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Dark field tin crystal

Posted: Mon Apr 08, 2019 9:26 am
by georgetmacro
While carrying out more dark-field experimentation, the attached image of tin crystal structure surface at 200X magnification has me somewhat bewildered. My setup was simply under lighting on my Olympus CH with an 80A photographic (blue) filter over the light source, also a make-shift dark-field arrangement (not a dark field condenser) I also used a polystyrene reflector around the objective to give uniform lighting. Generally, with direct lighting, this crystal would be seen as black. I think the tin is combined with quartz but still would not appear like this under direct lighting. The digital camera was set on colour temperature auto so it should not be to do with the light-source colour rendition. The objective is one given to me only a few days ago. An Olympus A 20, 0.40 160/0.17. Can anybody suggest what may be going on here?

Re: Dark field tin crystal

Posted: Mon Apr 08, 2019 12:04 pm
by apochronaut
Without seeing a picture or two of the set up , it is difficult to say. Although your description is fairly complete, it isn't precise enough to cause me to zero in on specifics, only general comments.

Firstly, by installing a reflector in order fill the illumination a little, you are defeating the concept of DF, which requires no direct illumination of the subject. As well, since your subject matter is opaque, the camera is going to be sensing mostly incident light, I would suspect. You also didn't indicate how far the reflector was from the illumination, whether there was a distance at all or whether the polystyrene was a thin layer, wrapped tightly around the objective shroud. Is this a crop? Was there extraneous light coming from below, around the crystal?

Usually, where I have experienced such skewing of the colour balance, is when I have used an auto setting and there are mixed light sources. It has also been when there is incident light being used. The camera seems to get confused and can't process what it is sensing. The last time this happened, was when I was photographing some fruit and using a mix of both led and halogen lighting. There would also have been some daylight landing on the subject. Auto had a hard time deciding what to do.

As a guess, I suspect your reflector is introducing some incident light that is tainting the stew. Did you try running through some manual light source options?

Re: Dark field tin crystal

Posted: Mon Apr 08, 2019 6:04 pm
by georgetmacro
Apochronaut. Given that way back in 1982 thereabouts I only just scraped through geology 101 within my first (BSc) degree know pretty well zilch about the subject. I have however read that tin, tungsten and gold often occur together. Could this be a chance occurrence of gold amongst the tin I wonder? I have observed an insect under exactly the same lighting arrangement using the same objective and the colour is as expected, i.e., normal. To answer your question "was it a crop", the answer is no, it is the complete image. The polystyrene is the base of a cup with a hole cut the diameter of the objective.

Re: Dark field tin crystal

Posted: Mon Apr 08, 2019 8:41 pm
by Scarodactyl
I am curious what you are looking at. I have a feeling it isn't native tin. Cassiterite?

Re: Dark field tin crystal

Posted: Mon Apr 08, 2019 11:39 pm
by apochronaut
georgetmacro wrote:Apochronaut. Given that way back in 1982 thereabouts I only just scraped through geology 101 within my first (BSc) degree know pretty well zilch about the subject. I have however read that tin, tungsten and gold often occur together. Could this be a chance occurrence of gold amongst the tin I wonder? I have observed an insect under exactly the same lighting arrangement using the same objective and the colour is as expected, i.e., normal. To answer your question "was it a crop", the answer is no, it is the
complete image. The polystyrene is the base of a cup with a hole cut the diameter of the objective.
So you are thinking that the reflector is picking up some scattered rays from some gold in the sample and then flooding the sample with a golden tint?

Re: Dark field tin crystal

Posted: Tue Apr 09, 2019 4:30 am
by georgetmacro
I have spoken to the person that gave me the samples. I have also shown/given him a full-gloss image for his gem & rock shop. He has assured me that the image is of cassiterite as Scarodactyl suggested. Apparently, cassiterite is the main ore associated with tin crystal form.

Re: Dark field tin crystal

Posted: Tue Apr 09, 2019 5:35 am
by Scarodactyl
How transparent is it? Cassiterote has a metallic sirface luster but can be transparent beneath it, and its color is often amber or orange.