Creating & Printing Greyscale from RGB Images
Basically, there are two places where your RGB image can be converted into greyscale: You can accomplish the conversion in an image-editing program such as Photoshop, or you can convert your image to greyscale through the printing process. Typically, performing the conversion in Photoshop will provide you with more control, and probably with results more to your liking. However and especially with the newer enhanced tonal- and gamut-range printers, knowing about and being able to take advantage of your printing device’s greyscale output capabilities is just as important as knowing how to convert your image in Photoshop. In fact, you’ll always want to coordinate any RGB-to-greyscale conversion you accomplish in Photoshop with how you intend to print your greyscale image. Here are several methods for converting your RGB colour images and printing them in greyscale.
1. Print RGB to greyscale using the standard print dialog
Using this method, all you do is select File>Print from within your application - in this case Photoshop. If you’re printing to a basic printing device, such as a simple desktop inkjet printer, you’d simply click the Black Ink mode. If you’re printing to a black-and-white laser printer, no ink selection is required.
This is the simplest method for printing a greyscale image, but the one that provides you with the least amount of control and typically the least satisfactory results.
2. Convert RGB to greyscale using the mode menu in Photoshop
Here, we’ll open the RGB image in Photoshop and choose Image>Mode>Greyscale. When we use this method, we can at least see the results of the RGB-to-greyscale conversion of our images before printing them, but this way provides little control over the process. Sometimes you’ll luck out and achieve a good quality greyscale image, but typically this method produces a low contrast, unsatisfactory result.
3. This time, we'll select one of the RGB Channels
Use the Channels Palette (Window>Channels) to view our greyscale image. With this method, you have three choices. Click on a channel to make it active. Try all three of the channels to see which is best, then go under the Image menu and choose Mode>Greyscale to delete the other two channels.
Note: For a variation on this method, you can convert your RGB image to Lab Colour mode and then select the L channel to create your greyscale image.
4. Channel Mixing
Another way to use channels to make a greyscale image is to combine them in varying percentages using the Channel Mixer (Image>Adjustments>Channel Mixer). This dialog allows you to assign percentages of each RGB channel that will be used in creating your greyscale image. Make sure that the Monochrome box is checked to change your Output Channel to Grey. Once again, we’ll finish by selecting Image>Mode>Greyscale to create a single channel greyscale image.
This method (Image>Calculations) provides you with even more control than the Channel Mixer. Using the dialog, you can merge any two channels from any currently open images, in various blending modes, and at a specified opacity. In Calculations you can also apply alpha channel masks to protect various portions from the blending. (A word of caution: You could spend the rest of your adult life in this dialog experimenting with all the possible variations!)
Note: These five methods above are by no means the only ways of creating greyscale images from RGB colour images in Photoshop.
For the five methods above, we assumed that we’d be printing with one black ink-either with a black-and-white laser printer or using only the black ink of a CMYK print device. If, however, you intend to print your images on a commercial printing press, you have the option of creating greyscale images in multitonal modes, such as duotones and quadtones. Creating multitonal images allows you to print your greyscale images with two or more inks, providing greater tonal variation in depth. Here’s a brief setup tutorial to help get you started.
6. Commercial Print Duotones
Start by creating the greyscale version of the image that you desire using one of the methods above, or your own favourite greyscale creation technique. Then make a copy of that greyscale image and follow these steps:
Step One – Go to the Image menu and choose Mode>Duotone to activate the Duotone Options dialog. For Type, choose Duotone.
Step Two – Click the load button to locate and apply one of the black and grey Pantone duotone inks found in the Adobe Photoshop Presets: Duotones: Grey/Black Duotones folder.
Notice how a second grey ink is loaded, along with two curves to control the application of the two inks during printing. You can also create neutral grey tritone and quadtones in this same fashion.
7. Inkjet Duotones
Until very recently, we could only create duotone greyscale images by sending our image to commercial printing companies, because they were the only ones who had the capability of printing these additional inks.