dark-field microscopy

Sometimes on location shoots where I have to pack light I will bring two 3'x10' rolls of black butcher paper(from a craft store for 5c a foot), just roll the two rolls up tightly together and bring a roll of masking tape. It takes about two minutes to tape the two sheets together and tape them to the ceiling. Its cheap, doesn't take up much space and is light. Thanks for the information about the CTO gel, I'd never thought of it like that. Mike ages ago (permalink)

Just a note: I use a CTO on the key light and a CTB on the background. That gives the largest color temp difference. ages ago (permalink)

This a mathematical technique intermediate between direct and reciprocal (Fourier-transform) space for exploring images with well-defined periodicities, like electron microscope lattice-fringe images. As with analog dark-field imaging in a transmission electron microscope, it allows one to "light up" those objects in the field of view where periodicities of interest reside. Unlike analog dark-field imaging it may also allow one to map the Fourier-phase of periodicities, and hence phase gradients, which provide quantitative information on vector lattice strain.

Dark-field microscopy (also called dark-ground microscopy) describes microscopy methods, in both light and electron microscopy, which exclude the unscattered beam from the image. Consequently, the field around the specimen (i.e., where there is no specimen to scatter the beam) is generally dark.

Very simple guys... Remember its a little more difficult with a white background because "White" reflects light "Black" absorbs light. If you are using white you can not have any light spill on the white background. The more light that spills on the white background the more it will "CANCEL" out the blue or gelled light. People dont notice this effect that often because they often shoot on dark backgrounds where this problem doesnt effect the gelled light that much on black background. So when they go and "DO" what they have always have done on a white background they cant figuire it out. Remember light is light and if you put a gell light on a surface and any other type of light is "Competiting" or "Cancelling" out the gell light. The color or gell color will show up very faint or washed out or not at all depending how powerfull the competiting light is. ages ago (permalink)

In the context of lighting design, the back light is sometimes called hair or shoulder light, because when lighting an actor or an actress, backlighting makes ...

Dark-field studies in transmission electron microscopy play a powerful role in the study of crystals and crystal defects, as well as in the imaging of individual atoms.

You should be able to get any brightness of the background, you just have to make sure _only_ the gelled flash hits the background, and nothing else. Usually, you would position the subject as far away from the bg as possible to make sure your subject lights don't brighten background. -Marin ages ago (permalink)

While the dark-field image may first appear to be a negative of the bright-field image, different effects are visible in each. In bright-field microscopy, features are visible where either a shadow is cast on the surface by the incident light or a part of the surface is less reflective, possibly by the presence of pits or scratches. Raised features that are too smooth to cast shadows will not appear in bright-field images, but the light that reflects off the sides of the feature will be visible in the dark-field images.

Tim - You can make a white background come out black if you want to..... johnnyd's advice is good, that'll blue up sunlit backgrounds so a blue gel...... even better. A few things you could have done. 1. Get the subject farther away from the backdrop so that less key light hits it. 2. Feather the light so that it brushes the front of the subject but misses the backdrop. 3. Use a softox or an umbrella with an opaque backing (not a translucent - shoot-through). 4. I forgot what I was going to say...... Originally posted ages ago. (permalink) carlos.benjamin edited this topic ages ago.

Bright field dark field

In optical microscopes a darkfield condenser lens must be used, which directs a cone of light away from the objective lens. To maximize the scattered light-gathering power of the objective lens, oil immersion is used and the numerical aperture (NA) of the objective lens must be less than 1.0. Objective lenses with a higher NA can be used but only if they have an adjustable diaphragm, which reduces the NA. Often these objective lenses have a NA that is variable from 0.7 to 1.25.[1]

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Thanks guys. I did want to move the subject as far from the background as possible, to eliminate shadows on the background and to try to make sure the background was lit with only light from the background flash, but the environment was a cramped office that had only a few spaces where two people could stand beside one another, so I couldn't move the subject as far away as I wanted to. My work is mostly (all) location, and I tried to make my kit as small as DH's, but it's already a lot bigger. Unfortunately I don't think I can carry around another bag with stands and backgrounds in it, so I have to use found backgrounds. Johnny: Thanks, I'll try that. Sounds like it'll work well. This group is a great resource. ages ago (permalink)

Dark-field microscopy techniques are almost entirely free of halo or relief-style artifacts typical of differential interference contrast microscopy. This comes at the expense of sensitivity to phase information.

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Briefly, imaging[5] involves tilting the incident illumination until a diffracted, rather than the incident, beam passes through a small objective aperture in the objective lens back focal plane. Dark-field images, under these conditions, allow one to map the diffracted intensity coming from a single collection of diffracting planes as a function of projected position on the specimen and as a function of specimen tilt.

What about making a grey background come out black? ages ago (permalink)

One limitation of dark-field microscopy is the low light levels seen in the final image. This means that the sample must be very strongly illuminated, which can cause damage to the sample.

Dark fieldimage

I am no expert, but have you tried with a black background? In your set up, the white brightens probably the flash light, in particular with the other light from the umbrella. While a black background would not be as much "additive". see what I mean? ages ago (permalink)

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I agree with mortimer, a black seamless paper background will give a nice rich color when it is hit with a gelled light. I have used this technique quite often for portraits. I have never tried johnnydanger's technique of using a full CTO on the main light and setting the camera for Tungsten and then using an unfiltered flash on the background light, but it sounds like it would work really well. I might give that a try with the black seamless paper for a really rich color. Originally posted ages ago. (permalink) stellarviewer edited this topic ages ago.

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Annular dark-field imaging requires one to form images with electrons diffracted into an annular aperture centered on, but not including, the unscattered beam. For large scattering angles in a scanning transmission electron microscope, this is sometimes called Z-contrast imaging because of the enhanced scattering from high-atomic-number atoms.

The interpretation of dark-field images must be done with great care, as common dark features of bright-field microscopy images may be invisible, and vice versa. In general the dark-field image lacks the low spatial frequencies associated with the bright-field image, making the image a high-passed version of the underlying structure.

Dark-field microscopy is a very simple yet effective technique and well suited for uses involving live and unstained biological samples, such as a smear from a tissue culture or individual, water-borne, single-celled organisms. Considering the simplicity of the setup, the quality of images obtained from this technique is impressive.

Here's the scoop: Make sure the background isn't too bright. If the exposure on the background is too hot then the blue will be light, not a nice rich dark blue. The reason of course is that a dark blue is... well... dark. So make sure the strobe is underexposed some. And here's the kicker that really makes the blue pop. The secret is that a standard flash is somewhere around 5000k and a blue gel is probably somewhere around 7000-9000K. That's only a difference of a a couple thousand kelvin. Try putting a full CTO gel on your key light and setting your WB to the tungsten setting (indoor house bulb). A CTO is about 2800K or so... that shifts the color of the CTB background light WAY into the blue spectrum since the difference is then 5000k or so. Try it, that'll do the trick for sure. Good luck! ages ago (permalink)

There are lots of examples of examples of coloured backgrounds lit with strobes and gels, giving a nice graduated colour effect. In the Northern Short Course article (March 11) they gelled a flash and shot it through some water glasses as a cookie. In the review of the new Vivitar HV flash units (strobist.blogspot.com/2007/02/return-of-classic.html) it looks like strobes are gelled and fired onto a black background (is this correct?). I've created a similar effect using an ungelled flash pointed at a dark painted wall with good effect. Obviously I had to settle for the colour that the wall was painted, but it was a good colour. Last night I tried to colour a white background to get the same effect. The background was the back of a large cupboard, with a laminated plastic on it. The subject chose a blue from a Cinegel sample pack, a dark blue, and I started setting up. The most colour I could get was a light sky blue. I then pulled out four more dark blues from the sample pack and stacked them all up on the flash unit. I dialled the power up and down, put a snoot on the flash to try to get a streak, etc., and the colour was always pretty light like a daylight sky. I wasn't able to get a nice dark blue. The ambient was fluorescent (the location was an office), but we turned those off and the situation didn't change. I also had a reflective umbrella pointed at the subject, and some of the light from the umbrella would hit the background. Thinking about it now, the light from the umbrella might have been the problem -- maybe in the NSC and 285HV articles, the light that lights the subject doesn't hit the background at all? I'm sorry I don't have any numbers or pictures of the setup but I had five clients there for a variety of shots and I had to get things done. For the situation described above, we eventually settled on the white background (they realise I'm an amateur) but one client wanted the sky blue so we left that in for his shoot. 10:03AM, 11 March 2007 PST (permalink)

Dark-field microscopy has recently been applied in computer mouse pointing devices to allow the mouse to work on transparent glass by imaging microscopic flaws and dust on the glass's surface.

It comes directly from light sources but is also reflected off surfaces in the visual field. Luminance contrast is the ratio between the luminance values of ...

In optical microscopy, dark-field describes an illumination technique used to enhance the contrast in unstained samples. It works by illuminating the sample with light that will not be collected by the objective lens and thus will not form part of the image. This produces the classic appearance of a dark, almost black, background with bright objects on it. Optical dark fields usually done with an condenser that features a central light-stop in front of the light source to prevent direct illumination of the focal plane, and at higher numerical apertures may require oil or water between the condenser and the specimen slide to provide an optimal refractive index.[2][3]

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When coupled to hyperspectral imaging, dark-field microscopy becomes a powerful tool for the characterization of nanomaterials embedded in cells. In a recent publication, Patskovsky et al. used this technique to study the attachment of gold nanoparticles (AuNPs) targeting CD44+ cancer cells.[4]

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In single-crystal specimens, single-reflection dark-field images of a specimen tilted just off the Bragg condition allow one to "light up" only those lattice defects, like dislocations or precipitates, that bend a single set of lattice planes in their neighborhood. Analysis of intensities in such images may then be used to estimate the amount of that bending. In polycrystalline specimens, on the other hand, dark-field images serve to light up only that subset of crystals that are Bragg-reflecting at a given orientation.

Weak-beam imaging involves optics similar to conventional dark-field, but uses a diffracted beam harmonic rather than the diffracted beam itself. In this way, much higher resolution of strained regions around defects can be obtained.