An Overview of LEDs in Microscopy
Robert Pavlis, Girard, Kansas USA

What is a Light Emitting Diode (LED)

Bulk metallic elements consist of atoms with their outer electrons only weakly bound to them, so that they interact to form a band of mobile electrons. The electrons in insulators form bands too, but the outer electrons are all in a band that is completely filled. There is another band above it, the conductance band, but it has no electrons in it. Hence the electrons are not mobile and the material cannot conduct current.

The energy difference between the "valence band" the one filled with with electrons, and the "conductance band", the empty one, in a semi conductor is small enough so that electrons can occasionally be excited from the conductance band, leaving a "hole" in it, and a free electron in the conductance band. Now the "hole" can migrate in the valence band, and the electron can migrate in the conductance band, and electric current can flow.

It is possible to put elemental impurities in semi conductors so that the material contains either free electrons or holes. This is called doping the material. A p-type semiconductor normally has a group III of the periodic element impurity such as boron in it. The boron fits into the semiconductor structure, but lacks one electron, leaving a "hole".

An n-type semiconductor normally has a group V element such as As or P in it. The As or P has an extra electron, which has to go into the conductance band, since the valence band is filled.

If one take a p-type semiconductor and connect it to an n-type one and apply a potential to the junction current will flow readily if we apply the plus potential to the p-type semiconductor and the negative one to the n-type. The positive potential on the p side of the junction will cause the holes to migrate toward the junction, while the negative potential on the n side will cause the electrons to migrate toward it as well. When they meet the electrons will fall into the holes, liberating energy. In a light emitting diode, this energy is in the form of light.

By using different semi conductor materials different band gaps can be present which will result in light having different wavelengths. Because the energies of the bands are not too wide, the wavelengths given off by a given device of this sort tends to fall into a rather narrow wavelength range.

In recent years researchers have developed LEDs with a wide range of band gaps and hence a wide range of LEDs having different colours.

Because LEDs emit only a narrow spectrum of light, they were initially monochrome devices. However, by using a LED that emits blue light and coating it with a phosphor it is possible to make LEDs that give off light that does a fairly good job of approximating daylight. Although the match to daylight is not perfect, it is dramatically better than tungsten filament lamps!

The spectrum of white LEDs has a serious "notch" at about 500 nm. This can present problems in some applications, especially when observing birefringence colours using a polarising microscope. The radiation curves for these devices can be obtained from the Luxeon website. Their publication DS51.pdf is particularly helpful.

With most microscopes conversion to LED illumination can be accomplished in such a manner that one can switch back and forth between tungsten filament and LED illumination at will. This is especially useful with polarising microscopes.

Tungsten filament lamps give off a large portion of their radiation in the infrared. LEDs give off most of theirs at the band gap energy. They are thus far more efficient. Tungsten filament lamps have a filament heated to temperatures in excess of three thousand degrees, but LEDs are scarcely above room temperature.

Recently it has also become possible to make LEDs with far higher power than before, some commercially available ones now give off several watts. Because they are so efficient these high power ones are more than adequate for use as microscope illuminators.

Advantages of Light Emitting Diodes as Microscope Light sources

Disadvantages of Light Emitting Diodes as Microscope Light sources

What kind of devices are available?

Many high power light emitting diodes are available mounted on a star shaped piece of aluminium that can be mounted to metallic heat sinking surfaces. These are probably the best types of high power LED systems to use for microscope systems.

At this point in time there are a rather large variety of these devices available that have different optical and electrical characteristics.

When these devices are used as replacements for tungsten filament sources it is important to be sure that the size of the emissive surface of the LED is adequate. Many, but not all, have a square emissive surface that is four millimetres on a side. This is usually adequate for Koehler illumination systems, and, in fact, may be superior to the originals. This is dramatically the case for the LOMO OИ35.

White LEDs have a phosphor over the emissive surface, and if one examine one while it is illuminated the light will be quite uniform across its surface. Coloured ones, however, show closely spaced lines of illumination. The lines are close enough together so that this does not usually matter. With a few optical arrangements this may result in a banded apparance of the microscope images unless a diffuser is installed. (The illuminator can also be thrown slightly out of focus.)

The most common commercial LEDs are the ones produced under the Luxeon trademark. Series from this group include:

There are other vendors of similar devices, but Luxeon seems to dominate the market.

It is important to recognise that ALL of these have an heat sinking requirement. They all need to be mounted to a metal surface, and it is best to place heat conducting paste behind them. This is more important for high power models than low power ones because the high power ones have to dissipate more heat.

It seems very difficult to find vendors of these things in standard retail stores, however, there are several vendors with extensive lines of them that sell them over the Internet. Many vendors, however, sell LED devices that are really not appropriate for microscopes. Internet search engines enable one to find vendors quite easily.

Designing LED Microscope Illuminators

It is best to examine the optical system of the microscope to be converted to LEDs very carefully before considering purchasing components. If the microscope have some sort of Koehler illumination system the size of the emissive source is extremely important. Examine the dimensions of the current emissive source. In many case the LED can be somewhat smaller than this, but there are limitations to this!

Try to determine the best way to mount the LED so that its emissive surface is AT THE SAME optical position as the present source. Sometimes one can make a mount that will fit in a light bulb socket. This can require some careful machining, but often does not.

Many microscope lamps are mounted on a cylindrical mount that slips into an opening in the microscope so that the light is emitted forward from a light source at the end of the cylindrical mount. On microscopes of this sort it is almost always possible to machine a cylindrical LED holder that will fit into the microscope exactly like the original holder did. The Wild M20, M21, and M40 are examples of microscopes for which this is true. There are VERY VERY many others.

Decide how to route the electrical wires that will deliver current to the LED. For cylindrical mounts like described in the previous paragraph this may require boring a hole down the centre of the mount.

Decide on a metal to use for this purpose. 360 Brass is very easy to machine, but it is very expensive at this point in time, and it is not a particularly good heat conductor. Aluminium is far less expensive and is a better heat conductor. The pure metal and some alloys tend to stick to tools badly and are thus difficult to machine. Other alloys are quite easy to machine. An easily machinable Al alloy is almost always the best choice here. Note the cautions above about the size of the mount--it must have sufficient area to be able to dissipate the heat produced by the LED.

Make up a drawing of the device. Do not rush into making it. Think about the design a while before trying to execute it. If you do not have access to metal working equipment you may be able to find pieces of bar stock that can be cut with metal cutting saws to the right size. Unfortunately, however, some microscopes almost demand that LED holders be machined with great precision--precision requiring lathes, milling machines, and band saws.

Supplying Power to LEDs

Unlike incandescent lamps the resistance of a LED decreases with temperature. For this reason the power source must have a current limiting device or the LED will be destroyed by too much current. There are several options.

My preferred option is to retain the original microscope power supply when possible, and to obtain an AC "buck puck" with current adjustment or similar device to regulate the current. Others choosing this option should be certain to obtain this device with the proper current rating for the LED as supplied by the manufacturer.

Summary. Planning and making the conversion!

The very first thing to do is to examine the microscope that one is planning to convert to see just what is feasible. Some microscopes are very easy to convert, others much more difficult. Remember that the LED must be mounted on a conductive metal heat sink of sufficient size and area to carry away the heat produced by the LED to prevent its overheating. At this point the K2 White high current model is almost always the most practical device.

Design the LED holder. It is best to draw out a dimensioned drawing of exactly what you are proposing to do. Remember that the emissive surface of most high power LEDs is 4mm square, and that it must be located very near to the same point where the tungsten filament was in the original source. One must be careful also to plan the routing of electrical wires. Copper wire should be used, and it should be about 20 gauge.

Remember that if you want to use coloured light it is best to make a separate LED source for each desired colour with an appropriate wavelength LED. This usually is less costly than using filters, and produces a far narrower spectral distribution.

Check that the design does not have any flaws, and then it is time to fabricate the LED holder. This may require small versions of metal machine shop equipment--a metal lathe, a milling machine, and a band saw. It may be possible in some cases to mount the LED on a piece of flat simple aluminium stock, and to fabricate some sort of holder for this. Sometimes machining is, unfortunately, the only option, and, unfortunately, machine shop services tend to be somewhat expensive.

Mount the LED on the holder. One should use the heat conducting heat sink compound paste used in computers between the LED and the mount. The "Star" LEDs are designed to be held in place with 3mm screws. These need to be nylon screws. The holder will need to be drilled and tapped to receive these 3mm screws.

Attach the wires, and place the desired electrical plug on the far end. Connect the device to a suitable power supply and check to be sure it functions.

Install on microscope.


All comments to the author Robert Pavlis are welcomed.


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Published in the June 2008 edition of Micscape Magazine.

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