LED. New lights for old Leitz microscopes

By Laurent Delvoye

LED makes exellent light for Diavert. 

In the last few years Light Emitting Diodes (LED´s) have been perfected to the point that their application for general lighting purposes has become a reality. Not only their luminosity has enormously increased, but they are also available in a broad range of colours, including white and near-UV. Those properties make them of interest for the microscopist.

Other desirable properties are: low power consumption und thus low heat generation, their long working life and the fact that on dimming the colour of the light remains constant. After reading some articles in Miscape on the subject (www.microscopy-uk.org.uk) I decided to make a LED illumination as an experimental prototype for my Leitz Orthoplan. This first attempt proved to be very succesful, so soon two more were constructed, one for my Ortholux and one for my Diavert. Thes two have replaced the original illuminations.

Contruction of the prototype

The LED was ordered on line at www.led1.de This on-line shop stocks nearly all the types on today’s market. On this website datasheets and other important information is available. After some browsing I ordered the Prolight Power 3 Watt white type. Within three days it arrived. Construction could start. In my scrapbox I I found the Leitz connector ring, a tube for the lampbody and a simple focussing mechanism with the condensor lens still in.

Putting them together was relatively simple.  From a sheet of PVC (1 cm thick) I cut a with a jigsaw a few connecting rings., filed and sanded untill the inner- and outerdiameters  fitted the parts to be connected. There were holes drilled and tapped in the material. The focusing part was assembled with lampbody using M3 screws.  See figures 1a, 1b.

 

The LED was fastened was fasetened with two M3 screws with nylon washers (beware of short circuit!)  on a suitable cooler. A small amount of heat-sink grease was applied between LED and cooler. Although the total heat production is small, locally the temperature in the LED is high enough to require cooling. In turn, the LED-cooler assembly was mounted on the bottom of a tin can with a long M-3 screw. In the bottomplate of the lampbody I driled an oversized hole. With two coins and two nuts a primitive but effective centering mechanism was realized. See figures 2a, 2b.

On soldering the wires on the LED care should be taken with the polarity. To avoid problems a red wire is connected with ‘ +  ‘  and a blue with ‘ – ‘. Use a low Watt soldering iron – a 15W is okay. A 2 Ohm/2W resistor is soldered directly to the LED. A 2500 mA 5 V constant current adapter was stripped from its outer casing, its output ‘ + ‘ wire connected in series with a 100 Ohm wire-wound potentiometer. Put together in a plastic mounting box it makes a fine power reguylator for the LED. There is enough light for normal microscopy. Low power darkfield and phase contrast viewing are possible. For the latter a green 3W LED is recommended. Then no filtering of the 3W white LED is required, with the lost of light that goed with filtering. From the beginning this illumination worked so fine that I ever used it since.

And this is how it works on the Orthoplan.

Construction of LED illumination for the Orthoplan and the Diavert

As the foregoing LED illumination was mainly constructed from scrap, here I faced a different situation being reluctant to modify the existing lampholders of bothe microscopes. So I decided to use the defective lamps as a base for the LED’s. For that I wrapped them in a paper towel and crushed the bulbs gently in a vice. The interiors remained intact. The two wires, that spun the tungsten wire were clipped to the proper size and gently sanding with fine sanding paper to remove any insulating material. After establishing with a multimeter the polarity, red and blue wires were soldered resp. on the ‘ + ‘ and ‘ – ‘. For the Ortholux, the 1W LED-cooler combination was directly mounted on the lampbase with Marine-Tex, a two component filler material used in in boat repair. The same product for carbody work will probably work as well.

For the Diavert a narrow passage in the lamphouse posed a serious problem. Therefore as a heatsink a 10Ct Eurocoin was soldered on a piece of short 15 mm copper tubing. The 3W LED is mounted with two M2 screws, with M2 threads tapped directly into the coin. This was mounted in turn on the lampfitting with Marine-Texz. After some careful filing on the outer edges of the coin and the LED. It could pass the narrow ring and it fitted perfectly. Both illuminations perform splendidly! See figure 3.

An experiment with UV-LED’s

My friend Piet Houpt drew my attention to his fluorescence experiment with UV-LED exitation with marine algae. With autofluorescence, the performance is in general to weak. But with fluorochromes it can work. So I made an adapter with a small UV-LED for one of my Ploemopak attachments.  For coral skeletal selections it works. There is light! But barely so. We will have to wait until stronger UV-LED’s become available. In this respect RGB-LED’s are also interesting. In this type of LED three typoes of light can be seperately lit, in approximately the standard red-green-blue excitation of standard epifluorescence equipment.  So in principle a simple turn of a knob can change the excitation wavelenght.  See figure 4.

On january 15-2007 Laurent wrote…

LUXEON K2

My experiences with the newly marketed LED Luxeon K2 are impressive. I managed to build one in an Orthoplan illuminator using a resistor 1 Ohm-2W for 5V DC. Perfect! The light is more halogen, not as blue as the Prolight 3W,  while giving twice the output. On using the Heine condensor there is now more than enough light.

NICHIA has announced a super DV/LED and when it becomes available I intend to build it in an old style Ploemopak. I’ll keep you posted.

Laurent Delvoye was trained as a medical biologist. His main interest in the biology of tropical corals. He wrote several publications on coral reproduction and tgheir light biology. Since he will retire soon as a biology teacher in high school, he will be able tot devote a larger part of his time to the subject of his interest.

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