Troubleshooting calibration for colour management

by | 12 July 2018 7:30 am

By Ernst Vegt

In the early days of colour management, vendors of calibration software and devices did not always deliver on their promises of producing better colours with greater predictability. Everyone had different approaches to the emerging field, developing their own ‘secret sauce’ in the hope they could corner the market.

Today, however, this is no longer the case. Colour management helps sign companies, print service providers (PSPs), photographers and advertising agencies better control the reproduction of colours from input devices, such as cameras and scanners, to output devices, such as wide-format inkjet printers.

Nevertheless, colour management consultants are frequently called upon by these clients to help create a more sensible workflow and make sure all of the various input and output devices across that workflow behave as they should.

One of the first steps in these cases is to calibrate (i.e. profile or characterize) the computer monitors. Very often, the client will put out an old monitor calibration device and explain it has not proven effective, complaining the images as viewed on-screen appear nothing like the printed output. The question this raises is what target was used for the monitor’s calibration—and the client’s response to this question is usually a puzzled look.

The right settings
Calibrating or profiling a monitor involves setting various parameters, such as luminance (i.e. brightness) and gamma (i.e. tone reproduction curve). There are also options to set the colour temperature of the ‘white point,’ commonly with a range from 5,000 to 6,500 K.

Most professionals in the graphic arts industry set the monitor’s luminance to 120 cd/m², the gamma to 2.2 (now the standard for all monitors) and the white point to between 5,000 and 5,500 K and use its native contrast ratio for the optimal effect. These settings ensure the monitor is not too bright, in comparison to a print or proof, and the tonality is correct, with both delicate highlights and dark shadow details preserved. Greys should appear neutral.

The choice of white point may seem less intuitive, given most monitors appear much whiter at 6,500 K than they do at 5,000 K, but this is actually a bluer white. The colour temperature of the sun, as measured on Earth between mid-morning and mid-afternoon, is around 5,000 K and is considered a perfect basis for viewing. Part of what makes it ideal, of course, is the availability of sunlight all around the world.

Manufacturers of monitor calibration devices, however, have generally neglected to mention and highlight the high importance of controlling one’s ambient lighting to such a white point. The human eye is magnificent at adapting to various lighting conditions, but unfortunately, computer monitors are not. So, if graphics professionals use incandescent lighting for their ambient illumination, any print or proof is going to look too ‘warm’ relative to the image on the monitor; and if they use typical industrial fluorescent tubing, the opposite will be true, with the print or proof appearing too ‘cool.’

The right lighting
Fortunately, it is neither too difficult nor too expensive to resolve this issue by bringing some sunshine indoors. One option is to hang a two-lamp-capacity, 1.2-m (4-ft) long fluorescent lighting fixture about 1 m (3.3 ft) above the monitor, then install a couple of lamps that fit the situation’s needs.

Many lighting stores carry relatively inexpensive lamps that are very close to the colour temperature requirements of 5,000 K, with a colour rendering index (CRI) of 90 or better. A silver ‘egg crate’ grill will keep the colour output of the lamps accurate and control light spill, so as to keep the illumination well-focused over the work area.

A simple—though not entirely accurate—way to test for the correct lighting is with light indicator strips. In daylight or under 5,000 K lighting, little to no variation will be visible on the strips, but under other levels of illumination, two stripes will appear. These are created through the phenomenon called metameric failure.

With metamerism, two separate samples will appear to be the same colour and shade under one light source, but different shades under another source, due to the different shapes of their spectral reflectance curves. A common example of this effect is when one gets dressed in the morning in slacks and socks whose colours match well under incandescent lighting at home, only to appear much less of a match at the office under fluorescent lighting.

The term ‘metamerism’ refers to the difference between the two colours’ reflectance curves. To avoid it, it is best to compare samples under outdoor sunlight or indoor 5,000 K light.

Offset inks and the toners used in colour photocopiers and desktop printers have always suffered from metamerism. Early-model inkjet printers were also prone, but with newer formulations of dyes and pigments, along with the addition of light to mid-greys, the phenomenon has been significantly reduced.

A whole new view
After controlling for ambient lighting, noticeable improvements should be achieved in the way graphic arts professionals view and compare the colours of image files on their monitors in their daily workplace. To improve the situation further, the next steps are (a) to change the monitor’s default appearance to a mid-grey and (b) to paint the surrounding walls light grey, if they are not already. These measures will help prevent image perception and comparison from being unduly influenced by nearby colours.

By way of example, a publishing company’s creative department whose proofs were repeatedly being rejected, despite a significant investment in a commercial-grade viewing booth, eventually realized whenever the sun was shining, someone was opening the window blinds. As a result, the reflectance of a bright yellow building across the street was overpowering the controlled lighting in the viewing area. Once the nature of the problem was identified, it could finally be solved.

When such a viewing area is set up properly and lit by the aforementioned fluorescent luminaires with a colour temperature of 5,000 K, it will be all the easier for large-format graphics professionals to recognize and enjoy the benefits of colour management, seeing their work in a whole new—and better—light.

Ernst Vegt is a principal with Coast Imaging Arts of Comox, B.C., which provides certified G7 colour management and related services to sign companies, print service providers (PSPs), photo labs and other organizations. This article is partly based on a seminar he presented in April at the BC Sign Association’s Sign & Graphics Show in Burnaby, B.C. For more information, visit www.coastimagingarts.com[4] and www.bcsignassociation.com[5].

Source URL: https://www.signmedia.ca/trouble-shooting-calibration-for-colour-management/

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