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advantages and disadvantages of dark field microscopy

What is Advantages of advantages and disadvantages of dark field microscopy?

What is Advantages of advantages and disadvantages of dark field microscopy?

A dark field microscope is ideal for viewing objects that are unstained, transparent and absorb little or no light.

These specimens often have similar refractive indices as their surroundings, making them hard to distinguish with other illumination techniques.

You can use dark field to study marine organisms such as algae and plankton, diatoms, insects, fibers, hairs, yeast and protozoa as well as some minerals and crystals, thin polymers and some ceramics.

You can also use dark field in the research of live bacterium, as well as mounted cells and tissues.

It is more useful in examining external details, such as outlines, edges, grain boundaries and surface defects than internal structure.

Dark field microscopy is often dismissed for more modern observation techniques such as phase contrast and DIC, which provide more accurate, higher contrasted images and can be used to observe a greater number of specimens.

Recently, dark field has regained some of its popularity when combined with other illumination techniques, such as fluorescence, which widens its possible employment in certain fields.

advantages and disadvantages of dark field microscopy

RPR/VDRL/MHA-TP (Serologic Tests for Syphilis) Darkfield/FTA-ABS MicroscopyRPR / VDRL / MHA-TP

RPR/VDRL/MHA-TP (Serologic Tests for Syphilis) Darkfield/FTA-ABS Microscopy

A variety of serologic tests for syphilis are available, including:

VDRL (Venereal Disease Research Laboratory)
RPR (Rapid Plasma Reagin)
FTA-ABS (Fluorescent Treponemal Antibody Absorption)
TP-MHA (Treponema Pallidum Microhemagglutination Assay)

Each differs the others in the precise substance being measured, complexity, and specificity. All are satisfactory for use in managing syphilis. Abnormals may be:

Reactive,
Weakly reactive, or
Bordeline

Whenever a screening test (RPR, VDRL) is positive, a more specific test (FTA-ABS, TP-MHA) should be used to confirm the test and rule out a “biologic false positive.”

A negative or “nonreactive” test may indicate:

The patient doesn’t have syphilis
The patient has syphilis, but is so early in the course of the disease that the test has not yet turned positive. In these cases, the test may never turn positive if the patient is effectively treated.
The patient had primary syphilis, had a positive test, was effectively treated, 6 months have passed and the test has now reverted back to negative.
The patient had secondary syphilis, had a positive test, was effectively treated, 12-18 months have passed and the test has now reverted back to negative.
The patient has syphilis, but his/her immune system is impaired.

A positive or “reactive” test may indicate:

The patient has syphilis.
The patient had syphilis, was effectively treated, but the test has not yet returned to negative:
With primarily syphilis, it typically takes about 6 months for the test to turn negative.
With secondary syphilis, it typically takes 12-18 months for the test to turn negative.
The longer syphilis remains untreated, the longer it will take for the test to return to normal, and the less likely it is to ever return to normal.
The patient has a biologic false positive (BFP)

advantages and disadvantages of dark field microscopy

What is advantages and disadvantages of dark field microscopy?

What is advantages and disadvantages of dark field microscopy

Brightfield microscopy uses light from the lamp source under the microscope stage to illuminate the specimen. This light is gathered in the condenser, then shaped into a cone where the apex is focused on the plane of the specimen. In order to view a specimen under a brightfield microscope, the light rays that pass through it must be changed enough in order to interfere with each other (or contrast) and therefore, build an image. At times, a specimen will have a refractive index very similar to the surrounding medium between the microscope stage and the objective lens. When this happens, the image can not be seen. In order to visualize these biological materials well, they must have a contrast caused by the proper refractive indices, or be artificially stained. Since staining can kill specimens, there are times when darkfield microscopy is used instead.

In darkfield microscopy the condenser is designed to form a hollow cone of light (see illustration below), as apposed to brightfield microscopy that illuminates the sample with a full cone of light. In darkfield microscopy, the objective lens sits in the dark hollow of this cone and light travels around the objective lens, but does not enter the cone shaped area. The entire field of view appears dark when there is no sample on the microscope stage. However, when a sample is placed on the stage it appears bright against a dark background. It is similar to back-lighting an object that may be the same color as the background it sits against – in order to make it stand out.

advantages and disadvantages of dark field microscopy

What is Basics of advantages and disadvantages of dark field microscopy?

What is Basics of advantages and disadvantages of dark field microscopy ?

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. Hence the descriptor “darkfield”.

Darkfield is therefore 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.

advantages and disadvantages of dark field microscopy

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