Project Description

A conventional digital camera works by capturing light from an object and focusing it through a lens onto an array of sensors.  Cameras used by the general public operate in the visible range, 400 to 750 nm, with CMOS sensors.  They can be modified to image in a slightly wider range, from 280 to 1200 nm, if their infrared (IR) filters are removed [1].

Current IR cameras can operate in much higher ranges, from the near infrared (NIR) at 0.75 – 1.4 µm, to the far infrared (FIR) at 15 – 1,000 µm [2].  The average person, however, does not have the purchasing power to buy an IR camera, as one generally costs between $3,000 and $50,000 [3].  Even for companies or governmental institutions, this is a substantial sum of money.

The bulk of the cost of an IR camera is in its sensor array and the cooling system that regulates the array's temperature.  The array alone accounts for at least one-third of any IR camera's price.  The cooling system that helps reduce background noise and the effects of pixel bleeding is similarly expensive.  Unfortunately, there is no Moore's law for these sort of components.  Unlike transistors, IR sensors and their cooling systems are not likely to decrease in size or cost anytime in the near future.  Thus, while the camera's image processing chips will probably become cheaper in the next few years, no such reduction in cost can be predicted for the sensor components.  In fact, as the general trend in the camera industry is toward more pixels, it is likely that soon the cost of an IR camera will be almost solely determined by the price of its sensor array and this array's cooling system.

Therefore, to create an economical IR camera, the number of sensors and cost of their cooling system would have to be dramatically reduced, without severely damaging the camera’s resolution.  The technology to do this already exists; it is called compressed sensing.  In principle, a compressed sensing device needs only a single sensor—a single pixel.  Not only does the use of a single pixel remove the price of a sensor array from the picture, but also it has the potential to reduce the cost of the sensor cooling system.  Since a single pixel device does not suffer from the pixel-bleeding problem that necessitates a high-quality cooling system in regular IR cameras, in a single-pixel camera an inexpensive cooling alternative just for removing background noise could be used.

The basic operation of a compressed sensing camera is relatively simple.  Light from an object is focused by a lens onto a digital micromirror device (DMD).  A DMD is an array of tiny mirrors that can be tilted ±10-12º, from “on” to “off” positions [5].  In the “on” state, these mirrors reflect light from the object to a secondary lens, which focuses this light to the sensor.  Data from this sensor is sent to a signal processing unit, which manipulates data points to construct an image of the original object.  To generate a set of useful data points for image reconstruction, the mirrors of the DMD are flipped into a series of random configurations, in each of which approximately half of the mirrors are in their on position.

Single Pixel Camera Diagram

Figure 1: How a compressed sensing camera works [3].

A lab-table version of such a camera already exists.  In this form, however, it is inaccessible to potential consumers.  It needs to be integrated into a closed-form structure so that it is compact and portable.  It also needs to be adapted to operate in the NIR range, so that it will hold an economic advantage over a conventional form of camera and thus be a viable option for a much broader market space.

Figure 2: Lab-table version of a compressed sensing camera [5].