Optical whitening or “brightening” agents are used extensively in the paints, plastics, and textiles/apparel industries to brighten colors and make whites appear whiter. They work by absorbing energy in the UV region of the spectrum and re-emitting that energy in the visible (usually Blue) region of the spectrum. Anyone that has seen a white shirt “glow” under a “black light” has seen the effect very clearly. The brightening agents take advantage of the naturally occurring Ultraviolet energy in sunlight to create the effect. The difference is not always easy to see in natural daylight due to the brightness of the daylight source. By using a separate UV source, with a higher UV output than a daylight source would normally produce, it is easier to visually detect the presence of the whitening agents.

If comparing visual evaluations to data measured with a spectrophotometer where the results are not correlating, it may be that the instrument’s source has no UV energy or the sample was measured with UV excluded. In this case, the visual and measured data would be different if the sample was highly fluorescing. Having a separate UV source in the viewing booth would allow the observer to see this discrepancy very quickly. Unless the sample was then measured with UV included, the observer would know that the data will never match the visual evaluation for this sample.

Fluorescent daylight technology has made great advances over the past 15 years, and GTI Graphic Technology, Inc. has actually helped this along. The main advantages of a high quality fluorescent daylight source are:

• They will include UV energy in sufficient amounts for the simulation of daylight (filtered tungsten requires the addition of a separate UV lamp to provide enough UV energy to properly simulate daylight).
• They do not produce excessive amounts of heat (filtered tungsten produces over 5000 BTU’s of heat).
• They do not use filters (filters fade or change color requiring periodic replacement);
• Because they do not use filters, proper intensity at the correct color temperature is easier to achieve with fluorescent daylight.
• They consume less power.
• They do not require special cooling or ventilation systems to stabilize the lamp color temperature (filtered tungsten must keep lamps within a tight temperature range or the color output will change).
• They do not require high amperage circuits (20 AMP or more), which are expensive to install. They will run on a normal 10 AMP circuit.

But not all fluorescent daylight lamps are the same. GTI Graphic Technology, Inc. is one of the few suppliers of lamps that meet the industry requirements for color matching applications.

This depends on your application needs. Generally, if the industry you are working in, or the company itself, has a standard practice, that should be followed. The standard ASTM D1729-96 is a good starting point. If this is not available or you are developing your own procedures, determining which light sources to use can be very logical. Other times, it takes a bit of digging to determine which is the best source to use.

For most applications, a Daylight source (D50, D65, or D75) is used as the Primary source for color matching, since Daylight is a big part of our lives. Daylight sources generally have higher amounts of Blue energy. D50 (or “Equal Energy Daylight”) is used almost exclusively for graphic arts and photographic color matching applications. D65 (Average North Sky Daylight) is now the most common daylight source for all other applications, such as paints, plastics, textiles, and food. D75 (Noontime North Sky Daylight) was the preferred Daylight source up to 10 years ago, when it changed to D65 to correspond to the source most commonly found in color measurement instruments, D65. Some applications still require the use of D75.

The next most logical source is Incandescent. Since most people have this type of light source in their homes, it becomes a logical Secondary source. CIE has defined a standard Incandescent source, Illuminant A. It’s color temperature is characterized as 2856K. Incandescent sources generally have higher amounts of Red and Yellow energy. The Incandescent “Home Light” source used in all GTI Graphic Technology, Inc. color matching booths is Illuminant A.

Old color matching booths only had two light sources, one with high amounts of Red/Yellow energy and one with high amounts of Blue energy. If samples matched under these sources, they should match across the spectrum. With the development of fluorescent light sources, additional lamps could be added to aid in the color matching process, and increase confidence in the match.

The third logical source would then be one that emits higher amounts of Green energy. Cool White Fluorescent is such a source. Additionally, Cool White Fluorescent is found in many businesses and retail stores throughout the world. It has become the third commonly found light source. Cool White Fluorescent has a color temperature of approximately 4100K.

Additional light sources can also be used to meet the requirements of an application. For packaging and printed display applications, which are both Point of Purchase (store) and printing applications, the use of a D65 and a D50 source would be recommended. In some large retail establishments, the Ultralume 30 fluorescent lamp is common. This lamp has a color temperature of approximately 3000K, but less energy is needed to power it than Cool White Fluorescent. For large retail chains (which also have large electric bills when you add up all of the stores) the cost savings can run into the millions of dollars. Because it is used to light the store, the Ultralume 30 lamp might be a good choice as the fourth source to match under. Some companies require it to be the first or Primary source to use for color matching! Again, this is very logical since customers will be viewing the products they may purchase under that lamp. Another popular fluorescent lamp for store applications is the TL84 lamp. It also produces large quantities of light for less cost.

There are a host of other lamps available. Examples are Warm White Fluorescent or WWF (3000K), TL83 (3000K), TL835 (3500K) and many more. The ones listed here are the most commonly found lamps.