Large-format print production includes Colour Management to convert the digitally defined colours from the print job into printed colour. In the previous newsletter we presented a method for ink savings by building printer profiles. Using low maximum ink coverage and heavy black generation will lower the ink consumption.
In this article we clarify the impact of the most used colour conversion methods: ‘Perceptual’ and ‘Relative Colorimetric’ colour conversion. The conversion from FOGRA39 print job colour data into a large-format print on a Dibond sandwich plate is evaluated.
Fig. 1. Example of Colour Management settings in a print workflow: an ICC-conversion method is set for each colour type (RGB, CMYK, Gray).
‘Perceptual’ Conversion
Ink reduction is mainly achieved by conversion of dark colours. Using the FESPA Colour label methodology, we can view the effects of ‘perceptual’ rendering of dark colours (represented by colours with a lower Lightness value).
Fig. 2. ‘Perceptual’ rendering FOGRA39 colours into print on a Dibond sandwich plate: Dark original colours are transformed into lighter Dibond printed colours (with lower colour strength).
What is the impact of a lower maximum ink coverage when using the ‘Perceptual’ conversion method? For this purpose, several variants of Dibond printer profiles were created. These profiles were all set up with maximum black generation, but with lower maximum ink coverages (300%, 280%, 270%… up to 200%). A set of dark FOGRA39 test colours were converted with the different Dibond printer profiles. After each conversion, the colour differences between with the starting profile without ink reduction (350% maximum ink coverage) were calculated and represented by categories.
Fig. 3. The impact of ink reduction on colour quality when using ‘Perceptual’ conversion: a maximum ink coverage of 250% shows no single colour difference, compared with the 350%-profile. In the range from 240% - 220% maximum ink coverage, the colour differences could be acceptable (dE00 <2, only green and purple bars). Below 210% colour differences become visible (dE00 > 2, cyan and orange bars).
‘Relative Colorimetric’ Conversion
The Fespa Colour Label approach shows the differences of both conversion methods.
Fig. 4. Characteristic of ‘Relative Colorimetric’ conversion. This conversion method preserves common colours between FOGRA39 and the Dibond print: chroma and lightness of the printed colour are closer to the original.
‘Relative Colorimetric’ conversion by printer profiles with lower maximum ink coverage appears to be a lot more critical:
Fig. 5. A maximum ink coverage of 250% is the critical threshold when using Black Point Compensation
‘Relative Colorimetric’ conversion is critical for shadow image detail. This conversion retains common colours, but this has a negative effect if the print process cannot print as black as FOGRA39 offset-black. In this case, shadow details from the incoming data ends up ‘flattened’, with loss of shadow detail.
Fig. 6. In print processes where offset-black is not achieved ‘Perceptual’ conversion keeps and lightens the shadow detail, ‘Relative Colorimetric’ conversion flattens all shadow detail.
Nevertheless, a major advantage of ‘Relative Colorimetric’ conversion remains the maximum preservation of colour, it would be a shame if this were lost due to loss of detail in dark areas. For this purpose, Adobe has added an algorithm to Colour Management that resolves this issue.
Fig. 7. The check box in Adobe Photoshop to enable the ‘Black Point Compensation’ algorithm.
The characteristic of ‘Relative Colorimetric’ with the Black Point Compensation algorithm enabled.
Fig. 8. Characteristic of ‘Relative Colorimetric’ with the ‘Black Point Compensation’ algorithm enabled. Dark colours are converted in a similar manner as ‘Perceptual’, which preserves shadow detail.
If we take the ink consumption into account, ‘Relative Colorimetric’ with the ‘Black Point Compensation’ algorithm enabled, behaves similar as ‘Perceptual’.
Fig. 9. The impact of ink reduction on colour quality with ‘Relative Colorimetric’ with the ‘Black Point Compensation’ algorithm enabled.
Conclusions
The colour conversion method has a significant effect on ink reduction. Total ink coverage in converting a set of dark colours differs, even with print profiles without ink reduction (350%)!
Colour conversion by lower maximum ink coverage – printer profiles, shows an achievable 220% limit for ‘Perceptual’ and ‘Relative Colorimetric’ + ‘Black Point Compensation’ conversion.
‘Relative Colorimetric’ remains the most critical method: control of the printers’ black point in relation to the incoming data is essential to avoid quality issues. But if correctly used, this conversion can provide the most accurate colour results!
