Question

detail note on kohlar technique

Answer 1

Köhler illumination is a method of specimen illumination used for transmitted and reflected light (trans- and epi-illuminated) optical microscopy. Köhler illumination acts to generate an even illumination of the sample and ensures that an image of the illumination source (for example a halogen lamp filament) is not visible in the resulting image. Köhler illumination is the predominant technique for sample illumination in modern scientific light microscopy. It requires additional optical elements which are more expensive and may not be present in more basic light microscopes.

Answer 2

Answer:

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Kohler illumination

method of illumination first devised in 1893, Kohler illumination optimizes a microscope’s optical train to produce homogenously bright light free from artifacts and glare.

Working for the Carl Zeiss Corporation, August Kohler introduced this method as a replacement for critical lighting techniques which illuminated the specimen by using the collector lens to form an image of the illumination source on the specimen. The illumination was uneven and the filament of the light source’s bulb was imposed on the specimen.

While different solutions were attempted to eliminate this problem, from diffusing the light to lowering the light’s intensity, these remedial efforts had their own drawbacks.

Kohler’s method of properly aligning the incident light and perfectly defocusing the image of the light source allows the best possible imaging in an evenly dispersed light field.

Components

Kohler illumination needs a high density illumination source, field diaphragm, condenser diaphragm, and collector and condenser lenses.

Although all major manufacturers of high quality microscopes provide Kohler illumination, if a microscope doesn’t have Kohler illumination it can be retrofitted as long as certain criteria can be met.

A true Kohler lamp has a very large filament but a standard lamp can be used. If your microscope doesn’t have a field diaphragm your unit will need to allow for the addition of this and the necessary lenses mounted above the lamp. Also, you will need to be able to raise and lower the condenser.

Working Principle

In Kohler illumination, four separate planes combine to form conjugate planes in both the illumination and image-forming light pathways.

The lamp filament, aperture diaphragm, back focal plane of the objective lens, and the eye point which is approximately one centimeter above the top lens of the ocular, form the illumination conjugate plane.

The conjugate planes of the imaging light path are the field diaphragm, specimen, the fixed diaphragm of the ocular, and the retinal plane of the viewer.

In Kohler illumination the collector lens or field diaphragm collects light from the illumination source and focuses it at the front focal plane of the sub-stage condenser’s aperture diaphragm which, in essence, projects an image of the lamp filament onto the lens.