Part 1: Gray
For the first part in this series we'll look at the underdog, gray working spaces. Yes, they are less glamorous than their RGB brethren, but choosing the appropriate space for grayscale images shouldn't be an afterthought. Even if you work exclusively in RGB, you ignore gray spaces at your peril. Plus, they are an easy way for us to start thinking about working spaces more generally.
Simply put, a gray working space is two points and a curve. In profile jargon this is a white point, a black point, and a tone reproduction/response curve or TRC. The white point is generally D50 or D65. The black point is almost always 0. The TRC describes the relationship between tones. It is often a power function, i.e. gamma 1.8, gamma 2.2, etc., but it can also be determined by discrete points or another type of equation. One example of the latter is L*, the lightness component of CIE LAB space which makes the relationship between tones perceptually uniform. Gamma 2.2 is actually a decent approximation of perceptual uniformity (you can see Bruce Lindbloom's analysis here) and mathematically it's less complex.
Until recently, the default gray working space in Adobe Photoshop (Ps) has been "Dot Gain 20%." There are two problems with this. First, unless you are making halftones, you shouldn't use a dot gain based TRC (if you are, consult with your printer before assuming that a TVI of 20% for a 50% tone approximates the press condition). Second, even if you always work in RGB, Ps uses the gray working space to render the channel views for RGB files. So if you do any complex operations like making HSL masks, it's imperative that your RGB and gray working spaces have matching TRCs. The "Dot Gain 20%" TRC does not match either sRGB or gamma 2.2 profiles. Wisely, the default has recently been changed to "Gray Gamma 2.2" (though if your RGB working space is the default, sRGB, you'll still have a slight mismatch).
So, which gray profile should you choose? In most cases you can safely ignore white points and simply choose a profile with a TRC that corresponds to your RGB working space. If you primarily prepare images for the web and your RGB working space is "sRGB," set your gray working space to Adobe's "sGray." If you've changed your RGB working space to "Adobe RGB" then use "Gray Gamma 2.2." Not sure what your RGB working space should be? Don't worry, we'll take a closer look at that in Part 2. If, however, you fall into the minority of users that use the absolute rendering intent or have some other reason to match profile white points, the following table lists common gray profiles along with their characteristics.
|Adobe||Gray Gamma 1.8||D65||γ 1.8|
|Adobe||Gray Gamma 2.2||D65||γ 2.2|
|Apple||Generic Gray Profile||D65||γ 1.8|
|Apple||Generic Gray Gamma 2.2 Profile||D65||γ 2.2|
|Elle Stone||Gray-elle-V2-g10||D50||γ 1.0|
|Elle Stone||Gray-elle-V2-g18||D50||γ 1.8|
|Elle Stone||Gray-elle-V2-g22||D50||γ 2.2|
|Roy Harrington||QTR - Gray Lab||D50||L*|
I'd like to call special attention to Elle Stone's well-crafted profiles. They are publicly available via her website. And unlike other sources, she offers V2 and V4 flavors of every profile. I suggest most users stick with V2 profiles due to broader support, particularly when preparing images for the web.