Summary

This report had two objectives: on the one side, the aim was to discuss the most suitable RGB colour space for newspaper printing and, on the other side, to examine which RGB colour space should be assumed in the unfortunately common case of pictures with a missing RGB specification. The solution to both questions from a mathematical and physical standpoint appears to be simple, but from a technical and practical standpoint complex and to the contrary. The choice of the most suitable RGB colour space was done using a strategy of picking the one with the least scientific anomalies. Regarding the problem of pictures with a missing RGB definition, a pragmatic approach was adopted. A problem of a missing overview of the complex subject of colorimetry and with it the closely associated picture capture-, picture processing- and print processes was also addressed. Recommendations: Seven golden rules of RGB A set of instructions that results from the conclusions of this report as well as the acquired knowledge of the most frequent questions and procedures can be summarised in a set of RGB rules converted into the language of the practitioner: 1. Basically, for all RGB pictures to be printed, care must be taken to include the RGB colour space specification in a standardised form. The most suitable way is the embedding of an ICC2 colour profile, in which all the colour values are defined relative to the CIE XYZ tristimulus values system. An original RGB picture without an exact colour space definition cannot basically be expected to reproduce with accurate colour. 2. The original RGB colour space of a digital picture should be kept as much and as long as possible, as after any and every RGB conversion errors can result because of colour space limitations and digitisation. 3. Should the necessity for a conversion into a different colour space arise anyway (RGB to RGB), it is to be recommended that, instead of conversion of the RGB colour values of the picture, an external profile be attached. 4. If possible, for scanned picture data for newspapers and commercial printing, the colour space (profile) of the scanner should be used. 5. For digital camera pictures, use Adobe RGB (1998), which is clearly larger than the alternative sRGB. 6. If the RGB colour space of a digital picture is unknown, choose a RGB space by visual evaluation. 7. If, after a visual evaluation, when no fitting RGB colour space can be found, choose Adobe RGB (1998). Details follow for the individual process steps, illustrated here for a Canon 1D Mk.II camera and Adobe Photoshop CS2, version 9.0: 1. Before taking photographs, the camera colour mode should be set to Adobe RGB (1998). 2. For the picture processing in Photoshop (we refer to Adobe Photoshop CS2, version 9.0), we recommend the following procedure: > In “Color Settings” in Photoshop select “Adobe RGB (1998)” as the RGB Working Space. > As a picture is opened in Photoshop, it recognises and accepts an embedded profile (e.g. of the digital camera), with the selection of the radio button “Use embedded profile”. > In the case that no embedded camera profile is available, “Leave as is (don’t colour manage)”. > Afterwards, using the menu function “Edit/Assign Profile/Profile (button)”, the RGB profile that delivers the best results should be chosen. > While saving a processed image, the appropriate profile should be embedded.