This is a big one. How do you tell when to take a new image?
This will depend largely on the optical/sensor solution we are using. In the case of a line-scan camera, the answer is almost certainly a rotary encoder. A roller’s rotation is monitored to determine the length of film that has passed. When the proper distance has been reached, a signal is sent to the sensor that closes the current scan image file and opens a new one.
If we go with a full-frame capture system (like a DSLR or traditional imaging sensor), then we will have two choices:
1 - keep the motion steady
2 - intermittent motion (like a projector)
Option #2 implies more parts and therefore more points of failure. If, however, we can design it well this is a valid option.
If we go with option 1, the concern becomes shutter speed of the sensor. Because the optical system is located very close to the film, blur becomes an issue with a constantly moving subject.
The solution is a balance of light (brighter = faster shutter speeds), processor speed (how many times per second are we checking for our sensor for a signal to take a picture), and sensor choice (dependability at high speeds and with various film stocks).
Right now, Kinograph uses a mechanical switch that is triggered by bumps on a sprocket. This insures that the signal to take a picture is always lined up with the film’s position in the gate (since the sprocket hole guides and bumps are on the same roller). It’s cheap and pretty reliable so far. It has not been tested at high , speeds however. And, I’m told that these mechanical switches can wear out (which makes sense).
Ideally, the solution would have parts that don’t wear out fast, are cheap, and can handle a variety of film sizes. In my mind, the rotary encoder system is a good option because it meets most of these criteria. I have not, however, tested the ones I have yet.
Many have suggested the laser or photo-interrupter option. This was tested (both with and without an IR filter) in the development of the first Kinograph. It worked great until I got a 335mm print whose base celluloid was clear instead of black. I could not get dependable results in this case and had to move to a mechanical solution due to time constraints. I would be interested to hear if there is a solution to that issue.
The other issue with a laser is that it would have to be moved when switching film gauges. This is yet another moving part and therefore another possible weak point in the system. If, however, the laser was incorporated in the swappable gate, then it might be a good option.
What does everyone else think? Anyone want to do some tests?