Excellent Lighting: A Necessity, Not a Luxury

Article by Manfred Eickhorst, previously published in the ICA Gazette, website www.gemstone.org.

The beauty of gems to the human eye is determined by the gemstone's colour appearance. This beauty is the fascinating result of a unique interaction between light and matter. The human observer can indeed find all the visible colours of our universe in the interior and on the surface of gemstones. It is therefore no wonder that the type of lighting and its origin will have a corresponding great effect on the way humans assess the beauty of gem colours.

After millions of years of evolution, of course, humans feel most familiar with the way objects appear when visible under natural daylight. This source of light, however, is available only on an irregular basis, and does not represent a uniformly dependable standard. Fluorescent lamps, therefore, with their artificial daylight, have since their invention proved an invaluable aid in the sorting of gemstones. When appropriately used, luminaires with fluorescent lamps offer a uniform standard for illumination and colour rendering.

The atmosphere scatters light, predominantly blue light. At sunrise and sunset, when the sun is low, its light travels through a thicker layer of atmosphere. More blue from the sunlight is scattered. As a result, morning and evening sunlight are reddish.
White as perceived colour in the results of a mixture of the primary colours red, green, and blue. The more intensive one of these colors is, the more it will create a shift from the white impression into the direction of this colour.

But the major question arises: Which fluorescent lamp is appropriate for all gems? And the answer: There is no such optimal lamp for all applications. In fact, the great multiplicity of colour-producing elements in a gem require that a compromise is met - in the same manner in which a compromise is necessary when buyers and sellers demand" the perfect light" for some particular purpose.

The standard of quality for fluorescent lamps is exclusively their colour rendering in comparison to natural daylight. In this context, the colour of light - characterised by its colour temperature - is an entirely independent factor.

The selection of the colour temperature of lamps for use with gems is oriented to CIF stipulations for the standard illuminants 0 55 and 0 65. For diamonds, the colour temperature is 6500 K, a temperature definitely in the blue-white range. For coloured gems, the temperature is 5500 K, which produces a more neutral- white colour appearance. An International Colour Code now applies for the designation of lamps; this code stipulates the colour rendering index (CRl) as well as the colour temperature. If a lamp carries the inscription 015W /950, this signifies a lamp wattage of 15 watts. The number ,,9" means a CRl in the range of 90 ... 100. The double number ,,50" stands for a colour temperature within the range 5000 ... 5500 K. The prefix ,,0" indicates the daylight characteristics of the lamp. New lamps which carry no colour code are frequently inferior in quality.

An additional standard is the luminous flux emitted by a lamp. In considering this standard, it is important to remember that the quantity indicated is provided in reference to the length of the lamp. As a result, the typical total luminous flux from a conventional no-name, 15- watt lamp could be 720 lumen, over a length of 440 mm. With almost the same length, a high-end compact fluorescent lamp will produce 2000 1m. In the same sense, an illumination photometer will measure approx. 3000 lux from a conventional lamp at a distance of 300 mm (approx. 1 foot). With a state-of-the-art lamp type, the output will be approx. 6000 lux: i.e., almost twice as much.

Spectral power distribution for Osram Lumilux daylight colour 12/950; approx. 5000-5500K. All visible colours of the spectrum exist here with good intensity components. As a result this light effectively brings out all colours of colour stones. CRI is 96.

In working with gems, a high value of illuminance is essential for a number of reasons:

  • Fatigue and headaches will not occur so soon
  • Red colours are easier to evaluate
  • Good illumination is an aid for elderly persons, whose eyes require 50% more light than young eyes for the same visual efficiency.

Highly transparent gems - amethysts, for example, as well as bluish specimens - however, represent exceptions to these rules. For these stones less light is often more pleasant for colour matching. Progress in the technology of ballasts has resulted in a new generation of electronic ballast systems.

These devices now make it possible for the first time to dim fluorescent lamps on desktop luminaires. As a result, the user can adjust the illumination level to that which is most pleasant for his or her eyes and daylight conditions. These electronic comfort lights offer another essential characteristic: they are flicker-free which is a boon for daily work under artificial light. A welcome additional side effect are the savings in energy, and around 50% longer service life of the fluorescent lamps.

Experience gained with the new compact fluorescent lamps has revealed that the combination of 5500 and 6500 K lamp types in one luminaire represents the optimal kind of light for coloured stones. This arrangement makes it possible to create an optimal colour balance: another boon for the eyes and for professional work.

The window of time for 5000-7000 K daylight from a slightly overcast northern sky at medium latitudes provides optimal colour rendering.

About the author:
Manfred Eickhorst received his MSc from Hamburg Technical College of Applied Physics. He has developed, designed, and engineered specific lighting for gems, jewellery, and gemmology. His expertise regarding light and optics is demanded worldwide. The articles he publishes are well known contributions as professional references for the gemmological community.

Lighting Glossary

  • Daylight: A mixture of yellowish sunlight scattered from a bluish moderate overcast sky to form a combination producing white light. The colour temperature range is 5500 K ... 6500 K if ideal but this depends on geographic location, time, date, and atmospheric conditions. The glare-free diffused daylight obtained from the northern sky is ideally used for colour grading of gems.
  • Fluorescent lamp: A low-pressure mercury electric-discharge lamp in which a fluorescing coating (phosphor) transforms part of the UV energy generated by the electric discharge into light.
  • Illuminance E: Indicates the amount of luminous flux from a light source falling on one square meter. lt is measured in lux (Ix) or in foot-candles in the non-metric system. The relevant data levels depend on the distance from the lamp to the incident area. 10 lux are = approx. 1 fc.
  • Luminous flux: This is the rate at which light is emitted by a lamp. It is measured in lumen (1m).
  • Colour rendering: The effect of a particular light source is the visual appearance of a coloured surface, usually judged by comparison with daylight. A lamp may be attributed with a colour rendering index (CRI), which is a number between one and 100; the higher the number, the truer the colours. Quality fluorescent lamps for colour grading must feature CRI 90 or above.
  • Colour temperature: A term for the relative yellowness or blueness of light colour as measured in absolute temperature units (i.e., Kelvin with symbol of K).
  • CIE illuminants D 55, D 65: Designation for daylightequivalent fluorescent light sources with colour temperature of 5500 K, as best suited for coloured stones. For diamonds, 6500 K is best.CIE standard illuminants have defined spectral power distribution with respect to wavelengths. CIE = Commission Internationale de l'Eclairage.
  • Ballast: An electromagnetic or electronic device used with a fluorescent lamp to provide the necessary circuit conditions (voltage, current, and waveform) for starting and operating.
  • Electronic comfort light: Light source characterised by flicker-free operation and dimmable Iluminance. Further characteristics are satisfactory brightness, energy saving due to powerefficient electronic ballast and relative long lamp life.