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It’s all in the lighting

A simple fix for accurate colour printing

July 2, 2024  By Angus Pady

An example of how colours look under bulbs with different colour rendering indexes. Photos © Angus Pady

Over the years I have had the opportunity to visit hundreds of print shops across Canada. I’m usually asked to perform some tasks to help improve colour reproduction. I view colour management as a multi-step process. Each one has an impact and will influence the result. Creating G7 press curves is only one part of the process. Today, I want to discuss lighting because I feel it is one of the most important aspects but is often neglected. People purchase D50 daylight bulbs and believe that should be adequate.

I have personally seen how this plays out. In one shop, I was called back because the press operator said the curves were wrong. He felt everything was coming out too red. I returned, measured the sheets, and they passed G7. But when I went back out to the press and placed the sheet under his lighting it looked red-heavy. We then took the sheet outside to review. Under daylight it looked neutral whereas it seemed too red in the press booth.

In this situation, someone had replaced the tubes in the light booth with 5000k bulbs. Unfortunately, the bulbs did not reproduce colour accurately. One of the reasons for the inaccuracy could be the bulb’s colour rendering index (CRI). A bulb purchased at Home Depot will have a CRI of 70-80 but a unit purchased from the booth manufacturer or a reputable re-lamping source will comply with the required ISO 3664:2009 specifications.


Color rendering index

CRI is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with a natural or standard light source. Light sources with a high CRI are desirable in colour-critical applications such as neonatal care and art restoration. ISO 3664:2009 states that the general CRI of a light source should be 90 or greater.

D50 has similar amounts of red, green, and blue energy, as seen here.

D50 (5000K)

D50 is a near white light source used for visual evaluation in printing, packaging, photographic, and other graphic art industries. It is the specified source in ISO 3664:2009. It has similar amounts of red, green, and blue energy. It neither accentuates nor subdues colour, a prime requirement when viewing press sheets and original images (i.e.photographs) since they usually have many colours within the product to be evaluated.

Another important point is that fluorescent tubes degrade and lose their colour accuracy. This happens around 4000 hours, which is less than a year.

Many factors contribute to a mismatch in colour viewing. It is not the purpose of this article to dive into the intricate and highly technical aspects of colour. I want to highlight that lighting is one of the easiest and least expensive components in the chain to fix.

If your tubes have been in the booth for more than eight months, contact your press manufacturer and order replacements. Tell them you want tubes that conform to ISO 3664:2009. Once they are replaced, print a sticker with the replacement date, and continue to replace every eight months.

Angus Pady is a G7-certified expert that has helped customers resolve colour management challenges for over 30 years. He can be reached at

This article originally appeared in the March/April 2024 issue of PrintAction.

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