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One of the unique attributes of the Lab Color mode, is that it separates luminance information, from color information. As a result, the Lab Color mode can be helpful in producing a baseline black and white interpretation of an image. Let's take a look at what Lab has to offer. We'll get started by reviewing some of the information contained within our RGB color image. We'll switch to the Channels panel and then I'll click on each of the thumbnails for the Red, Green, and Blue channels. These channels contain information about the relative amounts of each color contained for each pixel. So we can see which pixels have a lot of red information, which have a lot of green information and which have a lot of blue information. Each of these channels, however, represents color information. In the Lab Color mode, Luminance Information and Color Information are separated.
Let's go ahead and convert this image to the Lab Color mode, so we can see what that looks like. I'll simply choose Image > mode Lab Color, from the menu, and the image is converted to the Lab Color mode. As you can see on the Channels panel, now we have a lightness Channel, which contains Luminance Information, as well as A and B channels. The A and B channels contain the amount of color on a blue-yellow axis and a green-magenta axis. But what we're really interested in is the Luminance Information. And as you can see, this represents a pretty good starting point for a black and white image.
To get to this information and be able to use it as the basis of an image, we'll simply split out the channels so we can work with only the lightness information. To do so simply click on the panel pop-up menu at the top right of the channels panel and then choose Split Channels. This will separate our image into three individual images based on the three channels that we had available. We don't need the A or B channels so we can simply close those without saving. As you can see, we now have the Luminance Information separated out as an individual image. Now this image is in the gray mode, or gray scale and we do prefer to work in RGB whenever possible, especially if we're going to add color information in any way at a later date.
So I would choose image mode, RGB for this image in order to convert it to an RGB image. Again, this just gives us the option of working in color down the road. I can then go back to my layers panel and start adding additional adjustment layers or other effects as I see fit in order to optimize the final image. Because the L channel in Lab is focused exclusively on Luminance Information, it very often represents a great starting point for a black and white image. With just a little bit of refinement, a conversion based on the L channel in Lab mode can result in a great image.
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