The Microscope Lamp: Performance Requirements.
There is one essential condition that a microscope lamp designed for Köhler illumination must fulfil, and a number of desirable ones. The essential condition is that the lamp condenser optics must be capable of projecting an image of the source large enough to fill the substage condenser of the microscope.
Image of lamp filament formed by lamp condenser in the plane of the substage diaphragm.
The main structural elements of the microscope lamp body are the source, in a housing which allows movements for centration and focusing, a condenser lens with an iris diaphragm, and a filter holder.
This is in turn mounted on a stand which allows the lamp body to be oriented and reliably clamped in any position.
Desirable attributes of the lamp condenser are:
- It should have as large a diameter as possible to enable illumination of the
large fields covered by low power objectives.
- It should be as powerful as possible in order to project the required filament image in the shortest possible distance.
- It should have the highest possible aperture (lowest possible f number) for the efficient collection of light radiating from the source.
- It should be well-corrected for spherical aberration so that all light collected from the source is focused into a single image plane, and so that even a small source placed at its focus is capable of filling it evenly with light.
- Its diaphragm should be close to the condenser field lens, and should be capable of closing down to a very small aperture -- small enough to encroach upon the field of the highest power objective.
Desirable attributes of the light source are:
- It must have the highest possible intensity (luminous flux emitted per unit area), distributed uniformly across its area.
- Its intensity should be continuously variable over the entire range required by the microscope without change in colour, uniformity or effective size.
Desirable attributes of the lamp construction are:
- Lamp tilt, rotate, and distance from the microscope must be easily adjustable, and once set, should be sufficiently rigid to resist accidental misalignment.
- There should be provision for centreing the source relative to the lamp condenser/diaphragm.
- Filters etc. should be easily removeable and replaceable, and preferably located well forward of the lamp condenser.
- The condenser/diaphragm assembly should be easily removeable, and its components easily separated for cleaning.
- Focusing of the source should be possible without altering the distance of the lamp condenser from the microscope.
- The lamp should not become too hot for comfortable handling.
- If the microscope requires different light sources, they should be easily interchangeable, ideally without upsetting the lamp alignment.
Not all of these conditions are met in any one microscope lamp. The nature of the compromises which must be made are dealt with next.
Reconciling the Requirements.
The difficulty of reconciling points 1-4 is underlined by the following observations.
- Powerful condensers have strongly curved elements which, by the geometry of spherical surfaces, restricts them to a smaller maximum diameter than less powerful condensers.
- Less powerful condensers, which can be larger in diameter, produce a smaller image of the source at any given projection distance, and must therefore be moved further from the microscope in order to fill the substage condenser. The further away they are moved, the smaller field they illuminate.
- Lenses suffering from spherical aberration project a less intense image of the source, as light from different zones of the lens comes to a focus in widely separated planes in the region of the main focus. Lenses of smaller aperture suffer less from spherical aberration, but are less efficient at collecting light from the source.
- Lenses of large aperture, efficient at collecting light from the source, are difficult to correct for spherical aberration.
- For lenses of any given power (focal length), larger aperture (smaller f number) translates into larger diameter.
In addition to these generalizations on condenser lenses, there are some specific conditions which must be met regarding the requirements of the microscope.
Microscope substage condensers are (or used to be) spherically corrected for a lamp distance of 250mm, and a slide thickness of 1mm. Unless the lamp is capable of filling the substage optics from this distance, the substage condenser cannot deliver an optimal performance.
This is not particularly important in the case of Abbe condensers, but is important in the case of condensers which are achromatic and aplanatic.
If the lamp is intended to provide brightfield illumination for a microscope having objectives from x10 to x100, without the need for readjustment, the most difficult part of the operation is likely to be filling the field of the low power objective.
If it is assumed that the diameter of the diaphragm in the eyepiece is 20mm, the field seen by an x10 objective will be 2mm in diameter. If the substage condenser in use has a focal length of 10mm (about average) and the lamp is situated 250mm from the substage condenser diaphragm, then the magnification of the substage condenser system is about x25 (250 divided by 10) and the diameter of lamp condenser required to fill the field seen by the objective is therefore 25 x 2mm = 50mm.
Very few microscope lamps have a condenser of this diameter, and it would be unusual for a 50mm diam. condenser to have a focal length sufficiently short to fill the substage condenser at a lamp distance of 250mm, at least with the size of light source (2-3mm) commonly available.
Having discussed some of the issues involved in the design of an optimized Köhler lamp, we now proceed to the discussion of a particular lamp which has arrived at its own set of compromises.