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.

Minolta DiMAGE 7i image sensor

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

Minolta DiMAGE 7i IR filter

It has some pretty funky op­ti­cal prop­er­ties.

Minolta DiMAGE 7i IR filter

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.

Glass cutting

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

Glass breaking

It took a few tries to cut the right size.

Glass cutting materials

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.

Sample IR image

I can’t wait for a sunny day!