Reflected Darkfield Illumination

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steveminchington
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Joined: Sun Jul 22, 2012 2:30 pm
Location: Bedford UK

Reflected Darkfield Illumination

Post by steveminchington »

As this is my first experience with reflected darkfield illumination, I have no benchmark as a reference, so I would just like to check if my result is what I should expect. Here are a couple of test shots of of my piece of print, first brightfield, then darkfield. The exposure on the brightfield was about 1/4 sec, but 15 sec on the darkfield. I am using LED illumination and with it on max brightness, I can't see anything in live view on the camera in darkfield. I had to focus in brightfield then change over to darkfield and keep increasing the exposure until I got an image, which ended up at 15 secs.

So the question is, is it normal to have such a big difference in exposure between brightfield and darkfield, and if so, should I be looking at getting a much brighter illuminator?

Any comments would be welcome.

Image

Charles Krebs
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Post by Charles Krebs »

There can be a large difference but 15 seconds is pretty long. However, what was the magnification and NA of the objective used?

I don't pay that much attention to exposure times, but I was able to check the original files for the fourth picture in this post. It was taken with a 20/0.46 objective, darkfield, and the exposure times were 0.3 second at ISO 100.
http://www.photomacrography.net/forum/v ... hp?t=25427

The light source was an "off-the shelf" LED spot-lamp bulb... about 12W if I remember correctly.


Did you use any polarizers on the brightfield image? Looking over the exposure times on a bunch of images made the my vertical illuminator, both darkfield and brightfield, image exposure times ranging from about .2 to 5 seconds. Some of the longer ones are from brightfield where crossed polarizers were used to reduce glare.

steveminchington
Posts: 215
Joined: Sun Jul 22, 2012 2:30 pm
Location: Bedford UK

Post by steveminchington »

Charles Krebs wrote:what was the magnification and NA of the objective used?
The objective is a Nikon CFI 5x na 0.13 BD using the darkfield cube on a Nikon Epiphot 200, Nikon PL 2.5x and ISO 200. I think the LED illuminator is 3w, as the controller says 3w max.
Charles Krebs wrote:Did you use any polarizers on the brightfield image?
No, I don't have any polarisers.

Your images are really something to aspire to and I particularly like the darkfield one as it shows the relief of the surface. I am thinking about using a 10w LED chip if I can find the right colour temp., but I will have to do a bit of reading up on making a dimmable controller. I'll do some more testing today as the weather is pretty horrible here at the moment.

Greenfields
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Location: Nottinghamshire, England

Post by Greenfields »

Can you tell us a bit more about your illuminator ?

Are you using an LED unit made specifically to work with the Epiphot or is it something else, like a Nikon Lamphouse with the lamp replaced by an LED. Is there a diffuser to spread the light from the LED ?

I am clutching at straws here but the Epiphot user guide refers to the use of a HBO mercury or Xenon lamp. These are very small and very intense sources of light which would be magnified by the system's collector lens and the optics of its vertical illuminator to cover the rear pupil of the objective. Bigger sources, like halogen lamp filaments, needed different optics.

Unless the LED was also almost a point source and in exactly the right position using it with the same optics would produce a very low level of illumination. A diffuser would make things worse.

Henry
Feel free to edit my images.

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