Colored Stone Light Source
Can the colored stone industry choose a standard light source from the multitude on the market?
11 Minute Read
Can the colored stone industry choose a standard light source from the multitude on the market? What does a worker sorting colored gems in a shack in Sri Lanka have in common with the salesperson in a tony Manhattan jewelry showroom? In both settings, the type of light they use will make the difference in the color and the cash value of their gems.
Color is the most important aspect of a colored stone, and differences in lighting will alter the way a stone's color is perceived by grader, dealer, and, ultimately, customer. When lighting sources are inconsistent, it can cost dealers thousands of dollars. At the same time, tricks of the trade that enhance certain colors for sale can cost the industry in terms of credibility.
That is why many involved with colored stones are calling for a lighting standard. "Your industry's crying for help because they have no standard available," said Nick Lena, product marketing manager for lighting manufacturer GretagMacbeth, New Windsor, New York, who has spoken to various groups on the issue.
"I was totally amazed at some of the horror stories I heard," said Lena . Gemstone dealers lose thousands of dollars alone in airplane fare traveling back and forth because the stone grading is incorrect, he said. "People who have never been color tested, in a dimly lit shack in a Third World country, are making color decisions" that determine whether their companies make or lose money.
"I have a partner in Tanzania and a cutting factory in Sri Lanka and an office in Santa Barbara …. There are too many disagreements on the color grading of what I receive and what I send off," said Dana Schorr, owner of gem wholesale business Schorr Marketing & Sales, who is an outspoken advocate of a lighting standard.
Another problem occurs when dealers use light to enhance the color of a stone to a degree approaching fraudulence. "There are people in the [American Gem Trade Association] shows that artificially enhance stones with light that will make them look good. I've seen people selling rubies with red light," said Schorr. "A colored gemstone's value is based on color. A ruby that's purplish has less value than [one that's] red. The change in value is tremendous."
Why has the colored stone industry lagged behind other industries, including textiles, plastics, printing, and automotive, in developing a lighting standard?
"One reason we don't have a standard is we have a fractured trade," suggested Schorr. "It has to come from a coalition in the trade. We have to work with scientific experts…. Once we have lights that really do the work, we go before the American Society of Standards and they set up a scientific committee."
The one thing that everyone does agree on is that the ideal lighting would be the one that most closely replicates daylight. Daylight is defined as the white light caused by a mixture of yellowish sunlight scattered from a bluish, moderately overcast sky.
However, even this simple stipulated point opens a Pandora's Box. Which daylight? Early morning or noon ? Is that noon in Bangkok or Manhattan ? And is it a clear blue sky, or the polluted skies over most major cities?
"We've agreed that indirect north, noon daylight is the best light, and is considered standard," said Schorr. "However, the spectral output of the sun changes from day to day, even moment to moment. You have this floating point" that is influenced by pollutants, moisture in the atmosphere, latitude, and time of year.
Natural, or white, daylight is composed of an approximately equal mixture of all the wavelengths in the visible color spectrum. An object gives the effect of color by absorbing certain wavelengths and reflecting others.
The gemstone industry generally uses two standard methods to quantify how light affects color: Color Temperature (CT), a convenient, one number method expressed in degrees Kelvin, and Spectral Power Distribution (SPD), which breaks a light source into its component colors and then measures the amount of light energy present for each color.
The SPD is represented graphically by a curve. Natural daylight will appear as a more or less even line, whereas most artificial light sources register peak and valleys where they are strong or weak in a particular color. A spike in red energy, for example, means that that light source enhances reds.
The CT scale relates the color of light emitted from an object to its temperature, measured in degrees Kelvin. "Think about heating up a nail," explained Philip Bradfield, Ph.D., vice president of light manufacturer Tailored Lighting Inc. "First, the color is deep red. Add more energy, it goes from red to orange, to yellow, to white. If you keep going, and it doesn't melt beyond white hot, the light would look a bluish hue."
Bradfield outlined the Kelvin scale: 2300K is horizon light, with warm orange tones; 4100K is a cool white fluorescent color; 4700K to 5700K is direct midday sunlight; and over 10,000K is a clear blue sky in the shade.
Color temperature is used in determining the Color Rendering Index (CRI), a commonly used measure of how well a light source reproduces a given standard light. Lamps with a CT below 5000K are compared to a tungsten filament (incandescent) lamp, while lamps above 5000K are compared to a correlated phase of daylight. The maximum rating is 100; lamps with a rating of 90 or above are considered good for color evaluation.
Approaching the Ideal
Remember when the jeweler would lead his customer over to the shop window to get a better look at her gemstone?
Today, gems are graded and displayed under any number of brandname lamps, which can be incandescent (including tungsten halogen and metal halide) or fluorescent. Every manufacturer maintains that his is as close as possible to the ideal of natural daylight. The choice is the dealer's.
The variety of light sources used by gem sellers can be surprising. "When I went to the Tucson [show], I saw hand held UV [ultraviolet] lamps, big fluorescent fixtures, desk lamps, and people selling fluorescent lamps you could find at Home Depot," said Lena . "Because I'm a scientist, I spent most of my time laughing. . . . It was kind of sad, and it was kind of comical."
Different light sources have different effects on the color of gemstones, either enhancing or dimming the true color. But although the effect of the various types of light on color is predictable, opinions on the best light to use for color grading vary.
The most neutral light, and the optimal level for grading colored stones and diamonds, is from 5000K to 5500K in color temperature, said gemologist Howard Rubin, creator of the GemDialogue System, a system for grading color using 21 transparent color charts. When you approach 6000K and over, the light source becomes too blue.
Achieving that level is more important than using a particular brand, he said, because lighting manufacturers combine different elements to get the balance of color. Rubin did recommend fluorescent lights in the 5000K to 5500K range for grading; halogen desk lamps "get too hot," he said. However, halogen lamps "come close to neutral and are used nicely in showcases. There's nothing wrong with using them to display merchandise."
"Not all daylight bulbs are truly daylight, because some have more yellow, a little more red," said Shannon DeMulin, a diamond specialist with Rio Grande in Albuquerque , New Mexico . "You could have two lights, look at a ruby under one and it looks beautiful, and under the other and it looks icky." DeMulin said her company uses General Electric daylight corrected bulbs to grade colored stones and diamonds. She also stressed that overhead lighting, not just the desk bulb, should be color corrected.
Incandescent lights, which are defined by the presence of filaments, have a yellowish cast because they have more energy in the warm (red orange yellow) end of the spectrum. Incandescent light will enhance yellow sapphires, for instance.
Tungsten halogen, a type of incandescent light, is typically used in many jewelry stores. The advantage is that it simulates home lighting and, because it is a point source, will show light reflected from all facets of the jewel. While the light will greatly enhance yellow red, it is deficient in blue-green, making those stones appear subdued.
Metal halide is a popular light source that produces a whiter light than the warmer and more yellow halogens. Lena criticizes it as lighting, however, because it has a "very high level of ultraviolet energy" with multiple spikes in green and orange on the SPD curve and little red energy.
Fluorescent lamps are low pressure mercury vapor lights with fluorescing coating, or phosphors. Fluorescent lamps with fewer than three phosphors will have spikes and depressions in the SPD; more phosphor coatings are added to eliminate color exaggeration.
At Stuller Settings in Lafayette , Louisiana , the gemstone department is satisfied with the 5000K to 5500K Philips fluorescent bulb they have been trying out.
"It's more color corrected and closer to daylight than a regular daylight bulb," said Joe Orlando, department director and buyer. "No incandescent or halogen replicates [daylight] as closely as the fluorescent."
At trade shows, Stuller uses standard fluorescent lighting in showcases and special halogens overhead at daylight temperatures for presentation. For display of alexandrite, which is either red or green depending on the light source, they use a special display box that alternates between fluorescent and incandescent light.
There are a number of companies which market lighting systems specifically to the gemstone industry, each of which claim to imitate daylight.
One popular model is the halogenbased SoLux daylight lamp, manufactured by Tailored Lighting Inc. The SoLux is a light source with a special filtering approach, available in four CTs: approximately 3000K, 3500K, 4100K, and 4700K. The 4100K, the whitest light, is suitable for colored gems because it is closest to daylight, said vice president Bradfield.
SoLux replicates natural daylight "as closely as possible," for an indoor light source, said Kevin McGuire, company president. "We're replicating to adjust to indoors with the shifting from rods to cones. . . . Our approach is it's best to have a man made source because it is constant as you use different amounts of light. [And] it's best to have a daylight replication because it presents all the colors equally."
Ott Lite Technology manufactures a filtered fluorescent light based on a "proprietary formulation" of multiple phosphors to replicate the optimal balance of the color spectrum seen in natural daylight.
"The problem with tungsten halogen is it's very hot," said Ott Lite President Fred Mendelsohn. "Anything very hot has to be high in infrared, which is high yellow and high red."
"Halogen by itself provides the sparkle but doesn't provide the true colors," agreed Lena of GretagMacbeth, which manufactures a filtered tungsten halogen desk lamp called SolSource in CTs of 5000K, 6500K, and 7500K, which is designed to provide color balance approximating daylight.
GretagMacbeth also manufactures the SpectraLite Viewing Booth, which has options for filtered tungsten halogen, incandescent, fluorescent, and ultraviolet light, and the Judge II, a small fluorescent viewing booth.
"In general, I'm skeptical about fluorescent lamps because major lamp manufacturers love to make high efficiency fluorescent daylight sources that are not suitable for viewing colors," said Lena .
He explained that fluorescent lights tend to be very weak in red energy, while they are good at rendering blues and greens. So while a fluorescent light may have a high CRI especially at temperatures of less than 5000K, where it is being compared to an incandescent light rather than daylight its effectiveness at rendering blues and greens "cancel out" its poor rendering of reds.
Looking for a Standard
What are the things to consider for an industry wide lighting standard?
Lena , who is consulting with Schorr on developing such a standard, said that companies will need a directional point source, a neutral surrounding, and light levels higher than the typical jewelry store. Personnel should also be required to be color vision tested, he added.
Neither would the industry have to "reinvent the wheel," he pointed out, referring to existing standards by organizations such as the American Society of Testing Materials, which has visual and instrumental standards for solid to translucent materials which could be applied to gems.
In terms of measurement and rating, Schorr questions both the CT systems and the CRI, calling them less than ideal.
"Both CRI and Kelvin are false standards, used to trick the trade and consumer…. I can get three fluorescent lights with the same Kelvin rating and to the eye each light looks different," he said. "Kelvin isn't without meaning. It's just too broad a range as a standard."
The CRI system is also misleading, he added. If a lamp is rated over 5000K, it is being based on daylight; but if the Kelvin is below 5000K, the color index is based on incandescent light. "How can we listen to a lamp [sales] person when … one is saying his product is great because it compares to daylight, while the other says theirs is good because it compares to [incandescent]?"
Most lighting manufacturers, Lena pointed out, will try to keep their color temperature under 5000K to avoid comparisons to natural daylight. "Yes, it's easy to get a 98 CRI when you're comparing [the light] to an incandescent," he said. "But if you push the temperature to 5000K, the CRI will drop to 60 or 70."
"The only truly accurate, reputable, measurable means of comparing light [sources] is the spectral power curve, or the spectral output of the color," concluded Schorr. "All the major lighting companies have the equipment. The technology is there, the ability to create a standard, to measure and have accurate reproducibility is there. We have agreed what the spectral output should be and what variation is allowable." But the industry still has a long way to go, agree lighting and colored stone experts, before it achieves a standard.
- Eickhorst, Manfred. "Excellent Lighting: A Necessity, Not a Luxury. ICA Gazette, January/February 1999.
- Webster, R. Gems: Their Sources, Descriptions, and Identification. 5th ed. Oxford , England : Butterworth Heinemann Ltd., 1994.
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