In the darkroom, solarization (or what is technically called the Sabattier effect) is achieved essentially by re-exposing the print or negative to light during processing. This made parts of the print reversed, turning dark areas light and light areas dark. The first time you create this effect in the darkroom it's very exciting. The first time you do it in the computer it makes you wonder how anyone ever accomplished anything in a darkroom.
There are lots of ways to digitally emulate chemical solarization-not the least of which is Photoshop's Solarize filter. While the principle is the same, the end result definitely isn't. A darkroom-solarized print is usually pretty dramatic. While Photoshop's one-step filter is fine, it's not great. With a few more simple steps, though, the digital effect can be outstanding.
Color photos can be used for solarization, but in terms of recreating a darkroom effect it's black & white pictures that are bound to be most identifiable. So I like to start with grayscale photos, which tend lend themselves to this approach too.
In Photoshop's Layers menu, choose New Fill Layer>Solid Color. The color picker will show a middle gray option, which is the perfect starting point. (You can modify this color with a darker or lighter fill in a later step.) The resulting layer will be totally gray, but adjusting the layer properties from Normal to Difference makes all the difference in the world!
At this point you could stop and call it a day. This solarization is already much better than canned versions, but to really push the envelope don't hesitate to keep going. If you want to adjust the fill color, double-click the solarized layer's thumbnail in the layers palette and choose a different gray from the picker, watching as the picture changes before your eyes.
After adjusting the color it's a good time to tweak the layer opacity to decrease the effect as necessary. This is another great stopping point, but for even more control I like to take it one step further.
Duplicate the original image to the topmost layer and invert it-making the whole image a negative. At this point dark images will look light and vice versa. Using the Magic Wand selection tool with a high tolerance-say 50 or 60-and the Contiguous selection box unchecked, select an area of the most extreme tones in the image (in my sample image, it was the brightest whites I could find). Inverse the selection and delete, leaving only the brightest whites. Adjust the layer properties to Overlay and the layer opacity as necessary to moderate the effect. My favorite reason for this added step is to further boost the appearance of the solarization and to help prevent the photo from appearing too muddy and flat.
Continue to boost the contrast with a simple auto-levels or auto-contrast modification to the topmost layer. Then flatten the entire image and run the auto-contrast again. That extra bit of contrast from the last step can make all the difference in the world, taking your digital solarized shots from passable to amazing-making any darkroom master envious of your skills.