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The Microscope Lamp.
Design Considerations for the Ideal Köhler Illuminator.  
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Filament image conjugates.
Conjugate images of the lamp filament throughout the microscope optical system.
An image of the filament is formed wherever the rays cross over.

This article is in four parts:

Background Requirements Lamp FO Source LED Source

The Microscope Lamp:  Background.

Apart from the microscope itself, the next most important item in the microscopist's armoury is the means by which it is illuminated -- the microscope lamp.
Whilst most modern microscopes have a built-in illumination system, the requirements of a free-standing lamp are considered in this article, on account of the greater flexibility of adjustment and alignment offered by this lamp type.
Readers wishing to skip this historical preamble should go straight to page 2.

Baker oil lamp. Hind and Randles, 1913. It was only during the late nineteenth and early twentieth century that electrical illuminants of various types became available for lighting the microscope. Until then, the only possible light sources were either daylight, gas mantle lamps, limelight, or some variant of the oil lamp. The illustration on the right is a lamp by the English firm of C. Baker from the late 1800s, and represents the ultimate refinement of the paraffin oil lamp for microscopical use.

The chimney is of blackened metal, with a rectangular aperture facing towards the microscope, covered by a glass plate ( a 3" x 1" microscope slide) which is easily removed for cleaning. Immediately in front of this is a holder for rectangular glass filters, and a powerful "bulls-eye" condenser lens with adjustments for height, distance, tilt and rotation.

The height of the lamp can be set by the uppermost of the two knobs to the right of the paraffin reservoir, which engages a rack running the length of the vertical supporting column. The knob below provides a short range of horizontal movement.
A flat wick was used, which could be turned broad-side on for use with low power objectives, and edge-on for high powers. A small amount of camphor was dissolved in the paraffin to produce a whiter flame.

It was a design of microscope lamp which was able to meet all of the criteria of the ideal Köhler illuminator except intensity. It provided illumination sufficient to the requirements of high-power brightfield (in a situation of low ambient light) but was inadequate for darkfields, especially at the highest powers.
For those who would like to refresh their memories on the procedure for setting up Köhler illumination, here is a link to the tutorial.

Old Microscope Lamps.

Here are some examples of source types and microscope lamps which have seen widespread use in the past, and a link to a table comparing the intensities of some of these sources.

Limelight. Limelight was invented in 1816 by Thomas Drummond, and by the 1860s was in wide use in theatres, allowing the use of spotlights for the first time. It also provided microscopists with a small, intensely brilliant spot of light well suited to illuminating their instrument. The spot was produced by directing a narrow flame of hydrogen and oxygen at a block of calcium (the lime) which burnt away in the process, necessitating constant adjustment.
The apparatus shown in the diagram (minus the lime) used oxygen from a compressed cylinder, and either coal gas or air saturated with ether vapour.
Gas lamp. Light is generated in this lamp by using coal gas to heat a mantle, this being a fabric bag impregnated with the nitrate salts of cerium and thorium (or beryllium, aluminum, magnesium) placed over the flame. The fabric of the bag is burned away, leaving a brittle and fragile lattice of metal oxides which become incandescent in the heat of the flame.
There are two problems in using this source to illuminate a microscope -- it is not very bright, and the structure of the mantle makes it difficult to evenly illuminate the field.
Arc lamp. The carbon arc lamp shown in the diagram ran from a DC (!) mains supply through a large heat-dissipating resistor (on the right) which reduced the mains voltage to the operating voltage (20V-65V) of the unit. The electrical discharge between the two carbon rods produced an incandescent crater in the positive (horizontal) rod, which was imaged by a suitable condenser lens into the microscope substage. This rod had to be advanced by hand from time to time as it burned away.
Nernst Lamp. The Nernst lamp was an early incandescent filament lamp which apparently had "a single straight filament broad enough to fill an objective with even illumination when a suitable condenser is used, and of sufficient intensity to use with high powers", though why it needed to be started "merely by warming the filament with a spirit lamp" the authorsHind and Randles. Handbook of Photomicrography. 1913. do not say.
Pointolite Lamp. The "Pointolite" lamp pictured here is not notably different in outward appearance from many other microscope lamps of the period (1930s). The bulb it housed was the unique element -- a tungsten arc lamp. When the lamp was up to operating temperature, the light was emitted by a tungsten electrode at close to its melting point -- "a small incandescent sphere about the size of a peppercorn". In spite of the ideal nature of the source itself, the lamp envelope was too large to allow the approach of a high-power condenser, thus creating difficulties in the illumination of low power objectives.
Baker research lamp. This lamp is typical of a generation of free-standing microscope lamps substantially similar to those in use today, using a low voltage bulb with a compact filament in an envelope small enough to allow the use of a high power condenser. Such a lamp can be used to illuminate the field of any objective from about x10 up to the highest powers.
The manufacturers of this lamp (the now-defunct English firm of C. Baker) have gone to the trouble of locating the lamp field diaphragm in its optically correct position -- between the elements of the condenser lens.
Glass rod lamp. The Baker glass rod lamp can be seen as one of the first attempts to illuminate a microscope using fibre optics, even though the "fibre" was only six inches long and half an inch thick. The light from a tungsten filament bulb was conducted down the rod by total internal reflection, emerging, none too brilliantly, through the lightly frosted end which could be fitted with a right-angle prism to allow incident illumination of the microscope stage. This device is best relegated to the category of things which seemed like a good idea at the time.

The next section deals in more detail with the ideal requirements of microscope illumination, and how closely these may be met in practice.