Primary Colours (RGB , CMYK) and the MacBeth Chart: International Standard Reference Chart.

This is a card mounted colour reference chart produced by Gretag MacBeth. The sales information for the product states:

The Gretag MacBeth colour checker chart is designed as a colour calibration chart. It can be used with colour management software to calibrate the camera in line with the monitor and printer to produce accurate colours.

The internet description above describes the chart. It is an A4 sized cardboard chart. which is photographed alongside an object to allow calibration of equipment.

The 24 squares include the three additive primary colours (the three primary colours of light) - red, green and blue. This is shown below by the yellow marker.

Also included are the three subtractive primary colours which are those of reflected light - cyan, magenta and yellow. These three subtractive colours plus black form the basis for printed process colours - shown by the white marker.

In addition are the specialist colours such as skin tones (top 2 lines of chart above) plus the reference colours for greyscale. Greyscale in printing is a tonal scale from white to black (bottom row of chart)  used to control the quality of both colour and black and white processing.

Primary Colours (RGB)

Primary colours are a set of 3 pure colours from which all other colours can be mixed. The primary colours of light are red, green and blue - the additive primaries.  These additive colours can be mixed to form all other colours in photographic reproduction and display monitors. When all 3 colours overlap in equal amounts you get white; when 2 colours overlap you get cyan, magenta and yellow - hence the term 'additive' as other colours are produced by addition.

RGB Mode.

This is used by all computer monitors, even when displaying another colour mode (e.g. CMYK). Data is converted to RGB for the screen display.

Each pixel in an image is given an intensity value (0 for black up to 255 for white). A vivid blue pixel will have values such as - red 10, green 30, blue 240.

Primary Subtractive Colours (CMY)

These colours are created when the additive colour components are subtracted from white light. Thus if an object reflects blue and green light but absorbs red it will appear cyan - the red colour is 'subtracted'. Objects appear black when all 3 additive colours (red, blue and green) are absorbed.

Cyan, magenta, yellow and black (CMYK) are the basic colours used in printing. [Black is represented by K from the term 'key' plate - a term used in printing technology].

CMYK Mode

This is used in image editing applications when preparing an image for printing. An RGB image is converted to CMYK to create colour separations.

Editing is done in RGB mode and the image converted prior to printing. Some applications allow a CMYK preview  to enable the image to be viewed in CMYK mode whilst available to edit in RGB mode.

When preparing to print images ideally all equipment - camera, monitor and printer are calibrated so that 'what you see is what you get'. Mis-calibration can result in great diappointment when your images are printed - images can appear duller, more vivid or simply 'wrong' when this occurs. This is always a risk with commercial printing of images, and as such the company used must be 'trustworthy'.

Reference:

The Digital photographer's A-Z. Peter Cope. 2002.Thames and Hudson, London. ISBN 0-500-54247-3