USING HIGH INTENSITY LEDS IN MICROSCOPE LAMPS
Tony Dutton first alerted me to the possibilities of using a bright LED as the source of a microscope lamp. Unfortunately I missed the Gossip at which he showed the use of these LEDs, but he was kind enough to email me with details of the specifications, source of supply and suggest circuit details. I duly ordered these from Maplins. I was delighted to find that Tony had not exaggerated their potential. At the maximum rated current of 25mV they are bright enough for transmitted light with a X20 objective, but I felt that they deserved a proper lamp condenser. An old eyepiece of X10 or X15 makes the basis of a good one, with the eyelens nearest the source. I was lucky to find just such a one in the 'box 'o bits', although with a diameter of .75" it was not a standard size.
The following pictures show how the lamp was constructed. I needed a non conductive tube in which to mount the LED with it's bare electrodes. I was wondering what to use when my eye fell on a dried up orange felt tip pen! The barrel of his proved ideal, and the LED is secured by stuffing this with Plastic Padding (sold by motor spares shops for repairing dents in car bodies). I think the pictures show the rest of the construction clearly. Grub screws secure the LED tube and lens assembly in the barrel when focus has been set. The barrel is drilled and tapped 1/4 Whitworth so a camera ball and socket can be used to mount the lamp on a base.
| The picture shows most of the details: the backplate on the barrel is bored to take the plastic tube holding the LED. The lens assembly is 0.75" in diameter. The eyelens and field lens are both about 1" in focal length |
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| Here is the lamp mounted on the ball and socket camera fitment, and in the normal position for use with transmitted light. The power supply can be seen in the foreground. |
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| And here is the simple power supply - a 9V PP3 battery with a 100 ohm resistor and 220k pot (100k or 50k would have been better but were not available at the time) used as a variable resistor in series, and switch. |
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| The lamp can easily be used for top light: here it is clamped to the stage of my Meopta portable and used to illuminate a slide of foraminifera. |
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The complete equipment: lamp, base, power supply together weigh just under 7oz (190g). Used for transmitted light a typical setting draws under 1mA. If my figures are correct for the capacity of a PP3 this should give 150hrs use. For dark-ground or incident light at 25mA this should provide at least 6hrs continuous use.
There are a number of advantages to this light source:
High efficiency with little or
no heat
Consistent colour temperature with variable power - cool daylight
A reasonable size and shape 'filament'
A very low power consumption
I am most grateful to Tony Dutton for all his help and advice in this project which has proved so worthwhile.