Parts of the Microscope

It is very important that we all agree on what we are going to call the parts of a microscope. As a repair person I have had to many callers tell me that the thingummy next to the small knob is broken. Well there aren't any thingummies on the modern microscope. What is worse is that a lot of times I could have told the user how to fix the problem over the phone if I just knew what the heck they were talking about!

The main module of a microscope is the frame. This is the module that contains the focus mechanism and the connecting points for other modules. The frame can be any of the types we discussed in Basics. The design and manufacture of the frame is central to the performance of the microscope.

In the bad old days the user sat behind the frame since the separate light source had to be in front. Now all microscopes let you sit in front of the frame. This makes it easier on the user since it is a lot easier to put specimens on the stage. The controls on a modern microscope are designed for use from the front. The top part of the frame of an upright microscope is called the limb. This supports the nosepiece, any intermediate pieces and the observation tube. The bottom of the frame is called the base and usually contains the electronics for the illuminator and the field diaphragm assembly.

Focus mechanisms move either the nosepiece (in AO products and inverteds) or the stage (in all other microscopes) to focus the image. The precision of the focus mechanism is critical to using high resolution objectives. If the focus mechanism isn't precise then it is difficult to focus the specimen and keep it in focus.

The fine focus can be either limited or continuous. Limited fine focus mechanisms can only focus over a limited range. These mechanisms tend to be very accurate but when you run out of fine focus you need to return the focus mechanism to the center of its range and focus as well as you can with the course to get the fine focus into range. This is a real pain, that's why most modern makers use the continuous fine focus. Continuous fine focus uses a gear reduction system to connect the coarse and fine focus. This means that the fine focus can be used through out the whole focus range. Modern continuous focus mechanisms work very well. I feel that the trade of convenience for performance is worth it.

The nosepiece attaches to the frame as a module or is mounted permanently to it. It is drilled and tapped to accept the objectives. A nosepiece is rated by how many objectives it can accept. The usual size these days is five but six place nosepieces are available and are usually a good deal.

If the nosepiece angles the objectives towards the user if is called a front facing nosepiece. If it angles the objectives towards the frame it is called a rear facing nosepiece. Rear facing nosepieces are the easiest to use although they can be more difficult to manufacture. It is easier to put specimens on a microscope with a rear facing nosepiece and it is more difficult to get your fingers on the front of the objectives and dirty them up.

Some frames have the nosepiece permanently mounted to it. This is to reduce cost and in some cases is a reasonable trade of. Removable nosepieces allow you to put another set of objectives on the microscope very easily. If you are doing a lot of techniques this is a help. Cleaning objectives is also easier with a removable nosepiece.

The objectives screw into the nosepiece. Objectives provide both resolution and magnification for the microscope. However they don't provide all the magnification or resolution. It is perfectly OK to unscrew and examine the front of objectives. In fact it is something you should do routinely.

The observation tube is attached to the frame above the nosepiece. I'm not going to even go into monoculars, a modern observation tube is a binocular at least. Observation tubes can also have positions (called ports) for one or more documentation systems (cameras, tv,etc). The observation tube has two places to put eyepieces, also called oculars. We aren't going to call them oculars because it is yet another confusing term. The eyepieces magnify the image and provide a place to put measuring reticule. Research microscopes usually have a place in the frame to put reticules. This allows the reticule to be photographed with the specimen.

Any module located between the limb and the binocular tube is called an intermediate piece. These include fluorescence, reflected light, multi-veiw and magnification changers modules. When you are calculating the total magnification of the microscope you must include any intermediate pieces.

The stage sits below the nosepiece and supports the specimen. Most modern stages are mechanical with low, coaxial controls. This means that there is a single control hanging down from the stage that moves the specimen. It is very important that the stage be flat and that it be at right angles to the objective. If it isn't the specimen will keep going out of focus as it is moved. This is more important as the resolution of the objective increases. If you constantly have to refocus as you scan a specimen call your service person, it may be fixable.

Manufacturers are bringing out stages that have tops made out of stainless steel and ceramic. These materials really reduce the stage tops wear and are an excellent idea. The increase in cost is more than paid back by their increase in longevity and productivity. The clips that hold the specimen are called the specimen holder. No surprise here! These are made in a zillion different styles. If you don't like one then try another. Some specimen holders have adjustable spring tension. This can help when the specimen sticks to the stage.

Directly below the stage is the substage assembly. This module includes the condenser carrier and support for any other condenser optics. If the microscope is supposed to be capable of Kohler illumination then the carrier must have the ability to focus and center the condenser. A student diffusion illuminated microscope will have a focusable condenser carrier but it will not be centerable.

A Kohler illuminated microscope will have a focusing knob for the condenser and two screws that center the condenser. The screws will be facing forward or backward depending on the microscope. On most microscopes there will also be a knob or screw that locks the condenser into the condenser carrier. This allows you to remove the condenser and clean it or replace it with another with out having to completely recenter it, at least you will be in the ball park.

Different condensers are required for different techniques. If you want to do phase you will need a phase condenser, the same for Nomarski or dark field. Some condenser can do a wide range of techniques. These are called universal condensers for the wide range of things that they are capable of. They are also called "pancake" condensers because of their flat, circular construction.

Located on the condenser is the condenser diaphragm control. This may be a lever or a dial either way this controls the contrast of the microscope image. It also controls part of the resolution of the microscope. What it doesn't control is the intensity of the microscope, that is controlled by the electronics.

The base of the microscope contains the field diaphragm. This controls the size of the illuminated field. The field diaphragm control is located either around the lens located in the base or behind it. The location varies based on the manufacturer. Either under the field lens or behind the frame is the light source. Each light source uses a lamp. While this may seem like the obvious statement of the year there is a problem here. There is no such thing as a microscope lamp. Microscopes vary wildly as to what lamp they use. It is a real good idea to make a note of the lamp type and place it in the file with the rest of the information about the microscope.

All microscope manufacturers list their lamps under their parts number. Don't take this too seriously. Ask the dealer for the generic description of the lamp such as "six volt, 20 watt quartz halogen" . Usually this will be written 6V-20W Q.I.. You can then order lamps from the least expensive sources. Microscope dealers sometimes don't have the best prices on lamps since if they buy them from the microscope manufacturer since they have gone through to long a distribution chain. There are darn few lamp manufacturers and most of them are very good. You can order lamps made by Osram and Phillips with impunity. Just get the best price by asking around and ordering in bulk. If you order for the whole lab you will get a better price than by ordering by ones and twos.

Some were on the base of the frame will be the on-off switch and light intensity control. These may be one dial or slider. The electronics of a microscope controls the light intensity. The condenser diaphragm controls the contrast not the intensity.

Most microscopes change intensity by changing the voltage to the lamp. This raises or lowers the intensity of the lamp. It also changes the color of the light that the lamp produces. Reducing the voltage makes the light more red while increasing it makes the light more blue. Mostly this isn't a problem, a blue filter will usually be adequate for most viewing. If your taking color pictures it can be a real problem. The usual way to control this is when doing color documentation is to keep the voltage at the optimum (ie. 6V for a 6V lamp). If you need to lower intensity then use a neutral density, gray, filter. Some research microscopes use only neutral density filters to control light intensity through very complex, motor drive systems. These really work but are very expensive.

What this means to you Know the parts of the microscope so you can communicate effectively. Know what lamp your microscope really uses.