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Getting good results when printing with backlit material can be challenging. Backlit materials are some of the most difficult media to profile.
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Have you ever worked with backlit material? If so, then you know that it can be difficult to get your images to look the way you want. Here are some tips to help you handle working with this type of material.
FICKLE LIGHT INTERACTION
Backlit images are used in many situations where eye-catching image reproduction is desired. They are eye-catching because by placing diffused light behind an image on printed media, a much greater dynamic range is possible than for a reflective image.
However, getting good results when printing with backlit material can be challenging. Backlit materials are some of the most difficult media to profile for two reasons—color measurement and ink application. Figure 1 helps to illustrate some of these difficulties.
Two general layers of a printed surface are depicted in the simplified drawing in Figure 1—a colorant (ink) layer and a media layer, or base. The ink layer can be composed using several different methods, each of which will affect absorption differently.
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Figure 1. In this illustration we can see the ways in which light can interact with a printed surface. The color we see on a surface depends on the wavelengths of light reflecting off the surface.
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On media that absorbs the ink (or where the ink infuses itself into the media), the ink layer becomes part of the media layer where the ink has been absorbed. In cases where the ink cannot be absorbed into the media, a special coating may be added to improve ink absorption. This is especially true with backlit media since most usually do not absorb ink very well. (Note: when printing with UV inks the colorant isn’t absorbed into the media—instead the ink layer is formed by the ink adhering to the surface of the media using a curing process).
The color we see on a surface depends on the wavelengths of light reflecting off the surface. These are generally the wavelengths of light that are not absorbed by the object. Figure 1 also shows various ways in which light can interact with a printed surface:
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You can achieve greater density with a backlit without using more carrier by greatly reducing—or eliminating—any light inks (e.g. light cyan, light magenta, light gray). Additionally, you may need to reduce the yellow ink plus any high-fidelity ink such as red, green, blue and orange.
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• Specular Reflectance (gloss)—Light bounces off the surface at the opposite angle unchanged.
• Absorption—Light enters the surface, bounces around, and is absorbed by either the ink or the media layers—thus raising the energy level (i.e. thermal temperature) of the surface.
• Reflectance—Light enters the surface, bounces around, and eventually leaves via the same surface unchanged at an arbitrary angle.
• Transmission—Light enters the surface, bounces around, and eventually leaves via the opposite surface unchanged at an arbitrary angle.
• Fluorescence—Light enters the surface, bounces around, is absorbed and then re-emitted at a lower energy level (with a longer wavelength), bounces around again, and eventually leaves via either surface at an arbitrary angle.
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Figure 2: Measuring spectral reflectance vs. measuring spectral transmission on backlit media. Properly measuring color on a backlit swatch requires a color measurement device that measures transmission rather than reflectance.
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The important difference to note between reflectance and transmission is from which surface the light leaves. For backlit media the primary interaction of light that needs to be considered is transmission. This is especially important when measuring color with backlit media. Figure 2 shows the difference between measuring spectral reflectance and spectral transmission of backlit media.
MEASURING REFLECTANCE
If a spectral reflectance measurement device is used to measure color of a backlit swatch (e.g. with an X-Rite i1 spectrophotometer), be sure to account for the aspect of transmission by placing a white backing material behind the swatch (causing the transmitted light to be reflected back through the media). The measurement device will shine light onto the surface and measure light reflecting off the surface.
There are two basic problems with using a spectral reflectance measurement device with backlit media that can result in unpredictable output:
• The backing material also will absorb some of the light, thus distorting the measurement of the white point as well as potentially spoiling the gray balance.
• The light can end up passing through the ink twice, thus resulting in darker measurements than are warranted for transmission. This can result in severe posterization as well as an overall skewed gamut.
In essence a spectral reflectance measurement device is measuring the color of a different viewing condition from what is intended, when using a backlit media. The measured color doesn’t necessarily have much to do with what we see in an actual backlit condition.
To properly measure color on a backlit swatch requires a color measurement device that measures transmission rather than reflectance. These devices place a light source on one side of the media and only measure the light that is transmitted through the media. Two common devices that are used to measure light transmission are the Barbieri LFP and the X-Rite DTP-41T spectrophotometers. Using a device that measures transmission (rather than reflection) is strongly recommended to get a good estimation of the color you will see on backlit media.
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For the best backlit displays, use a spectrophotometer that measures transmission, use the target inks and media in your color measurement, and shoot for the highest ink density possible without generating negative side-effects.
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ACHIEVING PROPER DENSITY
Aside from getting good measurements, achieving enough color density when printing onto backlit media can also become a significant issue. This is because the light only passes through the ink layer once. As a result, more ink is needed to achieve good density.
However, this can be a problem because the ink layer itself may have limitations regarding the amount of ink that the media will absorb without creating bleeding problems or other adverse image quality issues.
Ink is made up of colorants (dye or pigment) and a liquid carrier (water or some type of solvent). One way to increase color density involves using more colorant without using more carrier. This can be accomplished by greatly reducing—or eliminating—any light inks (e.g. light cyan, light magenta, light gray). Additionally, you may need to reduce the amount of yellow ink used; plus any high-fidelity inks such as red, green, blue and/or orange that would further introduce additional liquid carrier into the ink layer.
If you still find bleed issues, you can reduce cyan and magenta ink amounts, but try to keep the potential for reds and blues as high as possible without bleeding. The last place you will want to perform an ink reduction on backlit is the black (K) ink channel. However, attaining a solid black with back lighting is typically the most challenging part of transmissive materials, so reduce the black channel only as a last resort.
Another way of obtaining higher ink density is to use a printer that allows for more ink to be printed than would normally be used. For example:
• Some printers include special features that allow for higher-than-normal ink coverage using multiple strikes of ink to increase ink density.
• Some printers include multiple ink layers.
• Some printers enable mixing multiple K sources such as Matte K and Photo K
Remember that any increase in overall ink load may cause ink bleed or some other image quality degradation. Be mindful of the image characteristics that will be printed and carefully consider them while creating the profile.
CONCLUSION
For the best backlit displays, be sure to:
• Use a spectrophotometer that measures transmission.
• Use the target inks and media in your color measurement.
• Achieve the highest ink density possible without creating negative side-effects for your prints.
Being able to achieve high-density backlit images enables a greater dynamic range to be represented. When you look at a backlit image of a city skyline at night, the city lights pop out while also maintaining the darkness on the night sky. This extra dynamic range can mean the difference between a simple image and a stunning one.
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