Led light boxes for standardized lighting of coloured stones and diamonds
By Dr. M.S. Krzemnicki, first published in Facette 28 (May 2023)
In the past, coloured gemstones and diamonds were often examined in natural daylight, but reproducing such specific lighting conditions was challenging. Some impractical methods had been suggested, like grading only during specific hours or facing northward, especially during cloudy weather. However, modern artificial light sources that resemble daylight have been available for a long time, yet possess specific pitfalls such as: heat, noise, instability of colour temperature, flickering, aging of the diffusor, aging of filters, etc.
A collaboration between SSEF and the Department of Physics at the University of Basel recently resulted in the development of a novel light source for the purpose of colour grading of diamonds and coloured stones (see also SSEF Facette 2021, pages 42-43). Unlike many models that use incandescent bulbs or fluorescent tubes, this new light source utilizes advanced high quality LED light sources to create a natural daylight- equivalent illumination. Its Color Rendering Index (CRI) – a quantitative measure which reflects how close a light source reveals the “true” colour of an object compared to observation under natural light or a standard light source – is 98,4% at daylight (D65), thus exceptionally high and promising for colour grading purposes.
The temperature of daylight ranges from 5000 to 6500 K, and this variation can affect the perception of an object’s colour, such as that of a ruby. The colour temperature of a light source plays a significant role in how we perceive the colour of objects. It is thus important to develop standardized lighting conditions.
SSEF now offers two cabinets of different sizes for sale. Each cabinet is equipped with a specific light source allowing the user to easily switch between three different colour temperatures. The D65 illumination is provided with a normalized light intensity of 2’200 lux at 20 cm from the source (following the ISO standard 24016 for diamond grading). The other colour temperatures are useful for gemmologists for coloured stone grading and seeking to evaluate colour shifts or colour change (e.g. alexandrite-effect) of a gemstone.
Specific attention was given to designing a diffusor specifically adapted to diamond and gemstone observation. Indeed, whilst most of the available diffusors on the market are bright and will add reflected light on the observed gemstones, our diffusor is roughened with a specific mesh to bring a high comfort of observation. Additionally, our diffusor is made of glass, so that unlike plastic diffusors which alter the colour temperature of the observation light as they age, our diffusor will not change with time.
The developed light source utilizes advanced LED technology, producing a nearly continuous spectrum of visible light that closely approximates natural daylight. We have sold several such light boxes to members of the trade in recent months, who use this to maintain consistency in their colour examinations of diamonds and gemstones. Our innovative light box for colour grading is available in different sizes and is available through our subsidiary company SATT Gems (www.sattgems.ch).
The advantages include:
- New generation of high-quality LED (manufacturer life-time: 10’000 hours)
- No flickering
- 3 colour temperatures
- Electronic touch screen for the selection of the colour temperature
- Home-made glass diffusor specifically designed for diamond and gemstone observations
- CRI 98,4%
- Perfect distribution of the light intensity
- Inner walls of the cabinet in normalized neutral Munsel gray conditions