Digital BW, The Print

<austin@d...> wrote:

> > What's a color densitometer? As far as I know, density has nothing to do
> > with color.
> 
> Color is determined by "sensing" density (just like in a scanner or digital
> camera does), through RGB or other color filters, so in that regard,
> determining what color something is, is based on "reading" the density.

Hi, Austin.

You're just on the edge of understanding. Or maybe I'm on the edge of understanding what you're understanding. In any way, let's give it a little push, and someone will go over some sort of edge, somewhere.

A scanner is just like a color densitometer. It has three narrowband filters, so it can measure the reflectance of narrow red, green, and blue bands, and "see" the density of cyan, magenta, and yellow inks. This lets a scanner get the maximum color separation when scanning cyan, magenta, yellow prints, such as color photographs, or publications printed on a CMYK printing press.

A camera is just like a different piece of equipment, a colorimeter. The colorimeter has broader filters. It's red response extends all teh way from deep red to green (the peak is in the red), the green filter extends pretty much across the entire spectrum (but the peak is in the green) and the red and green responses overlap, so a pure spectra orange (for example) produces a reading in both the red and green channels. Blue, similary, covers a large portion of the spectrum, and overlaps the green (and even the red, a bit). The colorimeter responses mimic the spectral responses of the three different color receptors in the human eye, so a colorimeter can tell you what "human visible" color you're measuring.
 
> To answer the original question, for B&W, using an RGB analyzer, each
> channel will give the same value...that is what grayscale is.

Unfortunatly, that's not quite how it works. The color densitometer is designed for maximum color separation if the colors used are produced from cyan, magenta, and yellow dyes. In any other situation (including B&W photographic prints) it will typically not read equal CMY when shown a perfectly neutral gray.