S90 SticK EXPOSURE CONTROLLER for SHUTTERLESS IMAGE ACQUISITION Part 1 of 5SYNOPSIS
Most scientific imaging applications require prescribed selection of a scene illumination or excitation source. Under those conditions, image acquisition occurs in complete darkness (like a flash shot at night). In a digital camera, the mechanical shutter is used only to terminate light acquisition, not to start it as in a film camera. The mechanical shutter is always open before a shot takes place. Hence if the imager, i.e. the CCD (here the S90 PFP configuration), is operated in complete darkness, that intrinsically simulates a mechanical shutter if scene illumination can be turned ON and turned OFF when synchronized with the camera's exposure signal. Exposure (light acquisition interval, Figure 1) in a digital camera thus is the combination of "electronic shutter open" and mechanical shutter close. So what is an "electronic shutter"? What it isn't, is not an LCD in front of a CCD. To start acquiring scene photons after a shoot button press, the CCD first gets a "well dump" signal that purges existing collected photoelectrons from the CCD wells into the chip substrate, thus clearing the wells from the accumulated unwanted photoelectrons before the shot. That dump is extremely fast and can occur in a few hundred nanoseconds. At that point, the photographer's desired scene light accumulation begins depositing new photoelectrons into the empty wells, the equivalent of the curtain-open instance in a film camera. After the camera's Tv timeout, the mechanical shutter closes, and the CCD image photoelectrons are serially read out to the CDS (correlated double sampler) of the analog signal processing pipeline. The two prime purposes of the mechanical shutter are:
a) to terminate exposure, and
b) to prevent image smear from occurring during readout because scene photons would still be arriving on the CCD after Tv timeout had expired and would distort a clean readout if not obscured.
However, there is no readily available exposure start signal that defines the CCD well-dump on the S90. Serendipity has been favorable here in a discovery that has given us access to that signal, which ultimately resulted in this new SticK EXPOSURE CONTROLLER. The signal is found on the bottom speaker pad on the S90 keypad circuit board (described in a next post).
In order to set up correct speaker operation for the controller, the only enabled sound function has to be shutter (Fig 2, right panel). Also, shutter volume must be at the lowest level as shown.
Figure 1 suggests there are two state machines that have to be handled: a) normal shoot, left panel, and b) custom white balance CWB, right panel. My solution for (b) on the
5V SparkFun ProMicro 16 MHz AVR (same MCU as I used in the MECHA SIMULATOR/SHUTTER CONTROLLER) is to have a switch input that "arms" the normal shooting state machine (a) to accept a CWB operation. The nice side-effect of the manual arm signal is that you can use the push button to turn ON your light source for liveview. I have provided a reset switch input to turn OFF the light source, which readies the state machine for a normal shot. Thus I consider this a complete imaging solution for scientific applications.
The TEMPORAL EFFECTS of PHOSPHORS in LEDs
When using LEDs as illumination sources, some caution has to be observed. Color LEDs and white LEDs use phosphors and scintillators to emit light. Their fluorescence lifetime, e.g. intensity to drop to a value of (1-1/e)*(initial intensity) after power-OFF, is usually quite long, sometimes as long as several milliseconds. The lifetime effect also occurs at power-ON, but in reverse. So that behavior limits shutter speed Tv. Using a white LED I have excellent high-purity results with a Tv as fast as 1/30th second. For ultra high speed photography, you can trigger diode lasers instead of direct-driving LEDs. In fact you can use this solution to obtain smear-free images of up to 1/200,000th second (or faster) quite easily using laser diodes.
