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ledlyt - the fundamentals

The light source

It has been possible, for some years, to remove the bright-field condenser from beneath a microscope stage, and replace it with a red or green LED run at maximum brilliance, or even, for the curious, with a 'laser pointer' style red laser diode. On our way to the ledlyt, we examined these. There was more definition in all cases due to the close to monochromatic nature of these sources. Colour detail was of course lost, but interference/diffraction effects began to destroy visibility with the LEDs, and it was not easy to see anything else but diffraction with the red laser diode.

Within the last few years, white LEDs have appeared on the market. The spectrum is not entirely flat, since they are essentially a high-energy blue source, exciting a fluorescent-lamp style white phosphor, but they do produce a great deal of light, and virtually no infra-red, from a very small junction area. The normal LED shape has a carefully moulded, probably aspheric curve on the front of the clear epoxy package with the object of concentrating as much light as possible into a narrow forward beam. What we needed was an ideal 'point source', so the curve was just distorting things. We cut the curve off and optically polished the result flat. The result was indeed a 'point source'. To make a point source easily locatable in a microscope, it was potted at the end of a brass tube so the point was at the geometric centre of the tube, and the tube diameter was precision-ground to a consistent value, so all ledlyt tubes are as close to the same diameter as engineering can make them. The photo above is of the very end of a ledlyt tube.

With the ledlyt illuminator constructed, the next step was to mount it firmly, reproducibly, and adjustably on a microscope in place of the condenser. Working from the outside in, a brass mount was turned-up with the same closely-measured dimensions as the condenser. This had a wide centre aperture, in which the machined ledlyt holder was located, so the holder could be slid around under control of three centering screws, in order to position the centre of the ledlyt directly under the optical centre of the objectives. Any practical microscope is not perfectly engineered, of course, and each change of objective with the turret shifts the optical centre. Fine adjustment for each objective can be carried out with the centering screws if required. Since the ledlyt is close-tolerance lapped into its holder, it can be slid out at any time and replaced by another without disturbing the centering of the light source.

The photo above shows the main brass housing replacing the condenser inside the glossy black condenser ring on a Wild M20 microscope. The ledlyt is fitted to the centre holder, and lit.

The breakthrough

With the ledlyt centered, and adjustable in height under the slide stage by using the condenser raise/lower, a quite usable slide illumination system resulted.

We found, by much empirical work, that the addition of an optical stop, and a fine diffuser, both of critical dimensions and positioning, created controllable optical effects around the edges of most specimens. Simply put, they look sharper, and in conjunction with the achromatism of a condenserless system, and the completely even light field on X10 or higher power objectives, the microscope user can see more, more easily. Our two professional users so far are enthusiastic.

Weight considerations

A ledlyt with a brass external housing varies in weight according to how much adaption is required. A complex one can produce a ledlyt assembly weight of around 420 grams, of which about 270 grams is the brass housing. A typical condenser weighs around 260 grams, although cheap thin-aluminium cased units can weigh less than half this. The rack and pinion condenser mount is made to take extra load, in case there is a change of condenser, but if the mount has slack, or corners have been cut, the considerable extra weight of the ledlyt my allow gravity to lower the condenser mount. We have seen microscopes where plainly much less knob force was required to lower the ledlyt than to raise it, indicating the extra weight was getting close to overcoming the friction of the stage. We have not yet found a mount slack enough to allow the ledlyt to drop under gravity.

This potential problem has been overcome by making the external housing out of polyacetal rather than brass. Polyacetal, or acetal resin, is a hard slippery engineering plastic much used for gears, varying in size from those inside a clock or watch mechanism up to giant gearing on industrial rollers etc. Natural polyacetal is a clean white colour. The material was introduced by Du Pont as 'delrin' about forty years ago, and no age-related failure has yet been identified. Close tolerance machining of polyacetal is more difficult than brass, and so the typical external housing price is up from about $100 to about $120.

A polyacetal housed ledlyt weighs 260 grams or less, making the condenser stage feel just the same in operation, whether the condenser or the ledlyt is installed. Unplated brass dulls with age, but is a traditional microscope material. Poyacetal retains its white finish. Both brass and polyacetal housings will be available. Where the original condenser design means that a very thin section is required on the adaptor housing, a brass housing may be the only feasible answer. However, since such a housing would be in all probability small, excess weight should not be an issue. Examining all the microscopes on the market is not possible, so we have to fall back on prediction.