Color Rendering for Digital Print

We are going to demystify Perceptual Color, and CRI (Color Rendering Index) in terms that should lead to good decisions for color-matching in your digital print workflow. We are talking about the front end – not the back end color calibration. In other words, why what you see looks so different depending on who is looking at the color. Just kidding – sort of.

First, let’s look at what is complex and confusing, and then clear things up.

A famous photographer, Paul Outerbridge (1896 – 1958) once explained:


"What is color? No object of itself alone has color.

We know that even the most brightly colored object, if taken into total darkness, loses its color. Therefore, if an object is dependent upon light for color, color must be a property of light.

And so it is."

Take a look at the chart below right showing the output of various light sources on the visible spectrum, from the Olympus Microscopy Resource:

Note a couple of things. The curve of daylight – or Noon Sunlight – is full, and smooth. The curve of tungsten is similarly smooth, but not as full. Every other artificial source is comprised of extreme peaks and what are, essentially, gaps. The white LED curve is the only source that is even remotely close to daylight.

Spectra-of-Common-Light

When you go to Home Depot or Lowes or other hardware supply warehouse, the rows of bulbs always include those bright fluorescent bulbs, with the “temperature” listed in Kelvin (K). You are probably looking for 5000K for your shop, as that is said to be more like daylight. You may even spend a little extra for “Daylight” or “Full Spectrum” bulbs. More power to you! But this is not the whole spectrum of what you need to look for (pun intended).

More of the technical definitions:
A 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 an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as neonatal care, photography and cinematography.It is defined by the International Commission on Illumination (CIE) as follows:

Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.

Why CRI is MVP

5000k-light-differences

Look at two bulbs, their light output curves. BOTH are 5000K color temperature (bright and cool white). BUT, one is missing a whole lot of the spectrum. If missing yellow, then the object loses some yellow. If missing purple, then it loses purple. Not cool.

When Color Matters.... Go Shopping

So what should we do when it comes to figuring out if that “gray” color is blue-ish, red-ish, or brown-ish? You KNOW what I’m talking about. What type of light bulb is closest to sunlight?

Halogen bulbs are a variation of incandescent. They give the closest approximation of natural daylight, known as “white light.” Colors appear sharper under halogen light and the bulbs can be dimmed. They’re a little more energy efficient than incandescent bulbs, but are more expensive and burn at a higher temperature. They may be impractical in your shop. You have some viable choices – one will make your whole shop POP and colors vibrant. The other is a professional tool that provides many, many benefits throughout the shop.

Option One: You CAN shop for bulbs that list a high CRI. Think of it this way: If sunlight has a CRI of 100 – this is 100% perfect color rendering. A CRI of 90 or better is 90% of perfect. At Bulbs.com, a site the specializes in one thing (guess what), they list 13 bulbs with a CRI of 90 or greater. Most are at 90 CRI, but one has a spec of 98 CRI, a Philips bulb. Other high CRI bulbs include GE and Sylvania. Replace and dispose of your bulbs (PROPERLY).

Not in the trash!

Fluorescent-improper-disposal

Proper Disposal of Old Bulbs

If your business has 10 or more employees, owns or maintains a building at least 3,000 square feet or meets any of the other three described in the Mercury Disposal Control Act, it is illegal to throw away fluorescent light bulbs or other mercury-added disposal projects. Contact your waste management company for your local disposal procedure.

Option Two: Purchase a 3M Sun Gun. Many things make the 3M™ PPS™ SUN GUN™ Color Matching Light a unique and powerful tool in your shop. You can use it to determine if the customer is crazy or if you thought your gray tone was really gray, not lavender. The tool can be used anywhere in the shop, it is cordless and powerful. Specs include 4700K light, CRI (Color Rendering Index) = 99.35%. The Bulb is virtually identical to natural daylight. It will significantly reduce the chance of metamerism occurring and provide an exceptional representation of what the job will look like outside (in natural daylight).

3M-Sun-Gun-Large

It will set you back between $500 and $600, but is an excellent option if you don’t want to change all of your overhead bulbs. But if seeing is BELIEVING, then worth every penny.

Q. Will it work outside on a cloudy day?
A. Yes, the cloudier the better… It will help you see metallic and other subtle differences in color that you simply can’t see on a cloudy, rainy or dark stormy day.