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removing near-IR from the image

The HyperVCam Mobile has its Near Infra-red filter built into the lens assembly which means that it cannot be removed and used in the modified version for a microscope. I believe the QuickCam has (or at least used to have) a separate IR filter that could be removed and reused. If you are only capturing grey-scale images then the lack of IR filter may even be beneficial because the camera will be more sensitive in low light.
For colour images, however, the near-IR light is picked up by all the colour sensitive elements and ends up swamping all colour information (as the elements are very sensitive to IR). There are two solutions
  • Fit a near-IR filter - either a reflector or absorber. These filters have sharp cut-off filter responses and that seems to make them rather expensive, i.e. in the region of £100+ for a suitable filter. For my purposes it looked like canabilising a colour Quickcam was going to be the cheapest solution.
  • Use a non-IR emitting light source - I prototyped a white LED (on the end of a straw) in place of the bulb and this gave promising results. Cost of the solution? Roughly £5

As I was doing this on a tight budget I opted for the white LED solution. Using a near-IR filter is the preferable solution as the bulb can be a lot brighter.

White LED illumination

The bulb filament requires placement with millimeter accuracy so I decided to stick with as much of the existing holder system to hold the LED. I also wanted to re-use the AC transformer built into the microscope to avoid excessive wires and power supplies. The solution I went for was to I build a White LED replacement bulb.

The original assembly

My microscope has a Kohler Illuminator system with a bulb that has, I assume, a proprietary fitting (see image below). The bulb fits into the spring-loaded holder and the clip holds it in place. A ring around the bulb fitting fits flush in the holder.
originallight_thumb.jpg - 3255 Bytes
I needed a suitable holder for the LED like the original. I couldn't find any bulbs with the right connector so made one from a standard bayonet nightlight bulb and a 15mm tap washer. I removed the top off the bulb (by filing around it) and pulled out the filament. Here are the bits and the assembled holder. It does not fit quite as snuggly as the original bulb but does hold pretty firmly in place.
modingbulb_thumb.jpg - 2485 Bytes

The circuit I''ve used is the simplest AC-DC converter possible. The signal is smoothed as any stobing of the LED may cause banding on the captured images. The circuit uses a 470microF capacitor, a 330Ohm resistor, a 5mm white LED and a 50V bridge rectifier. Total cost less than a fiver. The circuit diagram is shown below.
circuit.gif - 717 Bytes
The next job's to get the circuit built and fitted into a bulb... this bit got a bit fiddly. Fiddly bits? Getting the circuit small enough and bent enough to fit in around the glass stem in the middle. Getting the circuit soldered to the filament wires (they're coated with anti-soldering stuff) and removing the epoxy I put on to hold the washer in place (it wasn't needed and interfered with its housing in the holder). Getting the LED seated over the center and getting epoxy to hold it in place.
circuit_thumb.jpg - 3181 Bytesmodedbulb2_thumb.jpg - 3647 Bytes
Finally I fitted the bulb and pushed the LED around gently before the epoxy set to get the LED perfectly aligned. Completed, one Infra-red-free light source for the microscope. The light is reasonably bright but cannot be dimmed by the variable Voltage supply on the transformer. The transformer van be varied to output between ~6.5V and 8V so, if I was to make another bulb, I'd place a tiny variable resistor in too to allow the Voltage over the LED to be taken down to a point where varying the transformer Voltage would have a significant effect on its brightness.
  dylski @ 2ne1 . com - Last Updated: 10th November, 2008