Dark Field Illumination

Darkfield Illumination Advantages

darkfieldillumination — Thu, 17 May 2007 09:16:46 +0000

All of us are quite familiar with the appearance and visibility of stars on a dark night, this despite their vast distances from the Earth. Stars can be readily observed at night primarily because of the stark contrast between their faint light and the black sky. Stars are shining both night and day, but they are invisible during the day because the overwhelming brightness of the sun blots out the faint light from the stars, rendering them invisible. During a total solar eclipse, the moon moves between the Earth and the sun blocking out the light of the sun and the stars can now be seen even though it is daytime. In short, the visibility of the faint star light is enormously enhanced against a dark background. This principle is applied in darkfield, also called dark ground, microscopy, and a simple and popular method for making unstained transparent specimens clearly visible. Such objects often have refractive indices very close in value to that of their surroundings and are difficult to image in conventional brightfield microscopy. For instance, many small aquatic organisms have a refractive index ranging from 1.2 to 1.4, resulting in a negligible optical difference from the surrounding aqueous medium. These are ideal candidates for darkfield illumination.

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Basics and Principles of Darkfield Microscopy

darkfieldillumination — Thu, 17 May 2007 09:16:06 +0000

A darkfield microscope is a magnifying device in which objects are lit at a very low angle from the side so that the background appears dark and the objects show up against this dark background. Therefore, darkfield is the method whereby the sample being viewed is actually in front of a dark background and light is being angled onto the sample from the sides. Under phase contrast conditions, the light coming through the specimen is shifted into two beams, one slightly out of phase with the other. This gets a little complicated to explain easily, but as far as equipment concerns you need two matched items in order to get phase contrast. One needs a phase annulus, and the matching lens objective. For instance, if you want 40x magnification phase contrast microscopy, you need a 40x phase lens, and a matched 40x phase annulus. If you want 100x phase, you need the 100x lens and the matched 100x phase annulus. Both the techniques of darkfield and phase contrast allow nearly invisible microorganisms within the blood to be lit up and seen. It also clearly delineates the blood cells. This method is in contrast to the standard microscope brightfield conditions where light shines directly through the viewed sample, and invisible particles remain invisible. To view a specimen in dark field, an opaque disc is placed underneath the condenser lens, so that only light that is scattered by objects on the slide can reach the eye. (more…)

Darkfield Microscopic Blood Analysis

darkfieldillumination — Thu, 17 May 2007 09:15:17 +0000

A darkfield microscope is a microscope intended to permit diversion of light rays and illumination, from the side, so that details appear light against a dark background as opposed to light passing straight through the specimen. If bright lights from the microscope pass directly through the specimen, the heat from the light source will kill the red blood cells faster. In addition, by diverting the light rays, a greater amount of depth and details can be viewed. Darkfield microscopic blood analysis as studied, is a technique that could be very difficult to locate among medical practitioners that use them. The FDA does not approve of darkfield microscopic blood analysis, thus hands of many doctors are tied. Viewing a fresh, natural blood sample, a sample not altered with any stains, etc., needed for normal microscopic exams, under the technology of a darkfield microscope, will reveal conditions of your blood not normally even considered during the diagnosis of a normal blood test performed in the office of doctor or in a lab. However, an increasing number of health professionals have found that the use of this technique allows inspection of cellular dynamics which as noted above normally escape analysis or diagnosis using orthodox medical tests. (more…)

Rheinberg Illumination

darkfieldillumination — Thu, 17 May 2007 09:14:29 +0000

Closely related to darkfield illumination is the Rheinberg illumination. The light stop is replaced by colored filter material. Surrounding this filter are filters of one or more colors that contrast with the color of the central filter. The central filter will be the same size as a darkfield stop would be for the objective in use. One must first work out darkfield setup for each objective before trying to work out the corresponding Rheinberg setup. Rheinberg illumination combines normal brightfield illumination in one color with darkfield illumination in another color. When light from the center filter strikes the condenser, it will throw a small, solid cone of light upward after focusing in the plane of specimen. Surrounding this cone will be a hollow cone of light from the surrounding filters. (more…)

Darkfield Illumination

darkfieldillumination — Thu, 17 May 2007 09:13:34 +0000

An optical microscopy illumination technique used to enhance the contrast in unstained samples is called darkfield microscopy. It works on the principle of illuminating the sample with light that will not be collected by the objective lens, so not form part of the image. This produces the classic appearance of a dark, almost black, background with bright objects on it. A bright specimen on a dark background is presented by darkfield illumination. Research microscopes are equipped with special darkfield condensers. Gladly, it is possible to improvise darkfield illumination on almost any compound microscope. Darkfield microscopy techniques are normally used by scientists to, due to the simplicity of the setup, are almost entirely free of artefacts. However the interpretation of dark field images must be done with care as common dark features of brightfield microscopy images may be invisible, and vice versa. 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 qualities of images obtained from this technique are impressive. The low light levels seen in the final image is the main limitation of dark field microscopy. This means the sample must be very strongly illuminated, and can cause damage to the sample. The path of light goes as light enters the microscope for illumination of the sample. The condenser lens focuses the light towards the sample. A specially sized disc, the patch stop blocks some light from the sample, leaving an outer ring of illumination. The light enters the sample. (more…)

Darkfield Principles

darkfieldillumination — Thu, 17 May 2007 09:12:43 +0000

Darkfield illumination is most readily set up at low magnifications even up to 100x, even though it can be used with any dry objective lens. Any time you wish to view everything in a liquid sample, debris and all, dark field is best. In darkfield microscopy, to view a specimen in dark field, an opaque disc is placed underneath the condenser lens, so that only light that is scattered by objects on the slide can reach the eye. Instead of coming up through the specimen, the light is reflected by particles on the slide. Everything is visible regardless of color, usually bright white against a dark background. Pigmented objects are often seen in false colors, that is, the reflected light is of a color different than the color of the object. Better resolution can be obtained using dark as opposed to bright field viewing. Sophisticated equipment is not necessary to get a dark field effect, but you do need a higher intensity light, since you are seeing only reflected light. At low magnification, up to 100x, any decent optical instrument can be set up so that light is reflected toward the viewer rather than passing through the object directly toward the viewer. To set up a dissecting microscope however, for dark field viewing, the specimen should be placed over an opening so that light reflects only from surfaces between cover slip and slide, not from a surface beneath the slide. You may need to make a stand to hold the slide. The surface beneath the opening should be a flat black. Turn off any built-in illuminator. Aim a high-intensity light source toward the specimen at an angle, from the top or side through a glass dish or jar. Using a high powered microscope, with a compound microscope, dark field is obtained by placing an occulting disk in the light path between source and condenser. (more…)