Colour Correction

Perception & Measurement

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Right The X-Rite Digital Swatchbook feeds data to its own program called Colorshop. Here you can find best matches from the Pantone system, check out your profiles or measure density, metamerism, Lab values and a host of others.

Initially we will concentrate on Lab colour. This is the underlying colour system used by Photoshop (regardless of the mode your image – both a CMYK and an RGB image will be worked as Lab values in the background by Photoshop). When it comes to send the colour to your screen, Photoshop works out the voltages to apply to the red, green and blue electron guns at the back of your monitor. Conversely, if you send the image to your printer, Photoshop works out how much cyan, magenta, yellow and black inks to spray onto the page. The fact that these two methods of making colour are so radically different is the source of much of the grief you get with imaging and reproduction. In Lab colour, the L value is the lightness, a is the red-greeness and b is the yellow-blueness (see the diagrams in the call-out box). Hence skin has Lab values of 66L, 14a and 15b. This indicates that the brightness is 2/3 of the way towards fully bright white (66% is almost 2/3 of 100%) and that the other predominant colours are a weak red and weak yellow. Negative values of a indicate greens, negative values of b indicate blues. Hence a colour with values of zero for both a and b is a neutral grey, of a density determined by its Lightness value.

In order to understand colour better, scientists plot colours on graphs. This is like a map and just as you can measure how far two places are apart on a map, you can measure how much two colours are different on a graph. Just as you can look at a map and say Denbigh is west of Liverpool, you can say that magenta is more towards a red than a cyan.

Crucially, the eye’s ability to judge colour varies depending upon what the colour is. You have a hard time deciding if a deep black has a blue bias or a green bias for example. It is easier to spot the difference between two reds than between two deep blues if they are different by the same amount (i.e. the same distance apart on the graph). Sadly for monchrome enthusiasts, the eye is most sensitive when spotting the difference between light grey values. This is why you are rarely satisfied with your grey scale from a 6-colour printer. Not only does the printer have a hard time getting the colour right, your eye spots the errors with great ease!

Be aware that Lab space is very big and that colours can be out of CMYK gamut and RGB gamut. If you examine the histograms of the a and b channels in Lab mode you will find only a small portion of the -128 to +128 scale is actually in use. This also means that small adjustments can be leveraged to produce quite large visual jumps in your image – so beware.

In the composite of histograms the top left is a full range RGB image (green channel), the top right is that same image converted to CMYK (cyan channel). The two middle histograms are the CMYK converted to Lab. The bottom two histograms are those from the panorama landscape showing a very narrow Lab range of values.

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Photo Quote: There are no rules and regulations for perfect composition. If there Were we would be able to put all the information into a computer and Would come out with a masterpiece. We know that's impossible. You have To compose by the seat of your pants. - Arnold Newman