Notes on using a cheap LED torch on a LOMO stereo microscope and a compound microscope.

by Les May, UK



I use a bulky Russian made stereo-microscope to examine plant material and bumblebees under incident illumination. The low voltage tungsten lamp supplied gives a very uneven spread of light. After a short period of use the metal lamp housing becomes  hot resulting in burned fingers. To alleviate this I have fitted a 40mm fan which blows cold air over the housing, Figure 1. This limits the rise in temperature to a few degrees. But it requires a separate low voltage power supply and the extra lead further restricts the circular travel of the lamp around the optical axis of the microscope. Being able to shift the position of the shadows is a useful aid in making correct identification of the material under the microscope.

Figure 1

Attempts to replace the tungsten lamp with a cluster of white LEDs have not been very successful. It is difficult to solder up to eight LEDs in sufficiently close proximity and almost impossible to place a reflector behind them.

I noticed that Lidl supermarkets were selling hand torches with four and five LEDs for £4.99. A five LED torch is shown in Figure 2 sitting on the stage of the microscope. It just fits in the metal ring of the microscope which previously held the tungsten lamp, Figure 3.

Figure 2

In use the light from the torch is very evenly spread. The intensity is more than sufficient at 5x to 32x magnification. It is perhaps marginal at 56x and a second identical torch is being sought. A lot of light seems to be lost in the optical system of the microscope.

Figure 3

I have used ‘swan-neck’ fibre bundle illuminators many times in my ‘day job’ and the illumination provided by the torch is far superior. It has the added virtue of being totally silent. The cooling fans of fibre bundle illuminators run very quietly, but the slight hum is fatiguing if they are used for extended periods.

I recently started to study mosses. Identification requires examination of the ‘leaf’ using a compound microscope. These are only one cell thick and the all important shape and size of the cells can only be determined by viewing in transmitted light.

A very nice filar micrometer eyepiece allows me to make accurate measurement of cell dimensions. My modern Westbury type microscope has built in illumination and a substage condenser, but it lacks the one essential quality a microscope must have, rigidity. It is possible to use the micrometer but it is a frustrating experience.

I also have a very solid Beck 47 stand which has neither condenser nor substage mirror, but the condenser holder is still there. The eyepieces and objectives from the Westbury fit of course but what to do about illumination? Largely out of desperation I decided to try the LED torch.

By mounting this at various angles and distances above and below the stage some interesting dark field and oblique illumination effects can be obtained. Shining the LED torch onto a sheet of clean A4 paper resting on the horseshoe base of the stand gives images of moss leaves which are very bright at 100x magnification and brighter at 400x than the Westbury. White paper is a very good reflector with 80% or more of the incident light being returned. Now if I can just find the right condenser...

 Comments to the author Les May are welcomed.




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Published in the July 2005 edition of Micscape.

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