by Mike McNamee Published 01/02/2012
Monochrome remains popular both with professional and the enthusiast sectors as well as the public at large. It should not have escaped your attention that our Photographer of the Year won with a mono image and that mono images were well represented in the winning selections. It is timely, therefore, that we split this printer review into two parts and that the mono bit comes in now - purely coincidence guv'nor!
Mono printing remains challenging for both inkjet and process printing, and manufacturers have sought various ways to overcome the potential short-comings. On the inkjet, in order to obtain a smooth structure to the image you have to use as many nozzles and colours as you possibly can. Traditionally, a neutral grey is made up from a mix of cyan, magenta and yellow inks with the addition of black to support the darker tones and remove any tendency for the 'mixed-colour grey' to produce a muddy, brown tone. A by-product is the ability to reduce the amount of ink used so that the paper is not flooded with ink - simplistically a 100% each of cyan, magenta and yellow adds a 300% ink loading, this can be replaced by 100% black for nominally the same effect thus reducing the total ink load. In reality the situation is more complicated. Colour inks are reduced by one of two processes, GCR-Grey Component Replacement or UCR-Under Colour Removal, but that is only part of the story as there are light magenta, light cyan, a bunch of light blacks and a few greens, oranges and blues to account for on inkjet systems! The best we can say at this point is that the ability to bring additional colours to bear on the problem allows the scientists and engineers to improve the rendition of an image with more 'degrees of freedom' solving a number of conflicting issues simultaneously.
The desire to achieve a truly neutral grey is driven by the fact that the human eye is very sensitive to shifts from a neutral - this is the 'colour' range in which we have the most discrimination. On top of this is the ability of the eye at different light levels. In the dark tones we cannot discriminate if we cannot even see them too well, at the highest brightness levels we see everything as blinding white! In between is the more comfortable lighting level and the mid tones that we are so sensitive to. Given that the 'strong colourants' are cyan and magenta, they have the greatest ability to cause colour shifts and so the ability to replace them right down the tone scale has obvious advantages for the monochrome print. The is the most common solution from inkjet makers. Cyan and magenta are reduced in the lighter tones with lightcyan and light-magenta and then further aided by a variety of light greys to support both the smoothness of the structure and depth of neutral tones. When printing in 'greyscale mode' cyan and magenta might be eliminated altogether. The flexibility of creating the same colour from a variety of inks also allows the colour engineers to optimise the metamerism in a print (ie reduce it), here yellow is traditionally regarded as a bad actor and some RIP solutions have minimised yellow in a mono output to reduce the slightly higher metamerism of a pure pigment ink.
What makes a good mono?
Assuming that you have high-impact content and good composition (a matter of personal preference) how does a monochrome rendering add to the experience? The important criteria are the paper surface, the paper-base tone, the smoothness of tonal transitions, the depth of the deepest blacks, the discrimination of those blacks in the deep shadow area and finally the rendering of the highlight tones. To this must be added the 'colour' effects, associated with multi-ink mono printing, grey neutrality and metamerism.
Paper The choice of paper, its surface structure and base tone are personal choices. However, because the white and near-whites in an image are made up predominantly of paper tones with a small amount of ink, it is vital that the contribution of the ink matches the colour of the paper. In a nutshell, if the paper is creamy then the inks in the very light greys should also be creamy if distortions are to be avoided down the tone scale. Put another way, a cream paper benefits from a warmer ink tone and a cooler, bluer paper needs a cooler, bluer ink tone. This match up is called 'mapping' and we refer to the colour gamut being 'mapped to the paper white'. Paper white is a complex metric (and its measurement) and so we have pulled the description out into a call out, if you failed your GCSE physics you might want to turn those pages!
Black Black separation is an important criteria for a quality mono print (or colour for that matter!). Successive dark tones of the image should be distinctly separated in the print. This is actually the purpose of 'Black Point Compensation' in your printer driver. BPC looks up the maximum black that a printer/ink combination can achieve (from its icc profile) and then shuffles all the blacks from that point upwards toward the highlights. The most likely effect of not using BPC when printing is blacks clogged up from about 50 RGB points downwards. A slightly bizarre outcome of this can be that a print has more punch if BPC is turned off, specifically if it contains no important shadow detail.
In our experience, shadows made from a decent profile block up at around 15-20 RGB points. This is a visual assessment made in a 2,000 lux viewing booth, in normal domestic room light the shadows would appear blocked up earlier - maybe at 35 RGB points depending upon room brightness.
In their colour auditing utility, ColorCheck-online (www.colourcheckonline.de), demand that there is a 1% difference between successive dark patches at 15, 10, 5 and 0%. In other words a 1% difference in print patches that are 5% different in the image file, an allowable compression of 5x. This is both achievable and sensible. When using very soft, absorbent papers it is possible for the surface to become so saturated with ink that it 'crystallises' on the surface, starts to reflect more and the density reading starts to climb again, so it is not a given that the black density will proceed downwards in an orderly fashion!
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