Thursday, February 4, 2016

Evaluating Non-Continuous Light Sources With A Color Temperature Meter*

*Or Venturing Down The Rabbit Hole Where Things Are Not As They Seem. 

I recently took some time to adjust the daylight pre-sets on my Zylight IS3Cs to match them closer to my daylight Creamsource LEDs.  Every gaffer knows you can’t trust your color temp meter with non-continuous sources.  What I found surprising was the extreme difference in readings between 2 daylight LED lights.

I’ve seldom use my Zylights and the Creamsources at the same time.  When I do, it’s immediately obvious the pre-sets for Zylights do not match the Creamsources.  I wanted my Zylight daylight pre-set to match my daylight Creamsources.

If you are not familiar with the Zylight IS3Cs, these are strange and awesome light panels.  What makes them strange is each individual LED on the panel is actually a four color RGBW emitter.  This causes strange color fringing.  Using a chimera will eliminate this.  Of course the farther the head is from the subject, the less this becomes a problem.

The awesome features of the Zylight IS3Cs are adjustable light balance (color temp.), adjustable color correction (green/magenta), and a full RGB mode for colors.

My color temperature meter is a Minolta Color Meter IIIF.  I bought it new years ago.  It is a great meter that has been super reliable and accurate, but accuracy is a relative thing.  The first thing any color temp meter manual will tell you is this; a color temp meter is not accurate with light sources that have a non-continuous spectrum, or a spectrum with large spikes.  A film or video test is the only accurate way to evaluate these kinds of sources.

For a gaffer, the first time I learned this years ago, I was thinking “What!  That’s the whole reason I wanted a color temp meter!”

Of course, with experience, a gaffer learns a great deal about evaluating sources using the color temp meter as a starting point.

So back to the Zylight and the Creamsource:  on the left is a Zylight IS3C pre-set daylight with chimera and internal baffle only; and on the right, a Creamsource Mini daylight spot:
Clearly to the camera, and more so to the eye, the lights are not color matched, but they are similar.  That is not what the color temp meter says.  The readings are surprisingly differently; not even close to each other.  The Mini’s are in the ballpark for 55K.  The Zylight’s reading don’t make any sense at all.  Clearly, the meter is being confused by spikes and gaps in the spectrums of the LEDs.

Here, I have adjusted the Zylight and matched it to the Mini by eye:
I couldn’t tell any difference between the 2 sources to my eye.  The camera sees a subtle difference between the two; maybe a little more magenta in the Zylight on the left.   Again the color temp meter is unable to accurately read the Zylight.  Though the Zylight's numbers are now closer to the Mini's, the readings are still surprisingly different.

As an experiment, I tried to adjust the Zylight’s color to match the Creamsource Mini using the color temp meter.  Of course this isn’t going to work:
I added all the blue and green possible to Zylight in the white light mode.  Even then, I was unable to match the color temp meters readings of the Mini.  The numbers are closer, but clearly, the color temp meter is taking things the wrong way.  The light is a strange turquoise color.  My color temp meter wanted me to add even more blue and green.

I’m pleased with the new daylight pre-set for the Zylights.  It closely matches my Creamsources.  I would never be able to absolutely match the 2 lights source.  Though my color temp meter is unable to accurately read the lights, it does tell me there are differences in the spectrums of the 2 lights.  I cannot gel color into a source with gaps in the spectrums, or accurately control sharp spikes.  It is not fully correctable, but it is manageable to where the differences are not obvious or seen.

The best we can ever do in any situation where we are dealing with different lights with non-continuous spectrums is to make the difference manageable and not obvious--when possible.  Some lights have such poor spectrums, that correction is not possible.

Some postscripts:  This experiment with color temp meter readings is not a criticism or evaluation of either light.  Both are quality professional lighting instruments.

On my wish list is a Sekonic C-700 color temp meter.  I love all the features of it.  Graphing the spectrums of the Zylight and Creamsource, would have been interesting with this test.  I almost bought a Sekonic last year, but I bought another Creamsource Doppio instead!

Lastly, I’ve found test pictures with a still camera to be invaluable over the years.  Curiously, the eye can be deceived when looking at a subject directly, but not when looking at a still picture (or a monitor).  Subtle dynamic difference often become obvious looking at a still picture.  The same is true for color.  Subtle color differences often become obvious looking at a still picture, or a monitor.

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