Post homework questions online and get free homework help from tutors.
Learn More
Improve Grades
Help Others
Save Time
Accessible 24/7
  153 People Browsing
Related Images
Which 'study break' activity do you find most distracting?
Talking / texting
Playing video games
Listening to music
Watching TV
Browsing the web
If you would like to vote in this poll, please login or register

Previous poll results: Where do you get your textbooks?
Posts: 13
Rep: 0 0
4 years ago
Hi could someone please help me out.
What comparisons (similarities and important differences in structure, function, usage etc.) can be made between dark field, phase contrast, and bright field microscopy.  Im finding phase contrast to be quite a challenging aspect and I dont really understand how it all works compared to bright field.  

Thank you,
As always, I appreciate all the nice people who make the effort to help me out.
Read 2273 times
1 Reply
4 years ago
Bright field microscopy is the simplest of all the optical microscopy illumination techniques. Sample illumination is transmitted (i.e., illuminated from below and observed from above) white light. The most common use of the microscope involves the use of an organism mounted to a glass microscope slide.

Common components
Base - Supporting structure that usually contains an electrical light source or illuminator.
Objective lens(es)- Magnify the image.
Oculars - Magnify the image from the objective lens. A microscope with one ocular lens is often called a monocular, a microscope with two oculars is called a binocular.
Arm - The support structure that connects the lens systems to the base.
Body tube - Sends light to the ocular lens.
Condenser lens - Directs light to pass through the specimen.
Stage - Platform that allows mechanical movement of a microscope slide.
Adjustment knobs - Course and fine focus adjustment.
The magnification of an optical microscope is only limited by the magnifying power of the lens system. However, the limit of magnification for most light microscopes is 1000x which is set by an intrinsic property of lenses called resolving power.

BUT, Dark field microscopy is a technique for improving the contrast of unstained, transparent specimens.[2] Darkfield illumination uses a carefully aligned light source to minimise the quantity of directly-transmitted (un-scattered) light entering the image plane, collecting only the light scattered by the sample. Darkfield can dramatically improve image contrast?especially of transparent objects?while requiring little equipment setup or sample preparation. However, the technique does suffer from low light intensity in final image of many biological samples, and continues to be affected by low apparent resolution.

Rheinberg illumination is a special variant of dark field illumination in which transparent, colored filters are inserted just before the condenser so that light rays at high aperture are differently colored than those at low aperture (i.e. the background to the specimen may be blue while the object appears self-luminous yellow). Other color combinations are possible but their effectiveness is quite variable.

On the other hand, Phase contrast is a widely used technique that shows differences in refractive index as difference in contrast. It was developed by the Dutch physicist Frits Zernike in the 1930s (for which he was awarded the Nobel Prize in 1953). The nucleus in a cell for example will show up darkly against the surrounding cytoplasm. Contrast is excellent; however it is not for use with thick objects. Frequently, a halo is formed even around small objects, which obscures detail. The system consists of a circular annulus in the condenser which produces a cone of light. This cone is superimposed on a similar sized ring within the phase-objective. Every objective has a different size ring, so for every objective another condenser setting has to be chosen. The ring in the objective has special optical properties: it first of all reduces the direct light in intensity, but more importantly, it creates an artificial phase difference of about a quarter wavelength. As the physical properties of this direct light have changed, interference with the diffracted light occurs, resulting in the phase contrast image.

Share This Topic
Similar topics that might interest you...
Cell Biology   5 years ago   colleen   bio_man   1 Reply   1060 Views
Introductory Courses   4 years ago   o1893220   nuria   1 Reply   268 Views
Cell Biology   2 years ago   cuchillo   archie90   3 Replies   213 Views
Computer Studies   2 years ago   Drakor   weeji88   3 Replies   280 Views
This topic is currently locked from adding new posts. Only administrators and moderators can reply. If you'd like to contribute to this topic, start a new thread and make reference to this one. Otherwise, contact a moderator for more options.