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LED spot lights and dark field Illumination combined

Last Updated: 13th Sep 2016

By Volker Betz

Some kind of mineral species are more difficult to photograph than others. This includes black crystals with high luster, white and colorless, transpartent crystals and often also light colored transpartent crystals. In the recent years I made a couple of experiments with home made dark field devices made from LED stiipes. See Older article . Prior to that I used mostly spotlights. But I was immpressed by the results Edgar Müller got with his dark field ring lights.

As a result of experiments, my conclusion is, that both methods, spot light and dark field ring lights have advantages for different types of objects and I wanted to combine both. This was not possible with the ring light I used before, because it was to high and several rings are needed to have sufficient light.

Old Version of ring light

LED Ring light with flexible LED stipes

So I made a ring light from aluminum LED stripes on a aluminium carrier. This have high power LED and one ring of 10 mm hight is sufficient for illumination, also if it is used with a diffusor screen.

New Version of ring light

LED Ring light with aluminium stripe carrier

For many objects, typical those of very complex structures, like the picture below, the ring light in darkfield position will do the job. Ring Lights can be made from this kind of LED aluminum stripes in any circumference from 30 cm to 100 cm in 10 cm increments ( diameter from 9-31 cm). The 1 m stripes a can be cut in minimum 30 cm pieces and careful bent. Both ends are then inserted in the a piece of aluminum profile and the power conneted. 10 cm segment can be individual switched.

Natrolite on analcime from Cairns Bay , Flinders, Vicoria, Australia. FOV 10 mm. Photographed with one ring light 10 mm high.

For less complex objects with prominent crystals, either spot lights or a combination of ring light and spot light will produce better results. The combination must be balanced in light intensity and also in color temperature. My experinece is that LED with ~5500 K produce the best color balance. Unfortunately only a small fraction of LED on market match this value. I found an GU4 spot of 2.5 W, 12VDC 5500 K which I adapted on a stand from stonemaster.eu with a cinch connector.

Spot light 2.5 W 12 V DC on a stand from Stonemaster.eu

LED spot 2.5 W 24 ° 5500 K on stand from stonemaster.eu

Up to 6 of this spots are used either alone or in combination with the ring light. The use of a softener screen is essential in most cases. More then 6 spots are difficult to handle, as the light effect of each spot influences all other spots and balancing all the spots is very time consuming.

Combination of Dark Field and Spot Illumination

Using a combination of spot and dark field illumination combined with a PE diffusor screen with a 40 mm Zeiss Luminar on bellows. The specimen is mounted on a sphere table with a magnetic foot, so it can be tilted in any direction.
Example picture of Haüyne photographed with a combination of spot and ring light.

Haüyne and Pyroxene Crystals from Mendig, Eifel. FOV 6 mm

The combination of spot and ring light illumination makes it possible tho show a maximun of crystal faces also from transparent crystals without masking the interior of the crystals with to strong reflections.

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