You can read the whole article in the book:Introduction to Medical Equipment Repair The book also contains a chapter on preventive maintenance, with recommendations on how to perform the various tests. Click here for more information. |
Repair of Laboratory Microscopes |
by Frank Weithöner |
The laboratory microscope is one of the most important instruments in a hospital laboratory. It can be found in even the smallest hospital. The microscope is used to examine body fluids and tissues. These examinations are used to diagnose diseases, detect infections and allergies, and identify parasites and bacteria.
Laboratory microscopes use a system of lenses to magnify images of small specimens. The light required for this is also focused by a system of lenses.
In addition to the laboratory microscope, there are two other types of microscope used in hospitals: the surgical microscope, which is used in ear, nose and throat (ENT) surgery, for example, and the slit lamp which is used by ophthalmologists to examine the eyes. (Introduction to Medical Equipment Repair - Slit Lamps)
Light microscopes are not only optical, but also mechanical precision instruments. The distances between the optics have to be adjusted, the apertures have to be changed
and the specimens have to be moved in three axes for observation. All this must be done smoothly and without play, while the entire instrument must remain steady and stable.
A typical laboratory microscope. It is a transmitted light microscope and also a binocular microscope as it has two eyepieces.
Types and applications
In laboratory microscopes, the specimen to be examined is placed in a thin layer on a transparent slide. This is illuminated from below. The trans-illuminated image is magnified in the lens systems consisting of eyepiece and objective. Because the light passes through the specimen, this type of microscope is called a transmitted light microscope.Operating microscopes and slit lamps illuminate the object from above or from the front. This means that the reflected light of the light source is used, which is on the same side as the observer. The illuminated object is then also magnified by lens systems. This type of microscope is called reflected light microscope. With reflected light microscopes, the magnification is generally lower than that of transmitted light microscopes.
The simplest laboratory microscope is the monocular microscope. It has only one eyepiece and one objective. This means that the microscopist only looks at the object with one eye. Even in the developing world, these microscopes are not used very often any more.
The standard microscope in hospital laboratories today is the binocular microscope. The binocular microscope has one objective but two eyepieces. This makes it less tiring for the eyes when long-lasting examinations have to be made. Binocular microscopes should not be confused with stereomicroscope. Both have two eyepieces, but the stereomicroscope also has two objectives. With a stereomicroscope two objectives deliver two slightly different images which form a spatial or stereoscopic image. With binocular microscopes the image through both eyepieces is the same and is therefore not three-dimensional.
Operating microscopes and slit lamps are true stereomicroscopes. They provide a stereoscopic view of the object. This is particularly important if you not only want to examine an object, but also work on it. Stereo reflected light microscopes are also used in electronics to repair circuit boards with SMD components, for example when repairing smartphones and laptops.
Operation
The design of all laboratory microscopes is similar. They all consist of the same components, which are described below. Therefore, the operation is also very similar. Someone with some experience in using a laboratory microscope will also be able to operate any other microscope.For a novice, however, it is not so easy to recognise the influence of the various adjustments on the image. The same is true for us technicians, who also need to identify possible faults. You should therefore take every opportunity to 'play around' with the microscope to familiarise yourself with the different adjustment possibilities.
In any case, it is always better to be shown by the microscopist exactly what is wrong with the microscope. This will avoid misunderstandings and unnecessary disassembly of components that have nothing to do with the fault.
In the following you find basic instructions for operating laboratory microscopes.
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Tips & Tricks: Slide for testing microscopes A test slide is essential for testing a microscope. If you do not know where to get one and how best to prepare it, just ask in your laboratory. The microscopists will be happy to make you a slide that you can use for all future repairs and maintenance. |
Basic operating information
The microscope is an expensive precision instrument that requires careful handling. The optical system is sensitive and incorrect operation and cleaning techniques can cause irreparable damage to the instrument.
Eyes should be relaxed during examination under the microscope. Eyes and eyebrows shouldnot touch the eyepiece.
It helps if the room lighting is not too bright. Do not touch lenses with your fingers. Do not leave the microscope with immersion oil on the objective. Always wipe the oilobjective clean immediately after use.
Cover the microscope carefully after use.How to start
First, turn on the light source and set the brightness to medium. Open the aperture of the condenser half way. Now adjust the eyepieces. Therefore push them together or pull them apart until you see only one light spot with both eyes. If you know your eyes' dioptres, you can adjust the dioptre at this point. If you do not know your dioptre, you can adjust it later when you have focused an image.
Place the slide on the stage and secure it with the stage clamps. Now choose an objective. Always start with a low-power objective. Look at the object slide and the objective
from the side and move the object stage as close as possible to the objective. Now look through the eyepieces and turn the object stage downwards, away from the objective,
until the image comes into focus. When you have found the focus, you can then change to a high power objective.Never raise the stage using the coarse focus adjustment during focusing. The lens may hit the slide and damage both the lens and the slide.
Do not use the coarse focus adjustment with high power objectives. Make only small changes in focus. If you are completely out of focus, return to a lower power objective to adjust the focus again.
Once you have found the focus, you can adjust the eyepieces. One eyepiece has a dioptre adjustment ring. Look through the other eyepiece, the one without the adjustment and close the other eye. Focus the image by moving the stage. Now close the eye and look through the other eyepiece. Focus the image using the dioptre adjustment ring.
While working with a microscope the eyes should be relaxed. Eyes or eyebrows should not touch the eyepieces. It is also helpful if the room lighting is dimmed.
Daily cleaning by the user
At the end of the day, the microscopist has to clean the microscope. Cleaning in this case means: removing dust, lint from clothing, pollen and fingerprints from the lens surfaces, and removing immersion oil from oil immersion objectives.Microscope users should never clean internal lenses, only the external surfaces of these lenses:
Objective front lens. Condenser front lens. Eye lens of the eyepiece. Upper illumination lens or blue filter of the light source.If only the exterior lens surfaces are cleaned, there is no need to remove the objectives, eyepieces and lenses. This has the advantage that no dust can enter the microscope.
Oil immersion objectives have to be cleaned immediately after use. Excess oil attracts dust and can drop onto the condenser lens. The user should wipe off the lens with a lens paper after using the objective.
Installation
When transporting, carry the microscope upright with both hands, one on the arm and the other supporting the base. The microscope should be placed on a stable and leveled bench or table in a clean environment.A new microscope should always be delivered with all cleaning materials. Check with the microscopist to ensure that sufficient lens paper, a soft brush and cleaning solution are available.
In tropical regions with higher humidity, the technician delivering a new microscope should also advise on the prevention of fungus growth on the lens systems.
(Introduction to Medical Equipment Repair - Fungal growth on optical lenses)
In regions where spare parts are difficult to obtain, it is advisable to deliver a spare light bulb with the new microscope. This is especially true for donated used microscopes, which are older and spare parts may be harder to come by.
Storage
In order to keep the optics of the microscope dust-free, it is important to cover the microscope when it is not in use. If the original plastic cover is no longer available, a plastic bag, such as a shopping bag, will also work. This also applies to storage in the workshop after repair.
Repaired microscopes in a workshop, covered with plastic bags to keep out dust and dirt.
In areas of high humidity, a plastic cover is not suitable. The cover would prevent the necessary air circulation, resulting in fungus growth on the lens surfaces. In these regions precautions against fungus growth should be taken.
Construction
To be able to work at the highest magnification, a very strong light source is required. A simple light bulb is not sufficient. Instead, a powerful halogen lamp with an internal or external reflector and a large condenser lens is used. This focused beam of light leaves the base of the microscope and reaches the object stage from below.
The parts of a laboratory microscope.
Before the light reaches the specimen slide, which is placed on top of the stage, it is focused by another lens system, the condenser. The condenser is mounted directly under the stage and is adjustable in height. With an iris diaphragm mounted directly under the condenser lenses, the microscopist can control the amount of light that passes through the specimen.
Now the light passes through a hole in the stage onto the slide with the specimen. The microscopist can move the stage up and down to focus the image, and forward, backward, right and left to select the desired detail.
As a result of the high magnification of the following objective and eyepieces, the microscopist cannot move the slide by hand. A mechanism with a gear reduction is needed with which the slide can be moved minimally and precisely. To do this, the microscopist turns two knobs, one for the x-direction and one for the y-direction. Two further knobs are used to adjust the height of the stage. One is for coarse adjustment, the other for fine adjustment.
The objective above the slide captures the image detail of the specimen and magnifies it. Since different examinations require different image details, different objectives with also different magnifications are needed. For this reason, microscopes have a revolving nosepiece that allows three or four different objectives to be rotated into the beam path.
The image then enters the microscope head. The head contains a system of mirrors and a prism that split the single image into two identical images for the two eyepieces. The eyepieces also consist of lens systems that magnify the images again before they reach the microscopist's eyes.
Magnification
The total magnification of a microscope is calculated by multiplying the magnification of the eyepieces by the magnification of the objective.|
Example: Eyepieces 10x, objective 40x. The total magnification is 400x. |
A magnification of 1000x is the highest magnification a standard laboratory microscope can achieve (eyepieces 10x, objective 100x). This is more than enough for all medical examination in a hospital laboratory.
Working distance
The working distance is the distance between the front lens of an objective and the slide when the image is in focus. The higher the magnification of the objective, the shorter the distance to the slide.Due to the short working distance of powerful objectives, focusing has to be done carefully in order to prevent damage to the objective and the slide.
While the distance of a 4x or 10x objective is still quite large, the 40x objective almost touches the slide.
Eyepieces
An eyepiece, or ocular, is a magnifying lens that enlarges the image created by the objective. Eyepieces often have 10x magnification, but 6x magnification is also used. They can be easily pulled out of their holders for cleaning or exchanging.Light microscopes for medical use are generally binocular microscopes. They have two eyepieces. Binocular microscopes are less tiring for the eyes than monocular microscopes but not necessarily better. Also with monocular microscopes all examinations in the hospital laboratory can be made.
One of the two eyepieces always has a dioptre adjustment ring. This is necessary because the eyesight of our left and right eye might be different. The microscopist first focuses the image by looking through the fixed eyepiece and then adjusts the focus for the other eye using the dioptre adjustment ring on the other eyepiece.
Binocular head
The binocular head contains mirrors and one or more prisms. The task of the prism is to double and divide the image for the two eyepieces.The microscope head also has mechanical parts. Since the distance between the eyepieces must be adjustable, the eyepiece mount is movable. This is necessary because the interpupillary distance varies from person to person. A scale on the microscope head shows this interpupillary distance in millimetres. Furthermore, the microscope head can be rotated on the microscope arm. To do this, a locking screw must be loosened.
Nosepiece
The nosepiece is located under the microscope head and holds the different objectives. Depending on the microscope, there are three or four. The desired objective is brought into position by turning the nosepiece. The nosepiece engages noticeably and audibly.Objectives
Common objectives have a magnifications of 4x, 10x, 40x, 100x. Three or four of these are screwed into the rotatable nose piece. The 100x objective is an oil immersion objective which is used with a drop of special optical oil. The other objectives with lower magnification are air objectives and must not get in contact with the oil immersion.The magnification power is engraved on the objectives. In addition, the different objectives are identified by coloured rings:
4x red 10x yellow 40x blue 100x whiteOil objectives have an additional black ring (for oil) or red ring (for oil, water and glycerine) in order to distinguish them from normal air objectives.
Oil immersion objective
A magnification of 100x places high demands on the optical system. To achieve this with acceptable image quality, oil immersion objectives are used. A transparent oil film between the sample and the oil immersion objective helps to keep the required light between the sample and the objective lens and to minimise refraction and scatter losses. To do so, the microscopist puts a drop of oil immersion on the specimen. Then the stage is moved upwards until the objective lens is immersed in the oil. Now the examination can take place. At the end of the day, the oil must be wiped off the lens. This is done with lens paper.Oil immersion lenses are specially designed and do not work without oil immersion. Normal lenses, on the other hand, must not come into contact with oil immersion. If a normal lens has accidentally come into contact with oil, the oil must be removed with lens paper and cleaned with cleaning solution.
Since at a magnification of 100x the distance between the objective and the specimen slide is so small that the front lens of the objective can easily come into contact with the specimen slide, 100x objectives are equipped with a spring-loaded front lens. When the objective stage touches the slide, the lens system pushes back. This prevents the slide from breaking and the outer lens from being scratched.
Oil immersion objective with a magnification of 100x. Note how small the front lens is. Also note the spring-loaded front that retracts when the lens comes into contact with the slide.
Stage
The microscope stage has two functions. The microscopist can move the stage up and down to focus the specimen and also move the specimen holder with the slides in the horizontal plane to select a different detail. Both functions are controlled by gear reduction mechanisms.
The drive at the bottom right is for adjusting the height of the stage and the drive at the bottom centre is for positioning the specimen holder in the horizontal plane. Both have an outer wheel for coarse drive and an inner knob for fine drive.
For moving the stage platform up and down a mechanic with a coarse gear and a fine gear comes into being. The mechanic is constructed in such a way, that it moves smoothly but also keeps the platform firmly in place when the knobs are not used.
To enable the slide to be moved in a horizontal plane, the slide is first fixed in place with the slide holder. The microscopist can then use two further knobs to precisely position the slide and determine the image detail. One knob moves the slide sideways (X direction) and the other moves it back and forth (Y direction).
Condenser with aperture
The condenser is located between the light source and the stage with the sample. It consists of one or more lenses that concentrate the light from the light source into a cone of light the diameter of the area being examined. The aim is to achieve uniform illumination over the entire image. For this reason, the condenser is adjustable in height in relation to the stage.The condenser has an aperture, or iris diaphragm, which is mounted under the condenser lens. With a small lever the microscopist can open or close the aperture in order to control the diameter of the light beam and thus the brightness of the light.
Base
The solid and heavy base gives the microscope stability. It contains the illumination system and the dimmer electronics with a brightness control knob and the on/off switch. On the top of the base, to the left and right of the arm, are two hand rests.Light source
All microscopes have built-in illumination. This illumination must be very powerful, especially when 100x objectives are used. Halogen bulbs of 15 W, 20 W or more are used as the light source. A dimmer allows the user to adjust the brightness as required.In some older microscopes, the lamps are designed for mains voltage. A dimmer, or phase-fired control (PFC), circuit is then used to control the brightness. (Introduction to Medical Equipment Repair - Phase-fired control (PFC)). Most microscopes, however, have low-voltage halogen lamps with an electronically controlled and adjustable power supply.
Today's light microscopes use an LED as the light source. The LED is virtually indestructible, uncritical to shock and vibration and uses less power from the power supply. This also allows the microscope to run for many hours on small batteries.
Filter
Many microscopes use a blue filter between the bulb and the condenser. It is needed because the light from the halogen bulb is not pure white but yellowish, especially when it is dimmed. The blue filter compensates for this colour error. A modern LED microscope produces pure white light and does not need a colour filter.Repair
Before working on a microscope, you should disinfect it first. If it is very dirty, it is also a good idea to clean it thoroughly first. Wear disposable gloves.Before you start with the repair:
Note accessories such as transport box and power cable on the job card to avoid confusionlater.
Clean and disinfect the microscope. Place a test slide under the microscope and examine it to confirm the fault. Make sure thatthe problem is really a fault and not an operating error.
If you are not sure how to operate the instrument yourself, consult the user manual. Repairing and maintaining a microscope requires time, space and peace and quiet. The work must be done very carefully and should not be done under time pressure. Make space before you start, clean your table and make sure you have enough light. Above all, take your time. Repairing optical instruments is not work, it is meditation.
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Note Microscopes are precision optical instruments. Incorrect lens cleaning can damage a microscope. If you are not 100 % sure of what you are doing - do not do it. Better to do less than too much. |
Do not disassemble a microscope without good reason. Dust and dirt can get into the optics. Loosened screws can also affect the alignment of the optical components. Mechanical parts can also be difficult to reassemble once they have been dismantled. In any case, it is worth making sketches showing the parts, their correct position and the order of disassembly.
Mechanical and optical work should be carried out completely separately. Do not work on both at the same time. Clean your tools and work area after working on the mechanics and
before working on the optics.It is a good idea to have a small toolbox just for optical work where you can keep all the cleaning materials such as cotton wool, lens paper, etc. and separate them from the other tools you use for mechanical work. (Introduction to Medical Equipment Repair - Cleaning optics).
When a laboratory reports a problem with a microscope, an accurate description of the fault is important. It is even better if the microscopist can show you exactly what the problem is.
If you need to take the microscope to the workshop for repair, use the microscope transport box if available. For longer repairs, it would be fabulous if you could offer a loan microscope (for a fee) for the duration of the repair.
Also check the laboratory conditions after the repair or maintenance has been completed. Make sure there is a microscope cover, that the microscopist has enough lens paper, and that the microscope is stored in a place protected from fungal growth.
Locating dirt on lenses
The optical system of a microscope is often complex and consists of several lenses. At first glance, it is difficult to tell if there is dirt or dust on the eyepieces, the objective or in the optics inside the microscope. To identify the problem, we move a single component and observe the result.| Action | Reaction | Result |
| Move the slide | Spot also moves | Dirt is on slide or cover slip |
| Swing the objectives beside | Spot also moves | Dirt on the objective |
| Rotate the eyepiece | Spot also rotates | Dirt is on the eyepiece |
| Loosen the head and rotate it | Spot also rotate | Dirt is inside the head |
Read more about lenses and objectives and how to clean them in the separate chapter: Introduction to Medical Equipment Repair - Optics.
Working on the microscope mechanics
The microscope is not only a complex optical device but also a sophisticated mechanical instrument. A microscope mechanism does not have rotating parts that turn at high speed and generate friction and heat, and therefore wear, but this does not mean that the microscope does not need maintenance. The problem with microscope mechanics is that most of the mechanical drives are linear drives that are open and unprotected. Dust, dirt and spilled liquids can easily get into the mechanics, so these drives need to be cleaned and re-lubricated from time to time.When dismantling a mechanism, you will need space on your workplace. Place the parts in the order of disassembly on your workplace or, better still, in a tray. Make sketches and notes of how the parts go together. Photos taken with a mobile phone will also work, but they are often not as clear as a good sketch.
Lubrication
Before lubricating mechanical parts, they must be thoroughly cleaned. After removing dirt and old grease, the parts are washed with alcohol, dried and then re-greased.
Use only grease for lubrication. Do not use oil. Grease produces a thicker lubricating film than oil, stays on the mechanic longer and does not drip or evaporate. Petroleum jelly (or Vaseline) can be used instead of grease.
All rack and pinion linear drives should be lubricated. These drives are:
Height adjustment of the stage platform. X and Y position adjustment of the specimen holder. Height adjustment of the condenser. The rails of the interpupillary distance adjustment.When you have finished working on the mechanics, do not forget to clean your tools and work area. Wipe off any traces of grease with alcohol or soapy water.
Tools
Tools and materials needed to work on microscope mechanics are:
Watchmaker's screwdriver set. Allen keys (hex), inch type for American microscopes, metric for all others. Grease. Manufacturers often recommend lithium-based grease. But this is not so important.In a pinch you can also use Vaseline.
Alcohol to degrease and clean metal surfaces. Steel wool to remove rust from metal parts. Cotton buds, paper tissue.Common problems
Objectives and eyepieces of microscopes in laboratories where no one takes care of the instruments can often be incredibly dirty and scratched. Also, sometimes objectives and eyepieces have been removed and dust and dirt has got into the objectives or the microscope. Read more about lenses and objectives and how to clean them in the separate chapter: Introduction to Medical Equipment Repair - OpticsFurthermore, drives also become more sluggish over time if dirt gets into the drives or the lubrication becomes gummed up. These are all consequences of inadequate care and missing maintenance. That does not have to be the case. Here are some typical faults in detail.
The stage contains mechanical drives that should be checked and lubricated from time to time. There is the mechanism that moves the stage up and down and another that moves the slide forwards, backwards and sideways on the stage. These mechanisms contain open and unprotected linear drives and sealed gears for coarse and fine movement. All ball bearings, racks, shafts and gears are lubricated with grease.
A very common problem in hospitals where there is no technical support is jammed and damaged linear drives. These drives are not as well protected as axial drives and dirt and spilled liquids can easily get into the drives. This leads to rust over time and at some point the mechanic becomes completely stuck. Further attempts by the user to move the parts can then break the mechanic. Affected are the rack and pinion of the stage platform height adjustment and the X-Y adjustment mechanics at the top of the stage platform. It can also happen that dried blood or glass fragments from broken slides get between the rack and pinion of the linear drives and are not removed. The result is the same: jammed mechanics with often irreparable damage to the teeth of the drive components.
When this happens, the drives must be disassembled, cleaned and re-lubricated. It is not enough to simply loosen the screws a little that hold the parts in place. The main problem remains and the teeth of the drive will be damaged over time.
The dirt between the teeth can be removed mechanically with a wire brush, steel wool or an old screwdriver. But be careful with brass parts as they are softer than your tools and you could damage the parts. It is better to soak brass parts in soapy water overnight before cleaning.
After cleaning and lubricating, the stage may need to be adjusted so that it moves smoothly and easily up and down, but also stays in the set position. This can be done by loosening or tightening the fixation screws of the drive.
Too late. Dirt between the teeth of this rack blocked the mechanic. Further attempts by the microscopist to move the stage broke three teeth. Fortunately, after cleaning, the rack could be mounted the other way round, as not all of the length was needed.
The lens surface of eyepieces facing the eye is often smeared with grease. This is caused by the eyelashes of the microscopist and is normal. The user will therefore frequently clean both lenses with lens paper.
Sometimes dust and dirt gets on the inside of the eyepiece facing the microscope head. This lens is located deeper inside the eyepiece tube and is therefore more difficult to clean. The best solution is to disassemble the eyepiece.
Eyepieces and objectives are often held together by tiny grub screws. Other types have no such locking screw, but a locking ring inside the tube that must be unscrewed. To do this, the ring has two notches into which a special tool fits. A pair of thin pliers or two slotted screwdrivers will also work.
Each eyepiece usually has two lenses. They are different and not symmetrical. They must not be interchanged or inverted. A sketch of the lens arrangement is useful for later assembly.
The parts of an eyepiece. The lenses are easy to clean when removed. Note the correct orientation of the lenses. They must be reinstalled in the same position.
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TIP! Never leave the microscope without the eyepieces. Dust will enter the binocular head. If it is necessary to remove the eyepieces, close the openings with the plastic caps from water bottles. A piece of plastic bag and a rubber band will also work. |
After assembly, you can check the eyepiece. Turn it over and then you should be able to use it as a magnifying glass.
A microscopist who takes good care of his microscope and regularly cleans the exterior lens surfaces of the objectives will rarely have problems with the objectives. However, if dirt has got into an objective or fungus has grown on the lenses, they will need to be disassembled and cleaned.
The lenses of an objective are held together by a retaining ring. This usually has a thread. If you want to unscrew the ring, you have to be careful not to put too much pressure on the objective housing. It will then deform and the ring will be even tighter. This is especially true if the ring is so tight that you have to use a multi-grip pliers. This is very critical because not only can the body deform, but you can also damage the lenses inside the body. If you have no other choice, work very carefully and use as little force as necessary. Also, wrap a strip of cardboard around the objective before gripping it with the pliers.
Once the retaining ring has been removed, the individual lenses and the spacer rings can be taken out of the objective body one by one. But be careful, the lenses are different and must be reassembled later in the same order and position. Make a sketch when disassembling and put the individual parts aside in the order of disassembly.
When oil immersion objectives have not been cleaned after use, they are sometimes so sticky and dirty that the spring-loaded front becomes stuck. In these cases it is necessary to open the objective and remove the inner lens system. It is not necessary to disassemble the inner optical unit. The components just need to be cleaned.
A spring-loaded oil immersion objective is in principle an objective that can move within another housing.
Scratched objectives cannot be repaired and must be replaced. Original objectives from the microscope manufacturer are usually very expensive and can be difficult to obtain, especially for hospitals in developing countries. However, spare objectives from other manufacturers often fit. These objectives, made in India or China, are a good alternative. In big cities, there is a good chance of getting spare objectives from a laboratory supply store.
In this context, the following should be mentioned. When changing to another objective by turning the nose piece, the focus point should be the same. If this is not the case and it is necessary to adjust the focus by more than half a turn with the fine adjustment knob, the objective lengths are different. This can happen when using lenses from different manufacturers. This can be corrected by using shims. Shims are thin brass spacers that are placed on the objective before it is screwed into the nosepiece.
The binocular head contains the most sensitive parts of the microscope, the prisms and mirrors. They are mounted together to form an optical and mechanical unit. This unit is perfectly aligned by the manufacturer and should not be disassembled. Do not attempt to make any adjustments. Only the manufacturer should change the individual components and carry out the necessary calibrations.
The three optical units can be moved against each other and can also be removed. But they should not be dismantled.
The binocular head can be easily removed for servicing. Only one locking screw needs to be loosened. Care should be taken when cleaning prisms and mirrors. The surfaces of prisms and mirrors may be silver plated and any mechanical cleaning attempt may destroy the coating. The optical components should therefore only be dusted with a soft camel hair brush or a squeeze bulb. Otherwise, clean the surfaces with lens cleaner and lens paper, avoiding any pressure on the prism and mirror surfaces.
Defects in the microscope head occur when the images for the right and left eyes are no longer congruent. Double images are the result and the microscopist complains of headaches.
The binocular head also contains mechanics that should be checked from time to time. It is the slide with the holder for the eyepieces. The two holders run between two rails that need to be cleaned and greased from time to time. A small amount of Vaseline is usually sufficient.
The guide rails should be cleaned and lubricated from time to time.
The revolving nosepiece does not contain any optics and the mechanic is not complicated. Nevertheless, you should not try to take the mechanic apart unless you have a good reason. The lower rotating part runs on a ball bearing, and if you are not very careful, dozens of steel balls will fall out and bounce around your workshop.
What can happen is that the nosepiece does not lock into position when a different objective is selected. In this case, the small steel ball that snaps into the position grooves of the rotating part is missing. The ball is usually held in place by a spring steel plate. If this is the case, you can use a ball from an old ball bearing.
Only a small steel ball locks the nosepiece into place. If it is missing, the lenses can no longer be positioned correctly.
The condenser and aperture, or iris diaphragm, form a unit that can be adjusted in height relative to the stage. The adjustment works in a similar way to a rack and pinion stage adjustment. The aperture is an iris diaphragm that can be easily opened or closed with a small lever.
The upper front lens is often very dirty. To clean the lens, remove the entire condenser unit.
The height adjustment mechanism should move smoothly. Sometimes the mechanism becomes stiff. Then the rack should be cleaned and re-greased.
The condenser is usually held in place by three screws. These screws can also be used to adjust the condenser so that the light beam hits the lens exactly in the centre. The centring of the light spot can be checked by placing a piece of paper on the stage.
The aperture usually works without any problems.
The condenser lens (right) with the removed aperture (left).
All lamps have a limited life expectancy. They break sooner or later and spare bulbs should always be kept in stock. This is especially true for microscope lamps, as they are often very special and not easy to obtain.
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Tips & Tricks: Extending the life of lamps Light bulbs last much longer if the supply voltage is reduced slightly. A 5 % reduction in supply voltage is virtually invisible but doubles the life. This is easily achieved by adding a resistor in series with the bulb. (Introduction to Medical Equipment Repair - Halogen lamps - Life expactency) And the resistor does something else. It also limits the high inrush current when a lamp is switched on (lamps act as PTCs), which in turn also extends the life of the light bulb. A resistor in series with a 12 V, 10 W light bulb would be: The current through the lamp and the resistor is: I = P / V, I = 0.83 A The voltage drop across the resistor is: 5 % = 0.6 V Resistor R = V / I, R = 0.72 Ω Power dissipation P = V × I, P = 0.5 W A suitable resistor would be 0.68 Ω , 1 W |
Three different microscope lamps. All three are very different and therefore also very expensive.
When replacing a lamp do not touch it with your fingers. Sweat and grease from your fingers will stick to the quartz glass and change the structure of the glass. The glass will become hazy and milky. Use a paper tissue to hold the bulb when mounting. A lamp which was touched can be cleaned with a paper tissue and alcohol.
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Tips & Tricks: Conversion to car halogen lamp When a special lamp is needed and not available the microscope can be converted to another, similar light bulb. Halogen lamps from cars for example are a good choice. They are powerful, everywhere available and cheap. In practice however it is not that easy because the lamp will probably not fit into the socket and also the filament position will be very likely different. This means, also another socket is needed and some mechanical work has to be done. But once the new socket is in place and the filament of the new lamp is in the correct position you will have from now on always cheap lamps available. |
In addition to burnt filaments, lamp sockets can also cause problems. Contacts can be burnt because of the high lamp temperature, high current and oxidised contacts. Dirty contacts can be cleaned with a fibreglass cleaning pen, but burnt contacts cannot be cleaned or repaired. The lamp holder must be replaced.
The correct socket must, of course, fit the lamp. This means not only that the socket must match the lamp type, but also that the filament must be in exactly the same position as before. If this is not the case, the focal point will shift. It will therefore be necessary to modify the mounting of the lamp socket.
Additionally the lamp socket has to be heat resistant. Only ceramic sockets should be used. These ceramic sockets for halogen bulbs come with connecting leads. These leads are crimped or brazed. Disconnected leads cannot be soldered to the socket. During use the socket gets too hot and the solder would melt. For the same reason, the leads are heat-protected with silicone insulation.
Out of focus due to wrong lamp position.
In order to check whether the lamp has been positioned correctly, you only need to fix a piece of paper on the microscope stage. The focal point must now be exactly in the middle of the opening and hit the centre of the lens.
Most of the microscopes have low-voltage lamps and therefore need a power supply. This is in principle an adjustable power supply because the light bulb is dimmed by adjusting the output voltage. The power-regulating component here is a transistor.
However, it is still possible to find microscopes that are operated with lamps for mains voltage. These also have an electronic brightness control. In this case it is a phase-fired control (PFC). The power component here is a TRIAC. (Introduction to Medical Equipment Repair - Phase-fired control (PFC)) Both power components are prone to failure due to the relatively high power dissipation and the higher ambient temperature of the lamp.
Situation in developing countries
A common problem, especially in developing countries, is the lack of an external power supply. Finding a replacement power supply is not easy as microscope power supplies are quite powerful (20 W to 80 W) and universal power supplies available in local shops do not provide this high current.Again, a solution to this problem is to convert the microscope to LED illumination. As the efficiency of an LED is much better than a standard halogen bulb, the power supply can be much smaller. Even a cheap mobile phone charger can provide enough power for an LED microscope.
A description on how to convert a microscope to LED you find here .
After the repair
When the unit is reassembled, clean the outside of the microscope with a soap solution. Do not use too much water. Make sure that no water gets into the mechanical parts. Do not forget to clean the mains cable as well. After drying with a dry cloth, the microscope can be disinfected with a spray disinfectant.Finally, the electrical safety test should be carried out as explained in the Maintenance section. (Introduction to Medical Equipment Repair - Microscopes - Maintenance) When the test has been passed, you only need to fill out the job card and the microscope can be returned to the laboratory.
Further literature
On Wikipedia you can find further articles about these topics:Aperture
Condenser (optics)
Eyepiece
Köhler illumination
Microscope
Microscope slide
Microscopy
Objective (optics)
Oil immersion
Oil immersion objective
Optical microscope
Stereo microscope
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You can read the whole article in the book: Introduction to Medical Equipment Repair The book also contains a chapter on preventive maintenance, with recommendations on how to perform the various tests. Click here for more information. |
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