2009-12-07

I finally worked up the guts to convert my Minolta DiMAGE 7i to take infrared (IR) photographs.

The image sensors in digital cameras are actually sensitive to IR light, but most cameras are made with an internal IR-blocking filter in order to enhance color accuracy and reduce glare. This makes it impossible to use most digital cameras for IR photography. If the internal IR-blocking filter is removed, however, the camera becomes sensitive to both visible and IR light. An IR-pass (and hence visible-blocking) filter can then be attached in front of the lens to take IR photos.

Typically the internal IR-blocking filter is a small piece of glass just in front of the digital image sensor. Every piece of glass that light passes through before reaching the camera’s sensor changes the image’s focus, so for the camera to maintain the same optics without the filter glass, a piece of clear glass of the same thickness must be left in its place. For this I decided to use glass from Edmund Optics that has a high efficiency anti-reflection coating, sold in 1- to 3-inch squares.

It took about two hours to work my way down to the camera’s sensor optics. Pete Ganzel’s documentation of Minolta Dimage 7i Disassembly helped a lot here. In the photo below, the IR-blocking filter is the blue-tinted window on the right. Visible through it is the green CCD sensor.

The IR filter is held in place by a rubber gasket just on top of the CCD, and pulled out easily.

It has some pretty funky optical properties.

The clear replacement glass from Edmund Optics ships with a blue protective cover stuck to it. A glass cutter was able to score it while the cover was still on.

I snapped the glass on the straight edge of the side of a large textbook.

It took a few tries to cut the right size.

It took only an hour to put the camera back together. Without any filters on the lens the white balance is noticeably mis-calibrated, but a quick test with an IR-pass filter attached indicates full responsiveness to the infrared light of my halogen lamps. In the infrared range, broccoli and cauliflower look almost identical.

I can’t wait for a sunny day!