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Posted by Stephen Eglinton  
Stephen Eglinton October 22, 2011 06:29PM
Hi all,

Fishing around for ideas on back-lighting gem crystals for photography. You have most probably seen the thing where the lens is at front and the torch is at the rear, with the crystal positioned between them.

Had in mind a backing with an adjustable opening, with the light source behind this opening.

Someone advised me to buy a photographic "iris" for this purpose.

I know the result i am after, but i'm kind of stuck with the gear to buy and putting it together.

Anonymous User January 02, 2012 10:57PM
Perhaps you mean what microscopists mean "darkfied" illumination?
You can get it with something like a focusing LED torch/flashlight, set to spot, shone through the specimen at an angle so it MISSES the camera. Any light the specimen scatters will be visible in the camera. (Polarise the light, then try with another polariser on the camera and it gets even more interesting!)
(Not "circular" polarisers, you really need linear - and the one on the camera's called an "analyzer", but its the same)

In a microscope you have a large diameter lens (condenser) on the other side of the subject, with the light source the other side of the condenser. If you put a black disc in the middle of the condenser lens, you get a ring of backlighting coming through the subject., all missing the lens.
Stephen Eglinton January 03, 2012 01:33AM
Justin, it's exactly what i was referring to and what i was after.
Thanks heaps for your help.:)-D

Mineralogical Research Company January 03, 2012 08:29PM
I have used fiber optic illuminators, illuminating the subject from the side, to do a passible job. But, the contrast is not great.

Proper darkfield illumination usually requires a special condenser designed especially for that purpose. Here is a link to detailed info on Darkfield Illumination. The last part of the article describes how a brightfield microscope can be converted to darkfield.

Anonymous User January 03, 2012 10:41PM
If the contrast isn't great doesn't that imply there was llight "spill" going on?

Years ago I used a pair of slide projectors with a hole punched through an opaque (metal foil) slide, focused on the subject from about 45 degrees, behind, one each side in each. The background came out blackety black on Velvia 50D while I was struggling to stop the highlights blowing (as usual with that film). More cntrast that I could handle. The light coming through my girlfriend's hand was interesting too. Mine was no good - I'm too dense, obviously:-S
Photographers call it backlight, microscopists have to have a fancy word for it ;) but sure, "Darkfiled" is what to look up. You can buy odd microscope condensers for 10 bucks on ebay, but any torch/flashlight put right behind the subject, with the middle blacked out, would work, to "varying degrees" - pun intended.
If you have too wide an angle of light ( which microscopists call a large Numerical Aperture Condenser) you get more internal reflections going on in a thick specimen, and quite different light at the edge of the specimen (Imagine a wine glass).
Another cheap source of a suitable, bigger light/condenser would be an old enlarger. (Black and white type, not color) which are practically valueless now of course. You'd cut some metal foil discs of different sizes and just stick them on.
Owen Lewis (2) February 23, 2012 12:14AM
Imaging through a microscope and for macrophotography (using a camera only to image an object as a range of a few cm)' require slightly different approaches. Since you don't specify, I shall confine my comment to macrography.

For 'backlighting' I'd suggest you try a small light table that uses a white LED as a the light source and a diffusing white translucent platform on which to set your subject above the light. For this to work, the subject needs to be:
- Translucent/transparent.
- Wider that the LED light source.

If the sample is smaller than the LED,, get a piece of black card or similar and make a small hole in it. Place the card on the light table with the hole centralised on the LED and with the centre of your subject placed over the hole in the card. Having several such cards with different size holes in them achieves what an adustable iris achieves but at less cost and complexity (that said, I do use adjustable irises integrated into both my microscope bases).

The Osram 'Pocket Dragon' is one such small, pocketable and battery-driven light source. It enables two levels of illumination without noticable shift in colour temperature. OTOH, you could make an equivalent box on the kitchen table with parts from Radio Shack/Maplins. When 'out and about', it's a great transmitted light source to use with a folding polariscope too.

Here are tthree pics at different angles of a small piece of dark Sapphire rough (9x8x6mm) taken using this technique. I think it brings out the strong pleochoism of this specimen (and its beauty) very well. And all with a lighting system that cost less than 30 bucks, shop bought! The hi-res shots (before downsizing for web-pic display) have even more impact when displayed full screen.

Try this technique and see if it works for you?


Edited 1 time(s). Last edit at 02/23/2012 06:25PM by Owen Lewis (2).
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Ron Gibbs February 25, 2012 07:37AM
I once built a portable dark field illuminator for a boom style microscope. The initial write up can be found with photos here: HomeMade Darkfield Apparatus. I used a circular LED for optical viewing then changed the bottom light to reflected a TTL driven flash when doing photography to get more light and auto exposure.Overall I was quite pleased with the results. Further down the same page (URL above) I showed how I eventually created a better reflector the flash.

The unit could be adapted to several different scopes and moved as needed. I think it ended up costing me about $125. The darkfield condensor came from AmScope and cost about $70. The LED which was only used for viewing with my eyes, ran about $50 on E-Bay.
Stephen Eglinton February 26, 2012 01:23PM
Reading with hugh interest!
Thank you guys for the help. I am now talking with my photographic shop regards the set-up, and ordering the gear.
Much indebtet to all.:-)
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