Experiment #4 Laser Microprojector

Introduction

A microscope is an instrument that uses a combination of optics to enlarge a specimen. Details in the specimen which are invisible to the naked eye are amplified by lenses and illuminated by a light source. By changing the alignment of the lenses different layers of the specimen can be visualized. Varying the amount of light passing through the sample illuminates the image by varying degrees.

In regular microscopy using a light microscope the light source is an incandescent light bulb. The light bulb is located in the base of the microscope. The field diaphragm then limits the amount of light which will shine up through the specimen slide resting on the stage. There is also a substage condenser which limits the amount of light illuminating the specimen so that details can be visualized by varying degrees of illumination. The image is seen by a single user looking through either one or two eyepieces located at the top of the microscope.

What if more than one user at a time would like to see the image of the specimen on the slide? A microscope can be turned into a microprojector very easily by the use of a single laser light source. Unlike regular white light from a light bulb, laser light is coherent over a longer distance, and so will retain its focus. By replacing the white light in a light microscope with laser light the image can be projected beyond the eyepiece of the microscope, terminating onto a screen instead of the eyes of one user. In this way, several users at one time can view a larger image projected onto a screen, instead of one user viewing the image through the eyepiece of the microscope.  

Background

• A microscope is an instrument that uses a combination of optics to enlarge a specimen. In regular microscopy using a light microscope the light source is an incandescent light bulb and only one user is able to view the image at one time.

• A microscope can be turned into a microprojector very easily by the use of a single laser light source. This allows more than one user to view the image at one time.

Purpose: to use a microscope as a microprojector, with the aid of a laser, so that more than one user can view an image of the specimen at one time.

Materials

1. Microscope – any light microscope can be used to create a microprojector.
2. Laser light source – any wavelength and output power level laser can be used for this experiment. The wavelength and output power level will determine the quality of the image obtained. A plug-in laser module or a battery powered portable laser can be used.
3. Laser safety eyewear, if working with a ClassIIIb (>5mW) laser light source.
4. Retort stand or other mounting apparatus for the laser light source.
5. Projection surface – the image can be projected onto a blank white wall, a white piece of cardstock, or any other surface where the image will be clearly visible.
6. If you are using a microscope without a substage mirror you will need a small front surface mirror and a retort stand or other mounting apparatus to hold the mirror in place.
7. Specimen slides – either pre-made or made with household specimens.

Laser light source: either a plug-in laser module or a battery powered portable laser can be used for this experiment. A plug-in module might be more convenient, allowing for longer continuous use. Different wavelengths of laser light will result in varying qualities of the image. This experimental write-up was based on the use of a 532nm (green) laser. Any output power laser can be used in this experiment. A ClassIIIa (<5mW) laser is recommended for maximum safety, however the higher powered the laser, the brighter the image will appear on the projection surface. NOTE: do not use a laser of too high an output power level as this could damage your specimen slide.

Safety: if using a ClassIIIb (>5mW) laser the use of proper laser safety eyewear is highly recommended. In this experiment it is necessary to align the laser beam carefully with the mirror and the lenses of the microscope. This alignment process causes multiple reflections, the paths of which are unpredictable.

Procedure

You will first need to determine the type and style of microscope that you will be using for this experiment. Some microscopes have a substage mirror already mounted in, while others do not.

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The main objective is to replace the incandescent light source in the microscope with a laser light source. The laser beam must be aligned in such a way as to pass through under the specimen slide, up through the objective lens, out of the eyepiece and onto a distant projection surface.

1. Disconnect the microscope from any power source and ensure that the light source in the microscope is off.
2. Mount the laser light source in an appropriate mounting apparatus.
3. If the microscope has a substage mirror direct the laser beam at a 45 degree angle toward the mirror such that the beam bounces directly upward at a 90 degree angle through the stage and into the specimen slide. If the microscope does not have a substage mirror and instead has the light source located in the base and focused by a field diaphragm, you will need to mount an external mirror. Use the mirror mounting apparatus to hold the mirror just above the field diaphragm. Align the laser beam and mirror such that the laser beam bounces directly upward at a 90 degree angle through the stage and into the specimen slide.
4. You will further need to refine your alignment such that the laser beam exiting up through the specimen slide then enters the objective lens.
5. If using a binocular microscope replace one eyepiece with an opaque cap. Direct one eyepiece at the projection surface. If your laser light alignment is correct you should now see an image of the specimen projected onto the surface.

If better image quality is desired, and the microscope you are using does not have a substage mirror:

1. Disconnect the microscope from any power source and ensure that the light source in the microscope is off.
2. Carefully remove the field diaphragm.
3. Open up the latch door on the underside of the base of the microscope. The incandescent light source will be attached to this door and will swing out of the way.
4. Place the base of the microscope on two stable surfaces high enough so that the latch door is open and out of the way. You will now have a clear opening up through the base of the microscope.
5. Place the laser light source (mounted in an apparatus) in the opening, directed upward.
6. Align the module to shine directly upward through the specimen slide on the stage, up through the objective, out of the eyepiece and onto the projection surface.
   
   

Aligning the laser light source properly is tricky and might take some time. If using a ClassIIIb (>5mW) laser the use of proper laser safety eyewear is highly recommended.

Once you have aligned the laser light source properly and an image appears on the projection surface you can manipulate this image as you usually would while viewing it through the microscope. Adjust the focus to get a clear projection image. You can achieve quite a large image size, easily obtaining images half a metre in size.

Observations

The following two images were obtained by leaving the microscope intact and mounting an external front surface mirror above the field diaphragm as in the setup images above. A <5mW 532nm green portable laser pointer was used as the laser light source. The projection surface was a white wall. Image size obtained was approximately 30cm in length.

Img.1 Root of cat hair

Img.2 Human intestine 40Xmag


The following images were obtained with the field diaphragm and the substage condenser of the microscope removed as in the setup images above. A 20mW 532nm green laser module was used as the laser light source. The projection surface was a white wall. Image size obtained was approximately half a metre in length.

Img.3 Human intestine 100Xmag

Img.4 Section of mouse brain 40Xmag

Img.5 Section of mouse brain 100Xmag

Img.6 Section of mouse brain 400Xmag

Img.7 Household yarn sample 100Xmag

Img.6 Horse heart muscle 100Xmag

Img.3 Human intestine 40Xmag

Img.3 Human intestine 100Xmag

Img.3 Human intestine 400Xmag

Explanation

As laser light is much more coherent than an incandescent light bulb, it retains its focus over a longer distance. Replacing the light source in a light microscope with laser light allows for the reflection of the image to travel a longer distance, beyond the eyepiece, and onto a projection surface. In this way more than one user can view the image of the specimen slide at one time.

Conclusion

A microscope can be used as a microprojector with the aid of a laser.

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