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The Bad Girls’/Boys’ Guide to bright field vs dark field

bright field vs dark field

What is bright field vs dark field?

What is bright field vs dark field

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.

bright field vs dark field

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)

bright field vs dark field

What Advantages and Disadvantages about bright field vs dark field?

No one system is perfect, and dark field microscopy may or may not appeal to you depending on your needs.

Some advantages of using a dark field microscope are:

Extremely simple to use
Inexpensive to set up (instructions on how to make your own dark field microscope are below)
Very effective in showing the details of live and unstained samples
Some of the disadvantages are:

Limited colors (certain colors will appear, but they’re less accurate and most images will be just black and white)
Images can be difficult to interpret to those unfamiliar with dark field microscopy
Although surface details can be very apparent, the internal details of a specimen often don’t stand out as much with a dark field setup.

Below are contrasting examples of dark field (left) versus bright field (right) illumination of lens tissue paper. Note how they both create a different style of image.

Dark field illumination Bright field illumination

Admit it, by now you’re curious to check out your own dark field! You can create one with minimal time and effort. Just read on…

bright field vs dark field

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