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

• 1. The light from white light emitting diodes is a far better approximation of daylight than are tungsten filament and tungsten halogen bulbs. Coloured ones produce a much narrower spectral distribution than tungsten bulbs, and thus light emitting diodes are far brighter than narrow pass band filtered tungsten light.
• 2. The light from a light emitting diode does not become more orange when dimmed.
• 3. Light emitting diodes emit FAR less heat.
• 4. Light emitting diodes last MUCH longer than any type of tungsten filament lamps.
• 5. It is quite easy to make portable power supplies for Light emitting diode systems, and because they are so efficient, battery supplies last far longer.
• 6. Some standard microscope bulbs are extremely expensive or virtually impossible to obtain.
• 7. When power supplies on microscopes fail replacements are often extremely expensive, and the power supplies for light emitting diodes are less expensive in many cases.

Disadvantages of Light Emitting Diodes as Microscope Light sources

• 1. Some people have become accustomed to observing objects with microscopes with the reddish light of tungsten filament bulbs and prefer it.
• 2. There are many microscopes that are extremely difficult to convert to light emitting diodes, either because of optical or physical design.
• 3. Many people who use older microscopes like to maintain them in original condition rather than try to modify them, even when the modifications are great improvements. Note, however, that many LED replacements can be done so the original light source can be retained and interchanged at will.
• 4. The heat sink requirements of high power Light emitting diodes means that they must be mounted on solid metal holders. Making such holders generally requires the use of metal working machine shop equipment that can include band saws, lathes, and milling machines. The metal holder must have sufficient area and ventillation so that it can dissipate the heat. Note, however, that the heat liberated is dramatically lower than from incandescent bulbs. Higher wattage LEDs obviously have greater heat sink requirements.
• 5. There is some evidence that looking at bright blue sources for long periods of time can be damaging to eyes. The published spectra of white LEDs would lead on to suspect that there should not be a real problem here. Still one should think carefully about using short wave length monochrome LEDs for long observation periods. (And computer software vendors should also try to cure their addiction to making so many blue screens!)

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:

• Luxeon Star The models in this series seem to be all 1 watt models. Different colours have slightly different maximum currents and voltage. Most handle about 350 mA and about 3 to 3.5 volts. These are adequate for many purposes, but not recommended for high power, phase contrast, or dark field illumination.
• Luxeon Star III. The models in this series are 3 watt models. Most can handle 1000 mA and about 3.5 volts, but specifications vary somewhat with different colours. These are ideal for most compound microscope illuminators.
• Luxeon Star V. These models are more expensive. The white ones in this series do not seem to be available any longer. In fact the only models commonly available seem green, cyan, and blue. Most can handle 700 mA and run at between 6 and 7 volts. Apparently this series is being replaced by the K2, see below.
• Luxeon Star K2. There seems to be a somewhat limited variety of colours available in this series, many more not star mounted. That white one comes in two varieties, high current and low current. The high current model produces 130 lumens and runs at 1500ma and 3.85 Volts, making it over five watts.
• Luxeon Rebel. These are a very recent model. They are very small and bright. The small emissive surface may not be good for many microscope applications. They make tri-emitter models of these with 3 emitters on the same star. One of these in white delivers an incredible 540 lumens. This would certainly be suitable for a fibre optic illuminator, but because the light comes from three nearly point sources, it is really not suitable for standard microscope systems.

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.

• The first option is to obtain a simple direct current power source such as standard cells in series and put a limiting suitably sized limiting resistor in series with the LED. One can also search around and fine a variable resistor to use for this purpose so that the LED can be dimmed.
• The second option is to obtain a standard laboratory power supply that is capable of current regulation and that provides voltages in the range between 0 and 10 volts, and currents of from 0 to 2 amps. These devices tend to be expensive, and one must be certain not to connect one to a LED with the current limiter set too high, or one can destroy a LED in an instant!
• There are special power systems available from most organisations that sell high power LEDs. The most practical, perhaps is one called the "Buck puck." These are really current regulators. They require a separate AC or DC power source in the 6V or so range. (There are both AC and DC models of these, the AC models can be used with DC power supplies.) These seem to be available in only 3 currents, 350ma, 700ma, and 1000ma. There are available versions of these that have a pin to adjust current for dimming. A wiring harness is available from the suppliers of these devices, and there is a special version for the ones that can be dimmed that has an attached variable resistor. Many AC microscope power supplies are very suitable for use with the AC versions of the Buck Puck. It is extremely convenient to retain the original microscope power supply in this way when one wants to be able to switch back and forth between LED and the original tungsten filament illumination. Instead of the original microscope power supply one can also obtain the special DC power supplies designed for electronic devices from electronics supply stores, being certain that the one obtained is capable of delivering enough current, and a sufficiently high voltage to drive the LED.
• There are many current limiting circuit diagrams that are available. One can buy the components and build one's own power supply.

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.

Enjoy!