Backlight Bingo

Printing reflective (conventional opaque, front-lit) graphics is relatively straightforward. Use your favorite substrate and laminate, add a tried and true ICC profile, and you’ll have a pretty good idea how the final print is going to look before it even goes into the printer. Not so with backlit graphics. There are innumerable mysterious and independent variables that can radically alter the outcome of backlits, and trying to find successful fabrication combinations for all of them may seem like a major gamble.

Two printer-independent variables are the type of light box and the kind of lighting coming from it. No matter what the backlit print looks like in your shop, it will appear completely different once mounted into the light box.

In an ideal world the light box would be shipped to the printer at the start of the project. The client would have unlimited time and budgets. The designer, photographer and art director would gather to view numerous test prints and samples until a mutually approved range of color, hue, value, intensity and saturation was reached. Unfortunately, that is just a crazy dream. In the real world, printers have a couple of days to pump out something that is supposed to “blow them all away.”

However, there are ways to improve the odds of creating “instant winner” backlit prints. First, try to get as much information as possible about the type of lighting appliance that the print will be used in. The type of lighting used is the most important issue. Fluorescent lamps are still the most common light source for illuminated signs, but they come in different color casts. There are literally hundreds of different “warmth factors” in the lighting world. The most often used flavors are generically referred to as “cool white”, “warm white” and “daylight”.

Cool white lamps give off a cool, bluish light, while warm whites exude a reddish glow. Daylight replication lamps give a more accurate rendition of the tones of sunlight, but are also more expensive — and therefore less often utilized by sign manufacturers. LEDs are becoming more commonplace. They also have their own unique color signature, as does neon. The death of neon has been talked about since the invention of the fluorescent lamp, but it’s still going strong and may show up as a light source in a sign box on any project, especially if the box has an unusual, non-rectangular shape.

Each of these lighting types will radically change the colors of the most carefully crafted backlit print and must be compensated for in the printing process. If the lights are cool, add warmth to the print and vice versa.

Photoshop image editing software is a favorite tool of many color experts. The Curves function is often used for color correction. Color-corrected curves for each lightbox type can be saved and applied to other files. (Use of some other Photoshop features may result in loss of pixel information.) The amount of color compensation necessary for each of these lighting types can be calibrated by using a baseline print that looks good in daylight, then altering it until it looks appropriate when backlit by each of these lighting types. Save the compensation information and apply it as needed to backlit print projects.
The amount of light transmitted by the fixture is also a key component. This light saturation is controlled by a variety of factors. The distance between the lamps and the print is one component. Fluorescent light boxes are available in models of 4", 6" and 8" thicknesses. The deeper the light box, the farther the lamps are away from the graphic and the more diffused the lighting is. The closer the image to the light source, the more light is transmitted through the print. Sometimes this also calls for extra diffusers to avoid “hot spots” (see illustration). LEDs and edge lighting techniques have made much slimmer light boxes a reality, and these usually have reduced light levels, which makes printing easier.

As more light passes through backlit graphics, the print needs to have a higher density to compensate. A print that looks great on the printer can easily look washed out when placed behind a strong light source. Different types of backlit media allow for varying amounts of ink saturation. Some types of printers and RIPs allow for print saturation levels to be controlled, but the media will have its own limits. Print a variety of samples using different media and settings to discover and record which of these processes will be best for each type of light box.

Often, light boxes are only backlit at night due to strong daylight conditions. A print that is intentionally dark and dense to compensate for strong backlighting will look terrible in a frontlit application. To avoid this problem, print the graphic twice using non-backlit print density with a thin translucent white backing between the two print layers. When the backlighting is off, only the top layer can be seen because the white inner layer blocks the second print. When the box is lit, the light shines through all layers, giving it the added density that backlighting requires.

The type of printer, inks and media being used also makes a radical difference in the appearance of the final print. Digital photo process equipment like Lambda and LightJet have the most scope for adjustment. Prints from these machines tend to have the most brilliant backlit qualities. They can easily be adjusted to almost any density and color range, but are not perfect. The prints fade rapidly, are not water-resistant and can be expensive. Dye-based aqueous inkjet inks look good and are inexpensive, but they also fade rapidly and are not waterproof. Solvent inkjet inks are inexpensive, waterproof and much more fade resistant, but they don’t backlight as well. Each type and brand of printing media and laminate will also give prints a unique color cast. Different applications will need different usage of each of these processes and medias.

The only way to make sense of all of these variables is to create a chart. Make a list all of the types of light boxes and lighting your clients use. Test numerous media and laminates and select a few “standards”. Quantify the color and density variations for each light box type. Fill in the spaces on the chart. This is a virtually foolproof way to print backlit graphics in a wide variety of circumstances.

Mitigating all of these mysterious, independent variables for every backlit graphic may seem like playing a game of chance with diminishing odds, but controlling processes and quantifying all of the factors involved can greatly increase the chances of getting a “bingo” on every card.