An Electronic Flash Illumination System for Photomicrography

by Ron Neumeyer

The system described in this article consists of an external illuminator linked by fibre optic cable to the microscope lamp house. There are several components; a light source ("the box"), a fibre optic cable, a rheostat controlled lamp power supply, a lamp adaptor and an electronic flash. The 100 watt halogen bulb generates enough light for most viewing applications, while the integrated flash produces razor sharp, true colour images, regardless of subject movement or instrument vibration.

The Box

The combining of light from the flash and halogen bulb takes place inside the "box". In reality the box is a standard metal housing sold by electronic supply houses. Picture 2 shows the inner workings of the box. The spectacle lens on the left side of the picture collects and focuses the flash pulse onto the receiving end of the fibre optic cable, the small metal tube almost touching the surface of the bulb.


The halogen bulb is located between the cable end and the condenser. I use a general purpose bulb, available from most hardware stores. The reason for selecting this type, rather than one designed for microscope use, relates to filament configuration. Most microscope bulbs have a flattened coil filament orientated left to right (right of Picture 3) which prevents the flash beam from reaching the cable. On the other hand, the general purpose bulbs have cylindrical filaments aligned from top to bottom, with widely spaced coils (left of Picture 3). This pattern allows the flash burst to enter the cable end, essentially unobstructed.

A 110 volt cooling fan is bolted to the inside of the box, opposite the bulb (available from computer or electronic supply houses), a pin base for the bulb is fastened to the bottom, and a metal sleeve for holding the cable in position passes through the end plate (dark coloured tube on the right of Picture 2). The fan draws air through an intake grill (Picture 1) directing it across the bulb and out on the opposite side through an exhaust grill. (A cooling fan must be installed with 100 watt-plus bulbs as they release a great deal of heat, especially when run at full capacity.)

Also visible in Picture 1 is the grey fibre optic cable connected to the box at one end, and the adaptor (described later) at the other . This particular cable is very flexible, rather like heavy gauge electrical cord. The plug and cord in the lower right connects the bulb to a 12 volt transformer (following section). The 300TL flash can be seen entering the back of the box. The grey wire coming from the lower left corner is the power cord for the fan (110-volt).

Lamp power supply

In Canada, building supply stores now stock a good selection of small, 12 volt transformers used to power general purpose halogen lighting. I use one to power my lamp. It can handle 150 watts and is about the size of a pocket flash light. Light output can be controlled by installing a household dimmer-switch between the house power source and the transformer. I have installed the transformer and output control in a separate housing as I am using a large mechanical dimmer to avoid electronic noise when using a video camera.

Bulb Assembly Adaptor

The bulb's socket assembly must come out in one piece in order to machine the cable adaptor. The aluminum adaptor is made to have the same outside diameter as the socket assembly. This allows the cable to be positioned where the bulb filament would normally sit. How this was done for the 250 lamp house is shown in Picture 4. On the left hand is the bulb and socket assembly, which fits snugly inside the lamp house, and visible on the far right. In the centre of the picture is the machined adaptor holding the fibre optic cable. The cable can be seen entering the device at the top of the picture. It passes through a hole drilled in the adaptor to match the its outside diameter. The output end of the cable is located at the end of the adaptor's "snout".

A close up view is shown in Picture 5 ( circular object in the centre is the cable end).

The adaptor is inserted into the lamp house so that the tip of the cable is located at the same point as the filament of the original bulb (the adaptor can be adjusted in the same manner as the bulb socket). A threaded set screw locks the cable in the adaptor (knob in Picture 5).

Flash

The head of the flash fits into an opening at the back of the box, with its Fresnel diffuser located near the focal plane of the spectacle condenser. The flat surface of the condenser faces the bulb, opposite the fibre optic cable (shown in Picture 2). Internal reflection conducts any light beam along the inside of the cable to the lamp house, where it enters the normal light path of the microscope.

The flash light pulse enters the fibre optic cable at the same point as light from the bulb. In other words, the pulse becomes part of the light energy carried along the cable. However, as its intensity is much greater then a 100 watt bulb, and with shutter speeds of 1/60 second or shorter, the pulse, not the bulb, imprints the image on the film emulsion. Light from the bulb, especially when dimmed slightly, does not take part in image formation. In fact, exposure time is determined entirely by flash duration (1/700 to 1/10,000 of a second), not shutter speed. As a result most moving objects can be captured without blur, and vibrations caused by the shutter will not effect in the picture. In addition, because the flash delivers light very similar to daylight the photographer can use any daylight balanced film without corrective filters.

Conclusion

Hopefully I have provided enough background detail to get you started on building a similar system for your microscope and camera. It may take some tinkering, but believe me it is worth the effort! Before closing I should point out that box-cable flash system can be used on different scopes, all that you need do is machine new adaptors. In fact, the cable alone can be used alone to illuminate objects viewed by reflected light, such as when using a stereo microscope. That about wraps it up, however if you need help, or have questions, do not hesitate to contact me by Comments to the author sent via our contacts page quoting page url plus : ('rneumeyer','')">Email. If need be I can send JPEGs of specific components.

Ron Neumeyer
11135 Kendale Way,
North Delta, British Columbia
V4C 3P7
Canada

 

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