0.2 Projector setup and issues

To implement a compressive sensing camera, the most vital element is the DMD. There are two practical alternatives for incorporating a DMD into a prototype:

• Use the DMD that is built into a standard DLP projector, or
• Buy a TI Discovery Board kit.

The overall system we worked with consisted of the projector and the surrounding components that can be seen in the figure below. The basic flow of control in this system is as follows:

• Laptop sends random frames to the projector
• Projector light, modulated by the frames on the DMD, is projected on to the target object
• Light reflects off the target back towards the projector
• Zoom lens beside the projector captures the reflected light and focuses it down onto a photodiode
• Photodiode output goes through a preamplifier and filter
• Filter output feeds into a DAQ
• DAQ output is sent back to the laptop for reconstruction into the target image

The image below shows the zoom lens that we used to focus reflected light onto our photodiode. This photodiode has both an NIR detector and a visible detector. Both have a 1.5mm diameter active area, onto which the zoom lens needs to focus all the light it takes in. We had great difficulty getting this focus correct. Ultimately, we ended up focusing it by connecting the output of the photodiode to an oscilloscope, then turning the fine and coarse focus knobs until the maximum signal was achieved. Even with careful focusing, we only obtained max measurements of about 100mV for the NIR signal, when all mirrors of the DMD were turned to their "on" position.

To combat the zoom lens focusing difficulties and try to achieve a greater signal at the photodiode, we investigated different lamp alternatives. The standard projector lamp we originally worked with, pictured below, has a power of 200W. Much of its output is not in the NIR range, however. Thus, we found a 100W bulb with its peak emittance at 1 micron, and ordered it. However, replacing the projector bulb with this new bulb proved very cumbersome. The new bulb required a different power supply (12V, 8A, DC) than the projector bulb, so we had to buy a separate power source for it. Additionally, the projector has a safety feature which requires that something be attached to its lamp socket, or else it will not run. So, we had to make a set of connectors for the original projector lamp so that we could remove the lamp from its place inside the projector. After all of this, though, in the end we found it prohibitively difficult to mount the new bulb in the projector, as we could find no good way to secure it in place.