Darkfield

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

Moderators: rjlittlefield, ChrisR, Chris S., Pau

Mo Vaughan
Posts: 70
Joined: Sun Aug 12, 2007 3:16 am
Location: Cambridgeshire UK

Darkfield

Post by Mo Vaughan »

Hi,
I have been looking at some fantastic shots, and been impressed with the photo's that have used 'darkfield'. Can someone tell me what is darkfield and how I go about achiving good shots using this method?
Thanks,
Mo. :-k
Thebeeman

gpmatthews
Posts: 1040
Joined: Thu Aug 03, 2006 10:54 am
Location: Horsham, W. Sussex, UK
Contact:

Post by gpmatthews »

Hi Mo - and welcome!
Let me see if I can attempt a simple explanation:

Usually what is described as darkfield or darkground illumination refers to a transmitted light technique. That is the light is shone from underneath the specimen, passes through the specimen, and is viewed from above through the microscope objective. This is how normal brightfield is set up. The difference with darkground is that the substage condenser (the lens under the microscope stage that focuses the light on the specimen) has a wheelstop or patch stop in the filter holder. This is just an opaque circle in the centre of a clear filter disc. The light falling on the object is thus a hollow cone of light, rather than a solid cone of light, with the object at the apex. In the absence of a specimen, the objective of the microscope is situated in the dark part of the expanding cone of light above the theoretical specimen position. No light enters the objective and you see just blackness. When a specimen is placed at the tip of the cone of light however, it is like it being in a spotlight and you can see it through the microscope bright on a black background. The light you see is only that scattered by the specimen. Hope that makes sense.

In setting up darkground it is important that the patch stop is central and of the right size (trial end error comes in here). The substage condenser has to be in the right focal position. Darkground is most commonly used in dry non-immersion sytems and it gets trickier at higher mags. Difficult to set set up for a x40 objective, for example, but quite easy at lower mags. There are also ways of using the technique at higher mags with oil immersion, but that, I think, is for another day.
Graham

Though we lean upon the same balustrade, the colours of the mountain are different.

Mo Vaughan
Posts: 70
Joined: Sun Aug 12, 2007 3:16 am
Location: Cambridgeshire UK

Darkfield

Post by Mo Vaughan »

Hi Graham,
Many thanks for the information. Its as clear as mud. Only joking Graham I did understand all of what you wrote. Where do I obtain a wheelstop or patch?
I am waiting for my camera to arrive from USA (its cheaper to buy from America), its half the normal price than in the UK.
Thankyou again for the information.
Happy snappies,
Mo. :?
Thebeeman

gpmatthews
Posts: 1040
Joined: Thu Aug 03, 2006 10:54 am
Location: Horsham, W. Sussex, UK
Contact:

Post by gpmatthews »

I make my own by cutting a disc of clear plastic and then sticking another opaque disc in the centre. I found it useful to prepare a set with centre stops of different size increments. they have other uses as well - see techniques such as COL for example (circular oblique lighting).
Graham

Though we lean upon the same balustrade, the colours of the mountain are different.

Epidic
Posts: 137
Joined: Fri Aug 04, 2006 10:06 pm
Location: Maine

Post by Epidic »

Transmission darkfield requires a special substage condensor. For vertical illumination, you need a special objective as well as a trinocular head. The vertical illumination uses a larger light path and the trinocular head needs a beam spliter to accomodate that.

Another way to think of the difference between brighfield anf darkfield is that brightfield is black lines on a white background and darkfield is white lines on a black background. You don't need a microscope to see it either. Take a large piece of paper and stick it on the wall. Shine a light on it. Hold up a wine glass and the glass will be defined by dark edges. Then tape a smaller black piece of paper on the white piece so that the black paper becomes the background, but leaves an area of white pager around it. The glass shape is now defined by whilte lines. Or to put it another way, brightfield uses destructive interference to darken edges to form an image; darkfield uses constructive interference to brighten edges to form an image.
Will

Post Reply Previous topicNext topic