Light Color Temperature

[Garret Diduck]

We used to have the ability to set light colors by temperature. How can we do this now if I don't know the RGB value of a specific K temperature? Is it possible to have this feature added back to nXt?

[Roy Hirshkowitz]

There's a color calculator here, if you must.  Select Calc at the top of the page, then CIE Color Calculator.  Fill in the line marked CCT with the known degrees K of the light source and press the CCT button.  It will fill in the other lines for you.  

Fair warning-- color temp can produce out of range RGBs (> 1 and < 0)-- you'll have to do something with these. I can write a quick calculator that attempts to handle these for you-- but no, it's probably not coming back to the product.  CT is a lousy way to represent modern light fixtures.  What we may work on instead, at some point and only if the demand is there, is broader spectrum rendering where more than three wavelengths are sampled.

[Garret Diduck]

It doesn't sound like you're too keen on CT for lighting so maybe my approach to lighting might be wrong then.

I have a customer who is very insistent on the renderings showing the correct lighting that is specified for the space. I am given the fixture type and use the IES files from the manufacturer. I am also given the bulb type, color range and wattage. I'm typically okay with the light output for incandescent, CF and LED as typically the specified lamp/bulb and the IES files match each other. I always run into issues with fluorescent  fixtures where the IES file and the lamp type specified for the project are different in their color range.

I typically just change the color of the light fixture and render, but the color is purely a guess on my part. So how do I set the color of the lamp correctly then without having to guess?

BTW: my first attempt at using the calculator threw me right out of range.

[Roy Hirshkowitz]

You can't accurately model the spectrum of a light source using the current technology in nXt.  Using an RGB color won't do it, nor will specifying the color using color temperature.  The best you can do currently is guess by slightly altering the color of the source from white to just off-white in a warm or cool direction.  IES files, BTW, do not contain useful spectral information.  I wouldn't represent to your customer that this is something you can model accurately.

The problem of output spectra is further exacerbated by the human visual system, which adapts to color.  For example, if you stand under a sodium lamp for a while, despite a very spiky yellow spectrum, the colors will begin to appear neutral as your "eyes" adapt.  This effect would need to be captured in a tone-operator.

[Garret Diduck]

I understand. The IES file contains the only the photometric information and generally none of the color spectrum information. 

Not a super big deal on my end about the color. I'll just guess at it the best I can. Thanks Roy.

[Roy Hirshkowitz]

NP.  Full spectrum rendering is certainly on the list to explore.