Kohler Illumination Research Articles

A light source for Köhler illumination.

THE advantages of Kohler illumination have recently been discussed by Barer and Weinstein1, and also the difficulties involved when a single auxiliary lens is used. They describe a new microscope lamp with a variable auxiliary lens-system which overcomes these difficulties. For most purposes, however, a single auxiliary lens can be used if a sufficiently large and reasonably uniform light source is available. This can usually be obtained without serious loss of light if a piece of ground glass can be placed very close to the filament of a suitable car headlamp bulb or projector bulb; but unfortunately, in most lamps, whether commercially available or home constructed, it is difficult to arrange this. A satisfactory method is to grind lightly a circular patch on the end of the bulb, using a mixture of fine well-worn ‘Carborundum’ powder and water on a glass plate. It is not suggested that this provides so versatile an illuminant as that described by Barer and Weinstein; but it does enable an ordinary socalled ‘research’ lamp to be used for Kohler illumination with medium- and high-power objectives, while if the auxiliary lens is of large diameter it will also illuminate most of the field of low-power objectives.

The Influence of the Condenser on Microscopic Resolution

An expression is found for the distribution of light in the image of two small apertures which are illuminated by a condenser of numerical aperture equal to s times that of the imaging objective. The resolving power of the system is written ρ=Kλ/(N.A.), and K is found as a function of s. It is shown that precisely the same result obtains for both critical and Kohler illumination.

An Extension of Toraldo’s Theory of Phase Contrast I*

The problem of phase contrast image formation with Kohler illumination is a special case of a more general problem which has been treated by Duffieux and his students: The Fraunhofer diffraction pattern due to the object is projected upon the phase pattern, and the diffraction pattern due to the latter constitutes the image. A semi-infinite plane covered with a uniform, transparent phase-shifting layer is to be regarded as a typical microscopic object. The variation in the appearance of the image as the light goes through different portions of the phase annulus is examined. The field is darkened on one side of the edge of the object and brightened up on the other side. If it is desired that the dark side shall be as dark as possible if a large portion of the annulus is filled with light, a quarter-wave phase shift must be introduced in the phase annulus. For light passing very close to the edge of the annulus, point-for-point image formation is no longer rigorously possible. As the annulus is made broader, the contrast becomes less pronounced.