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It is generally acknowledged that LED lighting is the path for backlighting (see your TV, computer, tablet or phone if you don't believe it). Problems emerge when backlighting is moved from the realm of electronics to print and packaging.

Using light is a clear growth market in the signage and POP/POS display community. But entering this market - and being successful - takes more than just flipping a switch.

How Are You Going to Succeed? Through Understanding
Understanding LED color specification and visual rendering is the starting point.

Corrected Color Temperature (CCT) is now commonly specified and controlled, and the color temperature of any light source or lighted panel dictates the perceived color and display brightness. CCT is expressed in degrees Kelvin, abbreviated K. The white LED CCT ranges from a warm yellow 2000K to a cold blue 6000K, and most often, nearly pure white (5600K+) light sources are selected for lighting signage. Major LED makers are even offering white LEDs specifically tailored to market and lighting locations within stores. The colors are given names like Meat (2000K), Bread (2400K), Vegetables (5100K) and Fresh (5400K).

Color temperature can be measured by using a handheld meter, such as a Sekonic C-500R meter. Readings can be taken in seconds with the push of a button. This can be helpful with verification and sample references. That said, meter readings are affected by proximity to the light source, the surrounding materials (reflective, color, etc.) and the ambient light conditions. This generally prevents a highly repeatable result.

Color LEDs present a specific wavelength. For example, green is 515 nanometers. This is a very narrow color, not a Pantone/PMS defined color. Colored LEDs present a very tight wavelength rather than a spectrum, so the use of colored LEDs limits the available colors. The narrow light spectrum may produce unexpected results - this is why white LEDs are so important to printers (see Figure 1).

Color Rendering Index
Color Rendering Index (CRI) is a more subjective and subtle specification. CRI is a scale from 0 to 100. It indicates how accurately a defined light source renders color when compared to a halogen light source, and measures an LED's capacity for producing the correct color appearance. The score is calculated by measuring the average difference in chromaticities between the LED and the CRI sample colors (See Figure 2). The lower the average difference, the higher the CRI score. A high CRI number is like a high grade point average for color quality.

An LED with a fixed 5600K CCT will produce a very different print appearance with a low CRI light source. Many early LEDs focused only on color and lumen output. The color rendering was ignored and thus the appearance of lighted printed material suffered.

Most LEDs score in the 80s (a solid B average). Fluorescent has CRI down in the 70 range. There are also significant color shift and brightness change issues over the life of a fluorescent light source. This puts LEDs at a distinct advantage for longer life uses.

Red is not only a very popular color with brands, but it is the hardest color to render accurately. So much so that an extra set of color samples (R9-R14, See Figure 2) are added to the color evaluation, even though they do not impact the reported CRI. Note that the use of these samples in any color creation, calibration and photography is crucial.

So Where is Your Solution?
No LED manufacturer can produce uniform, exact and specific colors in their white LEDs. Instead, they produce batches of LEDs and sort the output into bins, which are specified by each manufacturer. The light emitted by LEDs is characterized by color and brightness and then sorted into tightly defined bins that group finished LEDs of similar brightness (lumens or mcd) and color (K) together to ensure visual consistency. Select an LED supplier that reliably produces consistent product with a very tight binning process. This is your best path to producing repeatable results and correct color during production runs of printed overlay. White chromaticity "xy" coordinates create very tight color definitions, and top-quality LED manufacturers use sorting bins based on CIE127-2007 specifications. Luminous intensity binning is also done to this specification.

There are variables outside the control of the manufacturer that also affect the light seen by the viewer. The physical temperature of the LED and the amount of current applied will each cause a shift in the light output and color. For example, the same display mounted inside a freezer may look different from one mounted as an aisle violator.

If the display is battery operated, the color of the LEDs will change slightly as the batteries reach end of life. It does take some time for a LED light source to stabilize, but this time is generally quite fast and has significantly less change than other lighting technologies. Fluorescent lighting will have significant and uncontrolled color change and light output over their life. Fluorescent lights also operate in a much narrower temperature range with significantly shorter life. With LED lighting, there is significantly less change over time. LEDs are rated for 50,000 hours (five-plus years) of continuous operation with only minor color or light output degradation. They will continue to operate for much longer.

What's Coming?
As we try to produce beautiful displays and signage, the key for all of us is the addition of a violet component to the existing blue/yellow spectrum used to create white. This addition will make the current generation of LEDs for signage and print applications obsolete. The CRI will exceed 95, allowing LED light to mimic the broad spectrum color and even sunlight - the best light for the human eye.

Additionally, Professor Shuji Nakamura, Global Energy and Nobel Prize winner, has created a company named SORAA that is dedicated to GaN-on-GaN diode production for LEDs. This exciting new process will begin producing LED chips in a couple of years. The expectation is a five- to 10-time increase in brightness.

These future advances will further cement LED lighting and backlighting onto the mercury-free and ever-challenging marketplace.

Showing Off
By understanding CCT, CRI rendering score, brightness and binning techniques employed by LED makers, your projects can really shine.

Pictures and simulations cannot accurately represent the response of the human eye to light due to the varied nature of vision. Simulations, GIFs, point-and-shoot cameras or phones cannot replicate the human visual experience. The true path to understanding visual response to a lighting system is prototyping. Working with established LED lighting experts and using stable suppliers allows your expertise to show through.

As printers and consumers, you already know that the printing media, inks, print technology and techniques are vital elements to creating excellent solutions. Lighting brings forward your knowledge in opacity, translucence, dead-front methods and blocking light. The skill and art you bring to the technology discussion for LED, or any backlighting project, will separate you from the big-box print shop down the road or the internet-based competitors. The excellent service you already offer will make the needed proofs, prototypes and demonstrations possible, and adding the element of lighting to your skill set can move you ahead of your current peers.

This article appeared in the SGIA Journal, September / October 2015 Issue and is reprinted with permission. Copyright 2015 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.