1. What light sources should I use for color matching?

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.

2. Why is simulated daylight using a high quality fluorescent lamp better than simulated daylight using a filtered tungsten source?

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.

3. Why do I need an extra UV (Ultra Violet) source?

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.

4. Is UV energy in the daylight source necessary to determine good color matches?

Yes, especially if optical whitening or brightening agents are present, or might be present, in the articles to color match.

5. Are there special electrical requirements for any of the color matching booths offered by GTI Graphic Technology, Inc.?

No, they can be plugged in to any normal outlet with a rating of at least 10 Amps. However, due to the use of electronic ballasts for the fluorescent lamps, they should never be plugged into a battery back-up system (Uninterrupted Power Supply).

6. Why is the booth the Munsell N7 gray? Why not white or black?

Industry standards specify the surround for critical color applications should be Munsell N7 gray. A chromatic color (e.g., Red, Green, Blue) will influence and bias the color perception of the viewer. For this reason, an achromatic color (black, white or gray) surround is needed. Munsell N7 is a medium gray tone, about halfway between black and white. Therefore, it works well for almost all color matching applications. When extremely light colors need to be matched (light pastel colors for instance) a white surround should be used. For very dark samples or for samples with high gloss, a black surround should be used. Color matching booths from GTI Graphic Technology, Inc. have a Munsell N7 surround. When other booth surrounds are needed, GTI offers white and black panel inserts to quickly convert the booth from the Munsell N7 to the appropriate surround.

7. Are light booths available in something other than the Munsell N7 gray color?

Yes. Although color-matching booths from GTI Graphic Technology, Inc. come standard with the Munsell N7 surround, both white and black insert panels are available at reasonable costs. These fit into the booth quickly and easily and are easily removable.

8. Why can two colors match under one light source but not match under a different light source?

This is a common color-matching problem and it is caused by Metamerism. Because colors can be created by using different mixtures of pigments and dyes, the combination of dyes or pigments used will determine how well colors match under different light sources. The greatest effect of metamerism can be seen when viewing a sample under a predominantly Blue source (e.g., D65) and a predominantly Red/Yellow source (e.g., Illuminant A). If the same dyes are used, no metamerism should be seen. If different dyes are used to create the same color, metamerism is possible. When comparing different samples, one colored using dyes and one colored using pigments (evaluating a textile sample next to a paint sample for instance), metamerism is not uncommon. Finding the right “mix” of dyes and pigments is part of the color technologists job. Sometimes, metamerism cannot be eliminated completely, only reduced.

9. Why do colors look differently under different light sources?

Most all samples will appear differently under light sources having different color characteristics. A Blue source will accentuate blue colors and subdue reds and greens. A Red source will accentuate red colors, etc. This is called Inconstance. Two samples can match under different light sources but both still shift in color. Inconstance cannot be eliminated, only reduced using the proper mixture of dyes or pigments. It is very important to know it exists and observe its effect on the color of the sample to see if the inconstancy is overly objectionable. Keep in mind that a determining factor in color is the light source. If the source does not have Red energy, there will be no Red energy for the object to reflect and therefore will not appear Red.

10. Are color viewing booths built to some national or international standard?

Yes. Color viewing booths, used for color matching applications, should conform to ASTM D1729-96 (Society for Testing and Materials, an International orgainzation). This standard specifies the surround color of the booth, the light sources to be used, with their minimum color characteristics required, the light levels that are required on the viewing surface, and procedures. The daylight source, as specified by this standard, must have at least a CIE Publication 51 rating of B/C for critical color matching applications.

For information on ASTM and the D1729-96 standard, please go to the ASTM web site, http://www.astm.org.

11. What does the CIE rating mean?

The C.I.E. (International Commission on Illumination) Publication 51 describes a standardized method to assess daylight simulator quality for colorimetry and color matching. The latest update from 1999 provides a rating system for the daylight simulators. The system is divided into two parts. The first indicates the quality of the source in the visible spectrum (400-700nm). The second indicates the source’s quality in the UV spectrum (300-400nm). An A/A rating is the best and an E/E rating is the worst. A rating of “B” in the visible spectrum will give between a 0.25 and 0.50 CIELAB Delta E visible difference, better than most people can observe. ASTM D1729-96, the standard for visual appraisal of colors and color differences, requires a rating of B/C for critical color differences and appraisal applications.

For information on the CIE and Publication 51, please see the CIE web site, http://www.cie.co.at/cie/.

12. What is CRI?

CRI is the abbreviation for “Color Rendering Index.” It is a method of rating how well colors are rendered by a light source when compared to a theoretically perfect source. A CRI of 100 means it will render colors very well. A CRI of 23 means it will not. For color assessment applications, a minimum CRI of 90 is required. This rating system is becoming replaced by the rating system described in CIE Publication 51.

13. Why isn’t the Color Rendering Index a good method of determining a light source’s ability to reproduce color well?

When it was first developed, CRI was the best method to rate a light source’s ability to render color. Unfortunately, for non-continuous spectrum light sources, such as fluorescent lamps, the method does not accurately rate the source’s color rendering abilities. In fact, a high CRI can be produced by using three narrow spectrum lamps, one Blue, one Green and one Red. Because of this, the more stringent CIE publication 51 method was developed. GTI ColorMatcher® and Graphiclite® lamps achieve a Publication 51 rating of B/C, the rating required for critical color matching applications.

For further information on CRI, see GTI Technote #30 “What is CRI”.

14. What does D50, D65, and D75 mean for daylight sources?

The “D” indicates it is a daylight simulator. The numerical value indicates the color temperature of the lamp (5000K for D50, 6500K for D65, and 7500K for D75).

15. What daylight source should I use - D50, D65, or D75?

It depends on your application. D50 is used in the graphic arts and photographic industries for color assessment of imaging media. D65 is used for color matching applications in the inks, paints, plastics, and textiles/apparel industries. D75 is the old daylight source for these applications but is still used for some specialty applications (color vision testing for example). Most applications are now either D50 or D65. GTI Graphic Technology, Inc. offers lamps in each of these color temperatures, in an assortment of sizes.

16. If the lamp says it is a D50 lamp (or D65 for that matter) are they the same, even from different manufacturers?

NO! Like all products, lamps are manufactured to meet certain needs. For most commercially available D50 and D65 lamps, the manufacturers are trying to produce long production runs and low manufacturing costs to gain the highest profits. Many do not render colors as well as lamps produced specifically for color matching applications. Additionally, they may not emit enough Ultraviolet energy to accurately simulate natural daylight. Make certain lamps have a high CRI value (92 or better) and a CIE Publication 51 Rating of at least B/C.

However, having a high CRI or Publication 51 rating may not be enough. Light booth manufacturers have their lamps formulated specifically to be part of a viewing system. This includes the reflector, the lamps, the diffuser, etc. Slight changes in the formulation can create a slight color cast that will influence color assessment. When lamps from another vendor, even another viewing booth manufacturer, are used in place of the original equipment lamps, the color matching booth may no longer meet the ASTM and CIE Publication 51 ratings specified by the manufacturer. Always use the lamps specifically recommended for the viewing booth you have.

17. Is exposure to the Ultraviolet energy emitted by GTI’s daylight lamps harmful?

No. Scientists have divided Ultraviolet energy into three very distinct regions; UV A, UV B, and UV C. UV A (also known as “Near UV” because of its close proximity to the Visible spectrum) are the wavelengths from 315 to 400 nm. The skin is sensitive to wavelengths below 320 nm. GTI ColorMatcher® and Graphiclite® lamps emit very little energy below 325 nm. UV B and UV C energy are used in medical and biocidal applications.

18. Is “Horizon Light” a daylight source?

Not really. “Horizon” light is described as having a color temperature of approximately 2300K. It was originally considered to simulate daylight just before sunrise and just before sunset. However, this generally accounts for only ten minutes of sunlight each day. It was chosen mostly because it is extremely red in color, one of the extremes of color for color matching applications. It has been replaced by the standardized source, Illuminant A, which is a Red/Yellow source. “Horizon Light” has never been specifically standardized as an illuminant, like Illuminant A, D50, D65, or D75.

19. Is there much of a difference between Horizon and Incandescent Illuminant A?

In color temperature, there is only about 500K difference between them. Compared to D50 and D65, which differ by 1500K, the difference is very small. This is one of the reasons almost all color-matching applications now favor Illuminant A over Horizon.

20. Can people use light boxes from different manufacturers to compare samples and still get good results?

Yes, providing each booth meets the specifications of ASTM D1729-96, has the same surround color (i.e., Munsell N7) and has a CIE Publication 51 rating of B/C or better.

21. My booth has become stained with paints or dye colors after using it. How can I clean it?

For color matching booths from GTI Graphic Technology, Inc., small stains can be cleaned with mild soap and water. A mild solvent can be tried, but test it on a non-critical part of the booth (the underside for instance) to make certain the solvent will not remove any of the paint.

For larger or stubborn stains, a new booth component (e.g., booth floor) can be order and the piece replaced. Another option would be to order a panel insert in the appropriate color, much like the white or black inserts GTI offers.

22. If I cannot clean my booth sufficiently, do I need to get a new booth?

No. Replacement panels or insert panels can be ordered to cover the stained area. As another alternative, GTI offers the Munsell N7 paint in quart and gallon containers that can be applied by brush or roller.

23. Is there some way I can keep my booth from being stained in the future?

Yes. As an optional accessory, GTI offers a clear, optically neutral, polycarbonate floor liner for each of its light booths. It is easy to clean, easy to replace, and resistant to most cleaning solvents.

24. Can aides be used inside the booth to hold samples?

Yes. Some industry specifications and test procedures actually specify the use of such viewing aides. AATCC Evaluation Procedure 9 is one such example. Viewing aides, such as Multi-angle tables and single angle easels, should be the same color as the booth (i.e., Munsell N7), not interfere with the lights by casting shadows on the sample to be viewed, and should be easy to remove. GTI offers such viewing aides in a number of sizes to fit its wide variety of viewing booths.

25. What are the advantages of using such sample aides?

There are a number of advantages. The most notable are that they allow the samples to be presented at predefined angles. Unless the sample is being placed on the booth floor, slight differences in the viewing angle can change the visual perception of the sample. This becomes more of an appearance difference rather than a color difference. Textiles are particularly susceptible to this condition due to their inevitable textures.

26. The GTI ColorGuard II monitor information does not display correctly, what should I do to correct this?

Do not hit the RESET button! Small voltage spikes or fluctuations can cause microprocessor-controlled devices (such as the ColorGuard II) to exhibit minor display anomalies. This is a common problem for companies that use heavy machinery or equipment that use a great deal of electrical power, or in industrial areas of a city. To correct this, simply unplug the unit from the main power, wait five seconds and then plug it in again. If the problem continues to occur, place a surge protecting power strip between the main power outlet and the light booth. DO NOT use an Uninterruptible Power Supply (UPS) or battery back-up device, as is sometimes used with a computer. These can damage the electronic lamp ballasts used in the light booths. If the problem continues to occur after installing the surge protector, call GTI Graphic Technology, Inc. – your unit may need servicing (888-562-7066 Eastern Time).

27. I want to use different lamps in my light booth than the ones that came with it. Can I do this?

Yes, but only with lamps of equal size and wattage ratings. For instance, a T8 lamp should not be used in place of a T12 lamp (the lamp size will be listed on the lamp itself). An example of a lamp code is given below:

28. Can I reset the GTI ColorGuard II to display different light sources than the ones that came with the light booth?

No. This is set at the factory.

29. My CMB light booth never “turns off”. The Red light and the GTI ColorGuard are always on when the rest of the lights are off. Shouldn’t it turn off completely?

No. This indicates that power is being supplied to the unit and it is ready to use. If local safety requirements state that equipment must not have power connected to it when not in use or when the business is closed, insert a surge protecting power strip (not an Uninterruptible Power Supply) between the unit and the main power source.

30. The Hour meter on my MiniMatcher unit only works (accumulates time) when the daylight source is on. Is this normal? Shouldn’t it track booth usage in general?

Yes. The hour meter only tracks the usage of the Daylight source. It is generally considered the most important source for evaluation and color matching applications.

31. My MiniMatcher unit does not have an Hour meter. How do I know when I should relamp the booth?

The simplest way is to determine how much time the booth is used each day then divide that time into the total rated hours for the daylight source (2500 hours). As an example, if the booth is used 2 hours each day (on average) the booth should be relamped in approximately 3.4 years (2500 ÷ 2 = 1250 ÷ 365 = 3.4 years).

32. I have power interruptions in my area. Can I use an Uninterrupted Power Supply (UPS), like I use on my computer, with the light booth?

No! This type of power supply is actually a DC battery with a circuit that imitates AC voltage. It will damage the electronic ballasts used for the fluorescent lamps.