Converting Color to Grayscale

There are several ways to convert color images to black-and-white, but you should know that not every method gives you good print results. Depending on the images you’re working with and the type of print job you’re designing, the following may not be the best conversion methods, because they all require a bit of tweaking afterward. They are best used for images that are to be printed at a very small size, or for images used in a layered Photoshop composition. For highquality black-and-white print layouts and designs, the best conversion option is to either apply a new Black And White adjustment layer or use the Channel Mixer.

Convert to Grayscale Mode Choosing Image --> Mode --> Grayscale is the quickest and easiest way to convert a color image to black-and-white—but it’s certainly not the best. When switching to Grayscale mode, you’re telling Photoshop to go ahead and apply a standard formula for mixing the Red, Green, and Blue channels together. This generally results in a flat, low-contrast black-and-white image that is not balanced enough for high-quality print layouts or designs.
 
Desaturate To strip the color from every pixel in an image, choose Image --> Adjustments --> Desaturate, or press F+Shift+U / Ctrl+Shift+U . This is the same as moving the Saturation slider in the Hue/Saturation dialog box all the way to zero. After applying the command, the image is still RGB. Converting to Grayscale mode after applying the command retains more midtone information than if you had made the conversion without desaturating first.
 
Select a Channel You can also convert an image to black-and-white by using Channels. Preview the RGB channels by clicking on each one in the Channels palette, and select the one with the best contrast. This is usually the Green channel, which contains the most detail. Then choose Image --> Mode --> Grayscale to delete the other channels in the image. Or if you prefer, you can also copy/paste or choose Duplicate Channel from the Channels palette menu to save the converted image in a new document.
 
Gradient Map By choosing Image --> Adjustments --> Gradient Map, you can map a black-towhite linear gradient (default Foreground to Background colors) to a color image. This usually results in a fairly good high-contrast black-and-white image (good for newsprint layouts), but tends to darken the shadow areas a bit too much and blows out the highlights. To accommodate for this, you can lighten the shadow areas and darken the highlights by making either a Levels or Curves adjustment. Note that you can also apply a gradient map and the subsequent Levels or Curves adjustments as adjustment layers. If you choose to use adjustment layers, make sure that when you are through correcting the image, you make a flattened duplicate by choosing  Image --> Duplicate and enable the Duplicate Merged Layers Only option in the dialog box. Doing so retains the original image with the adjustment layers. To complete the grayscale conversion on the flattened duplicate image, choose Image --> Mode --> Grayscale.
 
Lab Conversion You can also convert a color image to black-and-white based on the Lightness channel in Lab color mode. Choose Image --> Mode --> Lab Color, and then delete both the a and b color channels in the Channels palette. This results in an accurate rendering of luminance values, but the image also lacks some contrast. Deleting the a and b channels automatically puts the image in Multichannel mode. Choose Image --> Mode --> Grayscale to complete the conversion.
 
Luminance Map It is also possible to make a grayscale conversion based on an image’s luminance map. To make a luminance map selection, F/Ctrl+click on the composite color channel icon (RGB or CMYK) in the Channels palette. Then choose Select --> Save Selection. In the dialog box that appears, choose to save the selection as a new file. The new file is automatically placed in Multichannel mode. Choose Image --> Mode --> Grayscale to complete the conversion.

What Is Lab Color Mode?
RGB and CMYK modes are both device specific, meaning that they don’t actually describe a color. An RGB or CMYK specification is like a set of instructions that your output device, such as a monitor, ink-jet printer, or printing press, refers to in order to produce color. Different output devices can interpret the same specifications differently. This is why color made up of the same RGB values can appear different on multiple monitors, or the same CMYK build can print differently on various printers. Lab color is device independent, meaning that it describes a color’s appearance rather than the components that make it up (RGB or CMYK). Images saved in Lab mode depend on you, or Photoshop, or your color management software to decide what RGB or CMYK values are necessary to display color on a chosen output device.

Lab images contain three channels: the Lightness channel, plus the a* (a-star) and b* (b-star) channels. The Lightness channel represents the luminance values available in the image. The a* channel represents how red or green a color is (negative values represent green, and positive values represent red). The b* channel represents how blue or yellow the color is (negative values represent blue, and positive values represent yellow).