE colour model with hues etc. set using X,Y coordinates.

Not only that but Lumonic say that the issues in the above article are removed, using channels to select the Current and Next colour to which a Transition is added to, avoiding the round trip through a whole lot of LED mixed colours on the way.

Bearing in mind that a pair of hands behind the ilumo range brought you the original James Thomas Pixel Range and the Pixeline 1044, I’d say Lumonic has to be worth a look.

Colour mixing image by Madcrow_Maxwell on Flickr

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Rob is a Lighting Designer and Moving Light Programmer and currently Senior Lecturer in Technical Theatre Production at Bath Spa University in the UK. He is also the Editor of On Stage Lighting and runs workshops in stage lighting practice.

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3 Comments

  1. Jason:

    I have a LED stage light system at work and slowly my Blue has gone out on each can or mostly out. Now the Green is going. Looking to see if someone can point me in the right direction.

    Thank You,
    Jason

  2. Andrew:

    Hi Jason, with LED PAR cans i found that by wiping down the circuit board with a cloth the lights got brighter, probably due to a build up of dirt and dust that had got in, – while you have the circuit seperate take a look at the fuses too, another area where i had a problem once upon a time

  3. Badrol:

    hi Jason,what color filter that suitable to make led red color become pink color.

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