Here at the British Library we digitise a lot of our collections to make them accessible to a greater audience, while protecting the item from over-handling. Digital images are a fantastic resource for scholars allowing users to spend as long as they like looking at details. But have you ever wondered how accurate the colour representations are? Is what you see on the screen the same as what you print out? We use digital microscopy on some printed material to show you that what you think is red, may not be… and why you should be careful in using printed images as replicas of originals.

Figure 1: Standard office printers use the CMYK colour space. 

Figure 2: Printers often call for replacements of cyan, magenta, yellow and black cartridge replacements.

If you work in an office you may have noticed the printer demanding for the cyan cartridge to be replaced. Seems like an odd colour choice – why not red, green or blue (RGB)? Most printers use the colour space CMYK (cyan, magenta, yellow, and black) for print output, and like all colour spaces uses a manipulation of these to generate other colours.

What’s a colour space?

A colour space is a combination of a colour model (such as the RGB model) and a mapping to that model called a gamut. It is essentially the range of colours that a monitor can display or a printer can print. Digital cameras and scanning devices create images using combinations of the three primary colours red, green and blue, and TV screens and computer monitors display images in the same way. The three colours are combined to create variations in hue measured on a scale of 0-255. For example R255, G255, B255 creates white, while R0, G0, B0 creates black. You may have come across similar numbers in Photoshop when the mouse is held over a certain colour pixel.

Very few images are captured as originals in the CMYK colour space as the files have only a third of the information that an RGB image has. This reduction in file size makes CMYK ideal for printing as it improves network speeds reducing the delay between the user clicking print and actually collecting the print out. Images captured in RGB must be converted to CMYK in order to print. A reduced file size means there is a slight colour change upon printing as there are colours in the RGB colour space that are out of the CMYK gamut.

The CMYK colour space

Below is an example of how our eyes are deceived by colour combinations. Figure 3 shows a print-out of a folio from a manuscript. When this image was printed onto a white A4 sheet it was examined in closer detail using a high resolution digital microscope. In Figure 4 the page has been magnified twenty times and we can start to make out a pattern in the background.

Figure 3: Original image onscreen.

Figure 4: Printed page at 20x magnification.

At two hundred times magnification we can see that what we think are different colours are in fact various overlapping dots of cyan, magenta, yellow and black.

Figure 5: CMYK dots at 200x magnification.

CMYK colour space works in the opposite way to the additive RGB colour space. In CMYK space the process is subtractive and inks subtract brightness from white (cyan, magenta, yellow and black are known as subtractive primaries) by layering cyan, magenta, yellow and black to achieve a certain hue.

 

Figure 6: Cyan, magenta and yellow lie directly opposite red green and blue on the colour wheel. (CC-BY-SA-3.0 author DanPMK)

So how do we interpret these tiny dots to be different colours? This is the printing technique known as half-toning where tiny dots of each colour in the colour space are printed in a pattern small enough that we humans see it as a solid colour. In a print-out where there is a region of pale blue as in Figure 7, what we are actually looking at is a half-tone print pattern of cyan dots. More cyan dots would make the contrast against the white background appear more saturated, and we would perceive this to be dark blue.

Figure 7: A region of blue is represented by a higher frequency of cyan dots.
 

Much of the printed material around us is a result of half-tone printing and has the same CMYK pattern when examined at high magnification. Take another example using this standard business card.

Figure 8: Standard printed business card.

Below are various magnifications of the card showing how the "red" colour is built up using layers of CMYK ink.

Figure 9: Business card at 50x magnification.

Figure 10: 100x magnification.

Figure 11: 200x magnification of "red" printed ink.

Figure 12: 200x magnification of black CMYK printed ink.

The controlled conversion between the colour representations of various devices is called colour management with a primary goal of obtaining good matches. This is why sometimes you see colour cards in digitised material. Converting images into different colour spaces is used as an imaging technique to reveal hidden or faded information, and we’ll be posting about that later.

As an educator you may rely on print-outs of paintings or illuminated manuscripts as learning materials, so don’t be disillusioned if your students aren’t impressed; it must be the printer! What a great excuse to visit our free Sir John Ritblat Gallery: Treasures of the British Library at the BL and experience the real thing.

Christina Duffy (@DuffyChristina)
Imaging Scientist