XVII. Light sources

The light we see in a microscope comes from the light source. This is made up of the lamp, power supply and lamp housing. Each one of these must be designed to work with the others. The lamp must have the correct power supply and the lamp housing must be designed for the lamp.

The lamp used in a microscope is rated by amount and by color. Amount is rated in lumens, the more lumens the better. Color is rated by the spectrum the lamp produces and by the Kelvin temperature of the lamp.

A lamp can emit a continuous spectrum of light or a discontinuous spectrum. A continuous spectrum means that there is light produced at each point in the visible spectrum and that the spectrum line is a curve, not a series of hills and valleys. A discontinuous spectrum means that there are areas in the visible were no light is produced. An example of this would be the Mercury burner arc source.

Degrees Kelvin is a way of measuring the amount of red or blue in the emitted light. If you dim a lamp (reduce the voltage to it) the light will get redder. If you increase the voltage the light will get bluer, not whiter. This is what degrees Kelvin measures. The sky is somewhere around 5000 deg. Kelvin to 7000 deg. Kelvin. A quartz halogen lamp is around 2800 deg. Kelvin. We humans see both of them as producing white light even though they are very different. Our eyes can be very easily fooled by the color of light, a camera can't be fooled. If you are doing photography or video you must know the Kelvin temperature of the light at the specimen plane. This is dependent on the Kelvin temperature of the lamp and any filters in the light path. If you use film that is designed for outdoor Kelvin temperature and you do not use the correct filter on your microscope the pictures will have an orange-red background. If you use indoor film with an outdoor filter the background will be blue. This is true for color video as well. You should check your video camera's manual and find out the procedure for setting the white balance.

Since Kelvin temperature changes by varying the voltage to the lamp it is critical that you keep the voltage at the rated maximum when doing photography and video. If you have to much light you should use a neutral density filter, do not turn down the light. Light sources come in three basic types incandescent, arc and fluorescent. Just to confuse you when I say fluorescent I mean the type of lamp in your lab ceiling not a lamp that triggers a fluorescent effect. Fluorescent lamps are used with stereo microscopes. The fluorescent tube is in a circle and built into a module that attaches to the front of the stereo. This produces a shadow free illumination at a very low cost. The lamp produces very little heat so it is easy to work with. However fluorescent lamps can't be dimmed so what you see is what you get. Because fluorescent lamps emit a discontinuous spectrum, they may look white but their not, they shouldn't be used for color photography.

Most light sources are incandescent lamps. These are the good old light bulbs we know and love. An incandescent lamp produces light by passing an electric current though the filament. The filament glows and this is the light we get. The filament has to be in a vacuum otherwise it would burn up. In a modern microscope they are invariably Quartz-Halogen lamps operating at low voltage, either 6 or 12 volts. Quartz-Halogen lamps produce more light with less current than other types of incandescent lamps. This means that they are more efficient. For their light output they produce less heat than other lamps and they also last longer.

Earlier types of lamps burned out because the filament got very hot and atoms of the filament material were blown of into the vacuum of the envelope. The filament material wound up on the surface of the glass envelope coating the glass and lowering the light output. This also weakened the filament. The glass envelope of a conventional lamp must be large and far away from the filament to take the heat of the lamp without melting.

A quartz-halogen lamp uses a quart envelope that is much more resistant to heat. This allow the lamp to be much smaller. The vacuum in the envelope contains some halogen gas. When the filament is hot the halogen gas traps the filament material and holds it in suspension. When the lamp cools the material is returned to the filament. Since the re-depositing of the material is not perfectly even the filament will eventually break but it takes much longer than a non-quartz-halogen lamp of the same output.

Some special handling is required for quartz-halogen lamps. You should never touch a quartz-halogen lamp. The grease from your fingers will damage the quartz envelope. When you change lamps you need to hold the lamp with the packaging material provided or with a clean tissue. Be careful inserting the pins on the lamp into the socket, they can bend and break. It is a good idea to rub the pins with emory paper before inserting them as they frequently have corrosion on the pins.

Quartz-halogen lamps produce a continuous spectrum of illumination. Most people find that quartz-halogen light looks whiter than other incandescent light. This is because quartz-halogen light is richer in the blue spectrum. When dimmed quartz-halogen lamps get redder just like any other lamp.

Power supplies for quartz-halogen lamps are built usually built into the base of the microscope. Modern power supplies can vary the voltage continuously across a wide range. Some research microscopes have a fixed voltage power supply and vary the intensity by using neutral density filters. This is great for photography.

Never use a higher voltage or wattage lamp in your microscope. It could kill the power supply. The best hope is that the fuse in the microscope will blow before the power supply goes. Always check the lamp itself since all quartz-halogen lamps look similar.

Arc lamps produce light by arcing (jumping) current between two polls. An arc light is difficult to start because it take a lot more voltage to start an arc lamp than to keep one going. Arc lamp power sources should have a specialized starting circuit. Once started arc lamps produce a lot of light. Zenon arc lamps produce continuous spectra illumination while Mercury arc lamps produce discontinuous spectra illumination.

In the old days arc systems used pointed pieces of carbon to make the arc. Since they were in air the arc was constantly eating the carbon. Either the user carefully and continuously fed the arc or a complex feed system was used to keep the arc going.

A modern arc system uses high pressure gasses to trap and return the arc filament material so modern arc lamps last from 100 hrs. for the 50 watt AC Mercury burner to 400 hrs. for a Zenon lamp. The problem is that if you abuse an arc lamp it can explode. If it does the lamp housing is designed to contain the explosion not necessarily protect the optics. Usually when an arc lamp goes it will destroy at least some of the optics in the lamp housing. Check with the microscope manufacturer or the index in this book for the rated arc lamp life of your lamp.

When a Mercury lamp lets go it puts a certain amount of Mercury into the air. You should evacuate the room for an hour and close any doors. Lab safety should be alerted if applicable. Call your repair tech. and let them know what happened. The tech. should take a look at the system and correct the problem that caused the explosion. Zenon lamps explode with equal force but with no noxious stuff.

The best way to prevent an explosion is to use a resettable timer on the power supply. The timer keeps track of the total time the lamp is turned on. A reset switch return the timer to zero when a new lamp is installed. If your power supply doesn't have a timer a good tech. can add one. The next best way to prevent an explosion is to keep a log. Make sure every user makes entries.

Make sure that you learn how to change the arc lamp. Each manufacturer is different so make the sales rep. train you. When you change the lamp carefully inspect the lamp socket. The enormous heat that arc lamps produce means that the sockets corrode. If you see a lot of corrosion call the repair tech.. If there is a little remove it with some emory paper.

When an arc lamp is replaced I like to run in for about an hour. This helps the electrodes break in, develop the pits that the arc actually comes out of. After an hour the lamp can be turned of and is all ready to go.

The repair tech. should carefully inspect an arc lamp socket for corrosion. Usually an arc lamp socket will have to be rebuilt every few years. While this is a pain it isn't all that difficult.

Arc lamps should be changed at their recommended intervals for reasons other than explosion. An arc lamp puts out a very consistent amount of light up to its rated life. After its rated life it can start to put out one heck of a lot less. Since people have a hard time judging relative light intensity you may not notice that the light has dimmed. It may cause you to misjudge a sample. There are standards available for checking fluorescent output. These are quite valuable but they are not a substitute for changing the arc lamp regularly.

An arc lamp needs heat and lots of it for the gasses in the lamp to operate correctly. This is why an arc lamp should not be turned on and off over a short time span. If you turn an arc lamp on and then turn it of after 15 min. it is like running it for an hour. If you are going to lunch then turn the lamp of, if you are going to get more specimens or a cup of coffee leave it on.

Power supplies for arc lamps are very specialized. They should be used only with the lamp they are designed for. Never mix and match lamps and power supplies. Make sure you know how to use the power supply. Some power supplies require you to turn the supply on and push a button to start the lamp. Others start the lamp when turned on. Both types work very well.

Most arc lamp power supplies generate radio frequency current that is rather powerful. This can effect computers in the vicinity. It is a good idea to turn of any computers or camera systems that are very near the power supply then turn on the lamp. After the lamp is stable turn on the computers and cameras.

Lamp housings contain the lamp, condenser lens and mirror, if any. A modern lamp housing is designed to provide good cooling for the lamp. Make sure that nothing is placed on the lamp housing that could cut of the flow of air. A lamp housing is designed to work in a given orientation. The lamp housing should never be moved out of its designed orientation. If you do the lamp could get hot enough to self destruct.

A lamp housing either is pre-centered or centerable. Modern pre-centered lamp housings allow you to just put a lamp in and go. Centerable housings require you to center the lamp. To do this you will need to know were the condenser and centering controls are. Consult your manual or manufacturers rep. To center the lamp in a transmitted light system place a piece of paper or a Kim Wipe over the field lens in the base. Remove any diffusers or ground glass, all centerable systems have this. Focus the image of the filament on the paper and center it in the circle of the field lens. You need to know were these controls are. Check the manual or the manufacturer's rep. The lamp housing may have an adjustable mirror in it. If it does center the mirror image of the lamp so the images just touch in the center of the field lens. Do not make them overlap. Centering the images side by side will produce the most light.

If you need to center a lamp in a reflected light lamp housing you will need a target. For a materials microscope a blank or mirror specimen work well. For a fluorescence module a white paper end of a H and E prepared slide is good. The white paper will auto-fluoresce as will the H and E stained specimen. The advantage to this is that you will have a full field of fluorescing specimen. Focus on the paper and adjust the condenser lens so that you can see the arc or filament. Place this in the center of the field. If there is an adjustable mirror adjust the mirror image so that the actual image and the mirror images are touching in the center of the field. Do not have them overlap. Now adjust the condenser lens so that the field is evenly illuminated. If you run out of adjustment room with the controls the chances are that you have not properly seated the lamp in the lamp housing. Turn of the lamp and let it cool, this will take at least fifteen minutes. Then open up the housing and make sure the lamp is all the way in its holder.

Now focus on the H and E specimen. These specimens autofluoresce at a medium level. This allow you to see how evenly the field is illuminated. If you can't get a evenly illuminated field call your repair tech. something could be very wrong.

Fiber optic light sources are usually used with stereo microscopes but are sometimes used with compounds for low power work. This light sources use a halogen lamp and fiber optic bundles to get light just about any were you need it. The advantage of the fiber optic bundles is that they can snake all over the place to get the light on the specimen.

Fiber optic glass absorbs large amount of infra-red spectrum light. This makes the light at the end of the bundle cool. These illuminators are great for heat sensitive specimens. Since the illuminator is well away from the specimen the heat from the lamp will not hurt the specimen.

Fiber bundles come in flexible, goose neck and ring light forms. The flexible bundles can be run from the light source to the specimen with the ease of placing an electrical cord. There are stands to hold the flexible fiber bundle and adjust it to suit your needs. Goose neck fiber optic bundles are fiber bundles wrapped with a material that lets them bend and stay were they are bent. They can be bent to point were ever they are needed. Two of these attached to an light source is called a bifucated fiber optic system. This is the most popular type of fiber optic illuminator.

A bifucated illuminator lets you adjust the angle and intensity of the light so that you can get the right amount of light and shadow. Bifucateds let you avoid glare of the specimen. A well built one is so simple to use you will wonder how you got along with out it.

Ring lights are fiber bundles with one end formed into a ring. This provides shadow free illumination. In a lot of instances this is a real advantage. These ring lights are called annular ring lights. If you are looking at a flat specimen this may be the tool for you. Some ring lights are made with a series or small ports rather than being annular. This provides some shadowing. Not as much as a bifucated but more than an annular. Each port is referred to as a "point". A four point ring light can be a good compromise for dissection or parts inspection.
What this means to you Light sources come in a wide variety of types. Pick the one that is best for your procedures. Arc lamps for fluorescence and dark specimens and quartz-halogen lamps for routine bright field.