Bright-field microscopy is useful for specimens, which possess a sufficiently high natural color contrast with the background, or for specimens that can easily be stained by dyes. Now, it is possible to increase the contrast by closing the condenser aperture diaphragm. This, however, results in a reduction of the resolution and introduces diffraction artifacts. The natural colors also become less visible, as the whole image darkens. To overcome these limitations of bright-field microscopy, different optical contrasting techniques were invented.

• Dark Field Microscopy: This is one of the easiest and cheapest contrast-enhancing techniques. The main light beam is not able to reach the objective (and therefore the eye), resulting in a black background image. Light is capable of striking the specimen, however. This light is then scattered into various directions, and is also picked up by the objective. The specimen will appear bright on a dark background. Dark-field illumination can be achieved in two ways. Either a specialized dark-field condenser is used, or a so-called patch-stop filter is inserted into the filter holder of the condenser. The patch-stop possesses a central black area which blocks the main light of the illumination system. The patch-stop may not result in a satisfactory image quality for all magnifications, it is advised to experiment with the size of the central black area. For more information: Darkfield Microscopy.
• Rheinberg Illumination: This contrast enhancing technique is closely related to the dark-field method. In this case the patch-stop filter is modified in such a way that the central black area is replaced with a strongly colored, transparent film. The color of the central area of the filter represents the background color of the microscopic image. The peripheral area of the filter possesses a different color. Specimens will then possess the color of the peripheral area. These filters can be easily made by printing the filter using a color printer on an overhead transparency.
• Phase contrast microscopy: This system was invented by Frits Zernike (who received the Nobel Prize for this invention in 1953). Transparent, colorless objects can differ from their surrounding medium (for example water, or the mounting medium) in that they possess a different refractive index. Using bright field microscopy alone, these objects would nearly be invisible. The phase contrast optics of a microscope is able to convert the differences in the refractive index into a difference in brightness. Depending on the system used, the specimens will either appear bright on a dark background, or dark on a bright background. Phase contrast microscopes need special phase contrast objectives and a dedicated phase contrast condenser. In many cases, the phase contrast objectives can also be used for regular bright-field work, with a slight decrease in image quality. Phase contrast microscopy is commonly used for the observation of bacteria, which are otherwise difficult to see.
• Nomarski Differential Interference Contrast (DIC): The theoretical background of this method is complex. The light of the microscope is split up into two beams by a specialized prism which is located beneath the condenser. One beam passes through the specimen, the other beam does not. The two beams therefore have to pass through different refractive indexes and are then allowed to interfere with each other. The result is an image which gives the impression of being three-dimensional. A cell, for example, will appear to be illuminated from the side, with one corner darker than the other. The individual cell organelles will appear to stand out (or be depressed). The 3-dimensional appearance is an illusion, formed by the shadows and highlights. The formed image is similar to oblique illumination.
• Polarization: This contrast enhancing method is commonly used when viewing bifringent speciems, such as starch grains, crystals and cellulose. The light from the illumination system passes through a polarizing filter and then through the bifringent specimen. These specimens are able to interact with the light in such a way, that the light is split into two components. This light continues, and passes through a second polarizing filter, where it is allowed to interfere. The specimens will appear as bright, colorful objects on a dark background. The colors can change when the filters are rotated. Dedicated polarizing microscopes possess a rotating stage and tension-free objective lenses. Possible tension in glass modifies the plane of the polarized light.
• Fluorescence: Certain specimens, such as chloroplasts or cell walls of plant cells, have the tendency to glow in a visible color when flooded with ultraviolet (UV) light. It is also possible to selectively stain the different parts of a cell with flurochomes (fluorescing stains) to visualize them. The UV light can either be passed through the specimen either from the bottom or from the top (“epi-illumination”). It is recommended to use fluorite objectives, otherwise the glass elements, the lenses, will start to glow as well.
• Oblique Illumination: In this method, the illumination system of the microscope is placed-off center. The light strikes the specimen from the side. The specimens appear darker on one side compared to the other side. It is also possible to use a patch stop filter which allows light to pass through only one side. The effect is, that the specimen seems to create a shadow and appears three-dimensional. See Oblique Illumination for sample images.
• Using Color Filters: Color filters absorb the complimentary color. A red filter will result in green chloroplasts to appear dark. A blue “daylight” filter is commonly used as well. It will absorb the red parts of the spectrum and will enhance the contrast of objects that possess a red color. The blue filter will also increase the resolution, as it allows only the passage of the shorter wavelengths.

Impression of a leaf epidermis on white wood glue. The stomata are clearly visible. Left: oblique illumination; Right: regular brightfield illumination. Oblique illumination gives the appearance of a 3-D surface structure.

Left: Home-made cardboard patch stops for oblique illumination. Notice the off-center hole. Top right: filter holder of the condenser; Bottom right: Commercial dark field patch stop for comparison.

Leaf Stomata impression in glue. The light appears to shine from the left, with one side illuminated and the other side in shadow.

Rotating the patch stop results in an image with different lights and shadows. The contrast of both images was digitally enhanced to increase the effect.

Oblique illumination only allows light to hit the specimen from the side. The main light beam is not able to reach the objective. This can be achieved by placing a patch stop into the filter holder of the condenser. These filters can be made of dark cardboard or other suitable heat-resistant material. The patch stop contains an off-center hole. The main light beam from the microscope lamp is not able to reach the objective. The specimen is illuminated from the side. This results in the image to appear 3D.

The best size and shape of the patch stop filter hole is best determined by experimentation. In any case, the hole should not approach the center of the filter, otherwise the main light beam from the lamp is capable of directly entering the objective, which weakens the effect.

The Koehler diaphragm is centered and in focus. The adjustment is correct.

The Koehler diaphragm is centered but out of focus. Raise or lower the condenser to focus the diaphragm.

The Koehler diaphragm is off-center. Turn the centering screws on the condenser to move the aperture into the center.

Koehler illumination was developed by August Köhler (1866-1948). This illumination principle greatly enhances the quality of the microscopic images (especially photographs). The illumination principle offers the following advantages:

• It illuminates the specimen uniformly without the need of a diffuser.
• It only illuminates the part of the specimen which is actually observed (at a higher magnifications a smaller section of the specimen). This reduces the heating of the specimen.
• It reduces internal reflections. This improves the contrast in photomicrographs.

The Koehler illumination must be adjusted before observation:

1. Rotate a low power objective (eg. 4x or 10x) into position. This will increase the field of view.
2. Insert a slide with a specimen and focus it.
3. Adjust the field iris diaphragm (the diaphragm of the light source) in such a way that its edges become visible. The field of view is reduced this way, only a small round part of the specimen is visible.
4. Raise or lower the condenser (not the stage!) and bring the edges of the field iris diaphragm (not the condenser aperture diaphragm) into focus. The focus of the specimen is not changed. Now both the edge of the iris diaphragm and and the specimen should be in focus. If the height of the condenser is not properly adjusted, then dust of the lamp will come into focus and disturb the image.
5. There are two condenser centering screws/knobs at the side of the condenser. Turn these knobs to bring the field into the center of view.
6. Now you can open the field diaphragm and start regular microscopic observation.
7. When doing photographic work, open the field diaphragm only as far as necessary. Opening it further will increase internal light reflections and result in a lower contrast. You need to observe the edges of the field diaphragm through the camera viewfinder. It may also be necessary to refocus the specimen when looking through the camera.

An opened field iris diaphragm increases the width of the light beam. This setting is used for low magnifications (large field of view)

An closed field diaphragm decreases the width of the light beam.

A closed field diaphragm.

• Uniform specimen illumination: Before the advent of Koehler illumination, a diffusing glass was placed over the light bulb. This had the disadvantage of reducing the light spectrum.
• Reduction in specimen heating: A heated specimen increases evaporation of the water beneath the cover slip and also reduces the dissolved oxygen, a potential problem when viewing live organisms.
• Reduction of light reflections in photographic work: Excessive light is eliminated reducing reflections inside the optical system. As a consequence the contrast of the photographic image increases.

The Koehler field diaphragm is designed to restrict the light beam only on this part of the specimen which is actually observed. Especially at high magnifications only a very small part of the specimen needs to be illuminated.