Enlighten the experienced gemmologist

Published in The Journal of The Gemmological Association of Hong Kong, 2006, Volume XXVII
by Manfred Eickhorst, MSc Physics 

The eyes of gemmologists become more experienced the older they become. On the other hand, vision acuity decreases. Therefore the elderly gemmologist needs increasingly more light for the evaluation of gemstones, but at the same time he is strongly irritated by glare. For this reason, today’s light sources are outstandingly powerful and even dimmable to allow an individually required lighting level and thus offering as much as possible illumination without glare. Additionally the visual task is facilitated by electronic, flickerfree light. These features are establishing a remarkable new viewing comfort for the specialist in gemmology.

The visual task is decisive for the quality of illumination. For the user, frequently only the illuminance is directly comprehensible. Contrast, glare and colour rendition are perceived indirectly and only when the achievement of vision tasks gets difficult. Working with gems and jewellery means handling very small-sized specimens and discerning, treating and appraising often smallest structures. In addition, the colour of light and the colour of the objects are of crucial importance for the operating process. From there the necessity arises to adapt the illuminance to the respective workplace. See fig. 1.

Figure 1: Rising degree of visual task difficulty and increase of illuminance level

Note that at a window geared to the north, approximately 6,000 LUX are measured as average of all times of day, seasons and weather conditions related to one latitude. The high vision demands lead to early exhaustion of the operator and thus to a reduction of performance and efficiency. This can be avoided by carefully adjusting the level of illuminance to the respective task. At the age of approximately 45 years the reduction of accommodation of the eye occurs, limiting the range over wichwhich objects can be brought into focus and also the adjustment of the eyes to strong varying illuminance levels.

Figure 2: Light scattering in the aging eye at (C) Cornea, (L) Lens, (VB) Vitreous Body, (R) Retina. The arrow length indicates the amount of glare originated by the respective area of the eye.

Furthermore the transparency of the eye-lens decreases due to metabolic deposits. The light scattering at opaque lens parts, especially with blueish light, leads to hypersensitivity to glare and is perceived as galling. See fig.2. This can also be observed with automobile headlights: blueish Xenon-bulb light is experienced as more glaring than normal white halogen light and therefore as uncomfortably irritating. Elderly eyes, as off 50 years, need approximately 50% more light intensity for the same performance of vision than those of human beings aged 25. See fig. 3. Again, more light often means more glare due to strong reflections. This is the most frequent reason for problems in vision at the workplace due to the illumination. By dimming the fluorescent lamps to the individual, most comfortable level, work can be carried out for hours without causing strain.

Figure 3: Need of illuminance in relation to the age of the eyes

Over the course of the day flickering light is more and more apprehended and a clear evidence for the additional strain for the eyes and the visual perception. This stress can easily be avoided by electronic high frequency operation of the fluorescent lamps. In comparison to the 50/60 Hz frequency of conventional magnetic ballasts, the much higher frequency of 40,000 Hz leads to flickerfree light comparable to that of incandescent bulbs.

The colour of the fluorescent lamp has to be chosen according to the hue of the gemstones to be illuminated. Artificial daylight of 6,000 to 6,500 Kelvin is most frequently in use for diamond grading. For coloured stone in general the specialist prefers 5,000 to 5,500 K. See fig. 4. 

Figure 4: Colour temperatures of natural and fluorescent, artificial daylight

The use of two separately controlled, dimmable, fluorescent lamps of 5,500 and 6,500 Kelvin is very efficient for colour matching. All gemstone colours from blue to red obtain a spectral portion of appropriate light. The individual dimming of the lamp power also results in a perfect mix of light colour, enhancing the colour rendering and accentuating the beauty of gemstone colours.

A change in the colour of light can also be refreshing for the eyes, when they had been exposed to a uniform light situation. It is a surprising fact that gem trade and grading labs use different colour temperatures. While the lamps used by coloured stone dealers normally rate 5,500 Kelvin, the labs are often using 6,000 – 6,500 K fluorescent tubes.

Figure 5: Spectral responsivity of the human eye and spectral power distribution of daylight-equivalent fluorescent phosphors, 6,500 Kelvin.

When judging gemstone colours, colour rendition of the fluorescent lamps is the key quality specification related to natural daylight. Starting specifically with yellow diamonds, rendering of all spectral colours of the fluorescent phosphors is less important as only the yellow range which has to perform high values. Additionally their high spectral luminous flux in this range is perfectly matching the Cape colours and the spectral responsivity of the human eye has its maximum level at the same colour range as well. So even lamps with low illuminance levels and poor CRI do not inflict the results of diamond grading that much. Therefore the illumination of yellow diamonds is quite simple compared to the fancy coloured ones and other gemstones. See fig. 5.

Figure 6: High power dimmable light of 2 x 80 Watt fluorescent daylight lamps at 5,500 and 6,500 Kelvin.

The coloured stone trade generally prefers natural daylight to the artificial one. But when forced to use fluorescent lamps, the rendition of all spectral colours in comparison to natural daylight is only sufficient with a Colour Rendering Index (CRI) of +90. As a consequence the experienced gemmologist is looking for a lamp specification triplet consisting of high power (Watt), specific colour temperature (Kelvin) and excellent Colour Rendering Index (CRI).

As a matter of fact the international lamp manufacturers are reducing their special daylight assortments due to small business compared to standard types. Consequence of which is individual manufacturing as only possibility to assure the availability of daylight fluorescent lamps following the above mentioned specification triplet.

Every member of the gem industry should realise that the accurate selection of his specific light source is exceedingly important to maintain his efficiency and his visual comfort – to avoid exhaustion and fatigued eyes at the end of a long working day.