Darkfield and Photomacrography

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augusthouse
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Darkfield and Photomacrography

Post by augusthouse »

I found this brief article this morning while searching for some information regarding Darkfield and Photomacrography.

Photomacrography with the Wild M420

I was particularly interested by the information or technique mentioned in Section 2.

I'm trying to get my head around the function/concept of the 'mirror base' as referred to in this instance.
Brightfield and darkfield transillumination is done by shining light into the mirror base.
In Section 1. the article mentions a "mirror transilluminator insert with transparent plate and a tipping stage". Is anyone familiar with this device?

http://web.uvic.ca/ail/techniques/wild_m420.html

Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

rjlittlefield
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Post by rjlittlefield »

I suspect that's actually two devices. A bit more of the paragraph reads like this:
There are three base inserts for the macroscope. There is a plain plate (with a white and a black side), a mirror transilluminator insert with transparent plate and a tipping stage.
So, I think the base inserts are:
1. plain plate
2. mirror transilluminator with transparent plate
3. tipping stage.

The mirror transilluminator insert may be similar to this Olympus model.

--Rik

augusthouse
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Post by augusthouse »

Thanks Rik,

What initially had me puzzled was the reflective properties of the stage in the photograph at the link and the word 'mirror' bouncing around in my brain.

So there would be a darkfield stop somewhere in that "darkfield transillumination" setup?


Craig
Last edited by augusthouse on Sat Nov 15, 2008 10:53 pm, edited 1 time in total.
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

augusthouse
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Post by augusthouse »

Here is a piece of equipment that achieves a similar outcome but goes about it in a different manner.

I have the fiber optic cable plugged into my 'flashbox'.

Image

Image

Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

rjlittlefield
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Post by rjlittlefield »

augusthouse wrote:So there would be a darkfield stop somewhere in that "darkfield transillumination" setup?
That sounds right. Somehow light has to hit the whole back of the specimen, but from a collection of angles such that if there is no specimen, none of the light has a straight shot to the sensor.

--Rik

g4lab
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Post by g4lab »

Here is a Zeiss tilting stage. It lists for $563 dollars.

Here is a link to a page on Schotts website which if you go down to the ringlight section will give you a .pdf of a Schott Ringlight.
These used to be made by Dolan Jenner but are not listed any more.
It was a black anodized housing and glass stage with a conic surface reflector and designed to be used with a particular ringlight.

Schott also made one which was smaller in diameter. Neither of those two are shown on the websites but I suspect you might be able to track them down if you tried.

Craig your outfit looks like a fairly good approximation You might consider making an annular reflector to go immediately above the ringlight.

The "Fust an Champeen" of Stereo Darkfield bases is the GIA Gemolite
which unfortunately only has about a 35 watt little qh bulb. Older ones had
an ellipsoidal reflector made of spun aluminum. Newer ones have a plated plastic toroidal mirror which is probably more efficient optically. But being plastic it probably precludes upping the wattage.

Here is one of the new style. The more low profile toroidal reflector is contained in the black finned aluminum substage. All of them have flippable stops for brightfield. Newer knockoffs are going to LEDs which is unfortunate. LEDs are not currently suitable for viewing gemstones especially diamonds.

These can be set on a transmitted light base with a 45 degree mirror or moveable mirror. If a 150 watt qh fiber optic illuminator is pointing at the mirror they work GREAT. With the dinky 20 watt qh bubb they are usually sold with not as good.
Similar item with iris
Pages 141-143 or so show the granddaddy of them all

augusthouse
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Post by augusthouse »

Thanks Gene,

The unit photographed in my previous post is a Schott-Fostec Darkfield Illuminator. It was $29.95 on eBay when I purchased it some time ago and it looked like it might be useful.

http://www.subtechnique.com/fostec/4PointIllum.pdf

The iris aperture is a separate item and it is just something I had laying around, probably don't need to use it; unless it becomes necessary to "adjust the darkfield spot size for light sensitive applications".

Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

Charles Krebs
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Post by Charles Krebs »

Here is a Zeiss tilting stage. It lists for $563 dollars.
Next time I have a few projects for my machinist friend to work on I think I'll have him "cut" a couple billiard cue balls in half and make a base plate with the proper radius. It wouldn't say "Zeiss" but would work nicely.

NikonUser
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Post by NikonUser »

Charles Krebs" wrote:Next time I have a few projects for my machinist friend to work on I think I'll have him "cut" a couple billiard cue balls in half and make a base plate with the proper radius. It wouldn't say "Zeiss" but would work nicely.
Perhaps a $20.00 mini ball head heavily greased would work equally well :roll:
NU.
student of entomology
Quote – Holmes on ‘Entomology’
” I suppose you are an entomologist ? “
” Not quite so ambitious as that, sir. I should like to put my eyes on the individual entitled to that name.
No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
Oliver Wendell Holmes, Sr
The Poet at the Breakfast Table.

Nikon camera, lenses and objectives
Olympus microscope and objectives

Planapo
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Post by Planapo »

Shush, Charlie and NU! :-$
Don´t you undermine our economy, depending so strongly on export! {-o<

--Betty :wink:

Graham Stabler
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Post by Graham Stabler »

I used to have something like one of those tilting Zeiss stages, you could supply air so that the half ball part floated. I suppose if you switched to vacuum that could also act as a clamp too. Unfortunately I have a horrible feeling I threw it away as at the time I could see no use for it. Boo hoo. It was from some sort of die bonder or wafer prober I think, used in semiconductor industry.

More recently I made a sort of micro version using a steel ball with a flat ground on it, that sat in a socket made from car body filler, "bondo" as you guys across the pond like to say. It was used for dissection submerged under fluid but to be honest I would prefer to tilt the whole thing with a larger stage and I had no meas of clamping. Ideally it would be height adjustable too so you could set the centre of rotation at the part you are looking at.

Graham

Graham Stabler
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Post by Graham Stabler »


g4lab
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Post by g4lab »

The Nikon Java applet is cute but it is propaganda. While it may be correct that light will directly enter the objective if one moves from a particular radius of curvature towards the straight line of a cylinder it ignores how competitively made (but nor really competing in the same markets) stereo darkfield units are designed.

The original Gemological Institue of America Stereo Darkfield unit probably developed in the 1930s or 40s had a spherical reflector with a lamp mounted in it and a blocking stop to shield the objective from direct light.

In the 1950s they cooked up a nice increase in efficiency by making an ellipsoidal reflector. You will recall that an ellipse has two foci and the bulb was located at one and the gemstone at the other. This was fairly easy to have spun in sheet metal and the interior surface polished. The only real disadvantage to an ellipsoid was that it increased the depth of the unit.

In the late eighties or early nineties they updated the design replacing the spun ellipsoid with a molded plastic toroid. The plastic is plated with chrome plating and if you didn't notice that it was plastic you would think it was a metal or glass mirror. The working surfaces are completely specular
more so than the old aluminum one. They are lower in profile than the old one. But they are a complete circle not segmented like the Nikon which must show some artifact of that segmentation in the illumination uniformity. How much I don't know. Like Betty's countrymen Nikon charges so much for that particular item that they have not worked their way through the economy to where a bottom feeder such as myself can get his paws on one.

With all due respect to all the great German, American, and Japanese (and anyone else) optical companies none of them ever really built anything like the GIA base. GIA equipped it with B&L or AO stereos. The units were designed for looking at diamonds and gemstones and while one can do this with other bases for ease of use nothing else even comes close.

There is a fellow in Oregon who makes a similar unit here http://www.gemproducts.com/

Similar units are also now coming from China from Motic who have their own website.

Some of the Chinese and other distributors and manufacturers are going to LED lighting which for Gems use is not advisable.

The two complaints that users consistently register are that when used for extended periods of time the units get rather hot so that if you drop a stone into the well you must have tongs ready to fish it out. Your fingers would get scalded.
Also the 30-35 watt lamps used don't furnish enough light for all uses. Generally enough not for dark or semitranslucent stones.

I have considered upping the wattage and adding a small size fan to draw the heat downwards and out. LEDs do solve both these problems but their spectral curves have shapes that are too funny to judge color by. I expect that in the future these matters will probably be standardized.

The subject of lighting for gemological use was addressed here in a fashion better than I could ever hope to do:

http://www.palagems.com/gem_lighting1.htm
http://www.palagems.com/gem_lighting2.htm

When Dr. Sersen wrote this article, (1990) LEDs in white were not available and Metal Halide Discharge lamps were not as available as they are now.

augusthouse
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Post by augusthouse »

Graham,

Thanks for the link to microscopy.u.com. I can see the item mentioned in my initial post.

Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

augusthouse
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Post by augusthouse »

Gene,

There is certainly some extensive and interesting information in the articles that you linked to at palagems.com.

When I was looking at the device I photographed earlier in thread, some of the images reminded me of something - a solar eclipse or a sunrise as viewed from space and then I began to read the articles on palagems.com.

Craig
Last edited by augusthouse on Sun Nov 16, 2008 9:08 pm, edited 1 time in total.
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

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