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Exploring Darkfield Illumination for Mineral Specimen Photography

Last Updated: 17th Sep 2015

By Volker Betz

In the process of mineral specimen photography the illumination of the object is one of the more time consuming parts. Optimising the position, intensity and softening a couple of light sources can be a tediously procedure until the optimal result is achieved. The use of a single light source can speed up this part of the specimen photography and help if a larger number of specimens need to be photographed either for documentation of a whole collection, or as a tool for web activities. A successful single light source was found in upscaled incident light darkfield illumination, which is a common illumination technique in microscopy, mostly used for flat and opaque objects. Darkfield illumination is also the superior method to photograph coins, as shown in the picture of a used 1 cent coin.

One cent coin (used) photographed with dark field illumination


Priniciple of incident bright field (left) and dark flield illumination (right).


For close up photography (fields of view of 1 cm and larger) mostly incident light bright field illumination is used and ring lights for that purpose are available from several suppliers.These are typical mounted around the macro lens and give good results for insects, flowers etc. but fail terrible for most minerals, especially such with a reflective surface. This was the reason I used in the past mostly a number of spot lights, which work very well, but need a lot of time to optimise position, softening and intensity.

Small darkfield ring light

Having seen excellent mineral pictures in the macro range (fields of view less than 10 mm) made with darkfield illumination ( those of Edgar Müller shown in mindat.com ) I started some experiments. First in the macro range with a home made 8 cm ID dark field ring light, made from a plastic cup bought in the super market and 4 coils (1 m) of white LED stripe (6000 K) glued at the inner wall.

Incident dark field ring light of 8 cm ID (home made).


The following haüyne picture was illuminated with this method, which replaces now spot lights in most cases. Depending on the object, it is used in combination with a transparent cylindrical foil as softener. This kind of illumination works well in most cases and is easy to use.

Haüyne and sanidine from In den Dellen quarries, Niedermendig, Mendig, Laach lake volcanic complex, Eifel, Rhineland-Palatinate, Germany. POV is 8 mm


Medium size dark field ring light

In the next step a larger 15 cm ID dark field ring light of 8 cm hight, was made from a cylindrical plastic container (from supermarket ) from which the cover and bottom was removed. It hosts a 2 m white LED stripe (6000 K) glued at the inner wall. Additionally the LED´s are covered with a softener screen to avoid reflection from single LED´s.

Incident dark field ring light of 15 cm ID (home made)


This larger ring light was used for specimen photography with a vertical camera mount. Instead a conventional camera stand, I used a home made stand made from aluminium profiles and corresponding mounting parts and pivots, available from internet sources for reasonable part coasts.

Incident dark field illumination with vertical camera mount


The specimen is positioned on a 30x40x15 cm plastic box, inside blacked with (black) self adhesive velour foil and covered with transparent glass. With the use of glass the background is black . If white or satin-finished acryl glass is used the background is white to grey. Coloured backgrounds are also possible. Illumination can be optimized by vertical variation of light position. It is mostly better to mount the object a few cm above the background, using a rod and putty. For some kind of specimen with very complex structure superior pictures are possible, with only short time used for the illumination step.

Natrolite from Ölberg, Hundsangen, Westerwald, Rhineland-Palatinate, Germany. Size 5x8


Chabazite from Wasson Bluff, Parrsboro, Cumberland Co., Nova Scotia, Canada. Size 80 mm x 70 mm x 15 mm


Larger dark field ring light

The 15 cm ID dark field ring light is limited in specimen size to about 7-8 cm, and the diameter to small to use it for horizontal and tilted camera mount. So in the next step a 25 cm ID dark field ring light was made. The tube was made from a 2 mm thick PE-HD sheet of 15x 80.5 cm. This thick PE-HD is available from internet suppliers. The sheet can easy bent and was screwed together forming a cylindrical tube of 25 cm ID and 15 cm long. Two 3 m LED stripes (6000 K) have been glued to the inner wall using separate power supplies. To soften the light a second tube was positioned inside the tube covering the LED strips. This 25 cm ID dark field ring light can be uses either for vertical of for tilted camera mount.

Tilted camera set up with with 25 cm dark fiel ring light


Larger dark field ring light on blacked box, view from camera position. Reflections from light background can be surpressed with a black sheet in background. A coloured sheet can be used for colour effects in background.


With this setup specimens up to about 12-15 cm can be photographed.

Quartz (Var: Rock Crystal) from Holzhausen, Nastätten, Taunus, Rhineland-Palatinate, Germany. Size 90 mm x 50 mm x 60 mm


Background effects

Matted glass as background
Transparent Glass as background
Light spot with blue back ground
Matted glass as background
Transparent Glass as background
Light spot with blue back ground
Matted glass as background
Transparent Glass as background
Light spot with blue back ground
Haüyne in a Sanidinite from In den Dellen quarries, Niedermendig, Mendig, Laach lake volcanic complex, Eifel, Rhineland-Palatinate, Germany. Size 70x90x50

How the background looks, can be controlled by the background material. Matted glass results in a white to black gradient in the picture, the same or similar with white or satin-finished acryl glass. Plain glass results in a black background and some mirror reflection of the specimen in foreground. Colour spots are possible with glass and and a coloured background (instead a black) with a balanced spot light, like that shown in one of the haüyne pictures above.

My two cents to back-ground art

Personally I do not like any extra colours to enhance backgrounds, but I am aware this is a method to make mineral specimen pictures more attractive, like make up in human life style. I also do not like complete artificial backgrounds made with post processing, but photographing a perfect background is quite difficult and so some some retouching of disturbing background elements like dust and unwanted reflections are essential. Black backgrounds are fine for lighter coloured specimens and fine for display screens. Its also fine for a single printed picture but often not optimal for a printed page with several pictures. White background can be an alternative for dark minerals but bears the risk for overexposed backgrounds, so special care is necessary for exposure time. The critical border is at about 5 % grey in background. Completely white or overexposed white background can cause trouble in post processing and also if a picture is prepared for print media. Mixing pictures with different backgrounds on a screen or print media can cause a non satisfactory page layout, so there is some reason for a universal background. This can be found by using a light grey or better a light blue grey. This is compatible with white, light, bright and intensive dark coloured and also black black specimens.
The following picture shows two cents in darkfield illumination with a 25 cm ID darkfield ring light with camera mounted in vertical position.

Two cents in darkfield illumination. Crop of a 10 cm FOV. The coin is mounted 35 mm above background in order to keep background out of focus.


Varations of background from top to bottom:

Glass on black background
Satin finished acryl glass on black background
Satin finished acryl glass on mirror foil
Satin finished acryl glass on light blue background

Variation of light, left to right: only upper LED ring on, only lower LED ring on, both rings on.
For the shown example all types of background work well, except the use of a mirror foil under satin finished acryl glass. This causes a extreme white background with to high contrast which can cause trouble in post processing. Some influence of background/object contrast is possible by using either only one or both LED rings.

Conclusion

Incident light darkfield illumination is a powerful method for close up mineral specimen photography. Most objects give good pictures while the time consumed to adjust lightening is minimised. For some objects the light is to soft and spots must be added to the incident light darkfield illumination or only spot lights used, returning to more time consumtion.

In any case incident light darkfield illumination is the superior method for fast documentation.

Remark to mineral pictures:

All close up pictures shown here are made with a canon D700 with a Canon Macro Lens EF-S 60 mm, always with f-stop at 5.6. This is the best choice for resolution. Depth of field is enhanced by focus stacking with the macro lens under control of Helicon remote. Number of stacks is typical 4-10. This can be done without extra equipment for stacking. The extra time for taking the stacked pictures and subsequent stacking with Helicon Focus is only about 60 seconds and worth to be done. The resulting pictures are much better than those one shot pictures with smaller apertures. All pictures are post processed with Helicon Filter and some additional with Adobe Photoshop essentials.

The home made camera stand I used is pictured below in detail. It can be made from parts available at internet sources.

Universal camera stand and its parts. Zoom into the picture to see details.





Article has been viewed at least 4382 times.

Comments

Hello Volker, I too have been playing with a very similar set up for micro specimens using cool white self adhesive LED strips, the same ones I used in my cabinet display, for my prototype I just used a simple battery pack to supply the power to the LED strips which had the benefit of stopping any lamp flicker which is sometimes apparent at high magnification with all but the more expensive electronic LED drivers. Congratulations on a first class set up, you have encouraged me to build a larger unit for cabinet size specimens.
Regards Peter.

Peter Trebilcock
17th Sep 2015 4:51pm
Thank you for sharing this information. I guess I am going to have to try darkfield illumination now.

Is it possible for you to share the internet sources for the parts to your home made camera stand?

I do not like extra colors used to enhance backgrounds in mineral photography either, in fact I find it quite annoying.

Henry Minot



Henry Minot
19th Sep 2015 3:45pm
Hi Henry,

I have a german adress, it will not help much. Unfortunatlely I could not send you a PM. Please contact me at Volker.Betz@zeolith-sammlung.de
Volker

Volker Betz
19th Sep 2015 6:47pm

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