Fixture Calibration

[David Gemmell]

hi Massimo,

Firstly, thanks and congratulations to you and all your fellow contributors to the work done on QLC+. I can only guess at the time spent on the project. Great job.

I am with a small amateur theater group in Brisbane, Australia. We have just upgraded our lighting system with LED's. The old system had a JANDS lighting desk and 2 x 12 channel dimmer racks. The new gear was imported direct from a manufacturer in China, at considerable saving. We have daisy-chained the new LED's on to the tail end of the old dimmer racks, and added
16 PARS (with 24 x 8w RGBW) for Keys and Modelling,
10 x 1 mtr wall washers (with 24 x 3w RGB each) for Cyc floods and back-lights, and
6 x Movers for effects.
The PC is windows 7 with an ENTTEC DMX box. We have also retained our Fresnel-based fixtures, used on the dimmer racks, so we now have a great LX system for a theater our size.

Theatre requirements for color accuracy are far more stringent than for rock n' roll concerts or the like.
So, as part of the new installation, we did LUX measurements of the fixtures, in particular the PARS.

Using a chase with 5 sec pauses, we logged and plotted 1 mtr LUX values at DMX 25, 50, 75 100 etc to 250 for each color, for each fixture.

Two issues arose from these measurements.

1: Intensity non-linearity on some single fixtures.
In some fixtures, some channels flat-topped before reaching 100% drive.
This meant that mixed colors changed mixture ratios as the fixture was part-dimmed.

2: Relative intensity differences between fixtures.
This meant, for example, that full red plus 50% blue on fixture A did not produce the same color mixture as fixture B.
These differences were also checked using an Android App called “White Balance”.

The following approach is a suggestion...
Each fixture color channel could have 2 data values stored:
DMX value for Maximum linear output of each color, and
Ratio of intensity of a color channel to the intensity of the reference color (eg red?)

All of this is leading to being able to set the RGBW values for a scene ONCE,
copying the same values to all required fixtures, and the shades produced by all fixtures will all match.

My question is this...

Is there some way of recording and using these calibration values for each fixture so that color mixing is both simple and accurate from fixture to fixture?
(or have I missed the point somewhere ?)


Regards

David

[Jano Svitok]

David,

first, let's make this clear: do you talk about different models or different fixtures of the same model?

second, you say that the intensity of the channel starts from zero, then rises (more or less) linearly up to some DMX value where it reaches max intensity, and then stays there. Graphically it would look like this:

Intensity
| ---
| /
|/
+--------- DMX value

Do I get it correctly?


Last, as far as I know, there's no such functionality in QLC+. I would love to have it,
but for the near future, I don't plan to implement it, since I have now other priorities that I want to implement. If I manage to do it, it might bright some foundations also for this feature.

I mostly dismissed the calibration because specialized measuring instruments are needed, and I don't think people have access to them. If you know of a cheap/DYI way
how we (the users of QLC+) can find out the calibration data for our fixtures,
it may make much more sense to implement calibration. I.e. I could bring my 2 kinds of
fixtures to a local university with a measurement rig, but that would be 2 of 300 types. I think the White Balance app might be a good start.

[Massimo Callegari]

About the linear fading, I noticed that too with chinese fixtures.
One thing we can do is to add fading curves like some logarithmic ones.
This has already been discussed in this forum but I still haven't had the chance to get onto it.

[David Gemmell]

Hi Jano / Massimo

thanks for your replies.

1: I am talking about calibrating for differences between fixtures of the same model.

2: Your little graph is exactly the situation.

Attached is a jpg of values from 2 fixtures, one RGBW PAR, and one RGB wall washer. These are the worst ones of each group.

You can check your own fixtures visually by stepping the DMX value of one colour at a time in increments of 25 and watching the incremental change of brightness.
Any non-linear problems will show up quite easily.
I am guessing, but I will bet this problem is quite common.

3: instruments for calibration are readily available on eBay for as little as $25.
Just do an EBAY search for "LUX METER".
The LUX meter approach is far more accurate than the White Balance method.

4: I am not sure how changing the fading curve will solve this problem?