Understanding Light Sources and Color Management
June 14, 2012
For art reproduction, I’ve put a lot of stock in past posts on using the same lighting that the artist uses in creating the work. Here’s why.
Take a look at these charts showing the output of various light sources on the visible spectrum, from the Olympus Microscopy Resource:
Note a couple of things. The curve of daylight – or Noon Sunlight – is full, and smooth. The curve of tungsten is similarly smooth, but not as full. Every other artificial source is comprised of extreme peaks and what are, essentially, gaps. The white LED curve is the only source that is even remotely close to daylight.
Now, here’s the comparison between what’s now referred to as “full spectrum” fluorescent, speficially, from the NLPIP Lighting Resource Center:
Two conclusions here… first, “full spectrum” is marketing BS, and not supported by the data. Second, fluorescent sources, of whatever type, have dramatic peaks in their output.
Now, let’s consider the process that gets color to our camera. Light falls on our subject. Our subject reflects that light, but, by virtue of the idea of “metamerism” doesn’t reflect necessarily the same wavelengths that fall on it. (Look metamerism up, if you think it’s simply a color shift that your prints make under different light sources. Besides being misleading, in a lot of cases the term is used flat-out wrong.) Our lens takes that reflected light and filters out some of it – even the best lenses – and our sensor filters the light again, breaking it up into R, G, and B values as well as luminance (measured with a second G channel).
The goal of Color Management is to get your colors, after this long trip and the many transitions, to a known, and predictable place. We want stuff we see as red to be red once they get to our computer, then finally to our print. Most importantly, we want a neutral point – gray – that remains gray throughout the process, and maps all out colors from that starting point.
So here’s the process. The artist is working away in the studio, with that nice broad spectrum of daylight streaming in from the North light studio window and skylight. As is often the case in modern studios, there are halogen or tungsten down spots shining on the work too.
The work reflects this broad spectrum in a fairly complete way, but with it’s natural limitations of the pigments being used in the paint. Let’s pick on purple. The pigments may not really reflect the exact wavelengths around that color accurately… but, it will reflect them in a consistent way. And that is what the artist sees.
If you light the work with a source that doesn’t “have” that range of wavelengths, the one around that color purple, for example, how can the work possibly reflect that wavelength? Those gaps in the fluorescent chart are what I’m talking about here. Every peak and gap represents colors that are too strong, or simply not there, in the starting point of your color journey from source to print.
The goal of Color Management is to adjust for those gaps and peaks, but, it can only do so much. In the case of the purple, our system is going to measure our purple patches, see they’re reading as very low, and adjust them by literally adding value in a simple lookup table. It’s not working from actual data, it’s simply looking at some numbers, seeing they’re low, and adding to them to smooth out the curve that it’s looking to build.
Good Color Management, especially camera and scanner input profiles and the RAW processing “profiles” of the X-Rite Passport system can get you consistent results from a consistent light source. It can iron out specific characteristics of your particular lights and lenses and sensors, and bring them into a range of the industry standards. It can’t, however, make a perfect match between a light source that has a full and complete spectrum and one that has dramatic peaks and gaps in it’s spectral response.
For a nice, simple visual demonstration of this, see my post here: Light Sources and Color Spectrum, Simply.
Which begs the question… if you’re trying to reproduce artwork, and want to be faithful to the visual experience of the artist, then why would you start by limiting your palette of illumination? You are, at the very beginning of your process, assuring that you will never have the same colors to work with as the artist had.
Here, for reference, is the NLPIP Lighting Research story, the last page, which has comparison graphs. Read the whole post, it’s very, very interesting.