Non-Photo Blue (or Non-Repro Blue) is a common tool used in the graphic design and print industry. It is a particular color of blue that will not be detected by Graphic Arts Cameras. This allows layout editors to write notes to the printer on the print flat (the image which is to be photographed and sent to print) which will not show in the final form. As well, it allows artists to lay down sketch lines without the need to erase after inking.
With digital scanning and image manipulation, non-photo blue fulfills its function in a different way. The artist can do his sketch and inking in the traditional method and scan the page. Most scanners will detect the light blue lines. However by shifting to greyscale and increasing the contrast and brightness the blue will disappear. Depending on the scanner and settings this exact process may be different for each user, but the concept remains the same. The difference between the non-photo blue and black ink is vast enough that digital image manipulation can separate the two easily, ensuring non-photo blue a place in the world of artistry for some time to come.