Basic Intent
This
lecture will focus on several recently developed technologies for Infrared
Sensing.
Modern thermal
infrared detectors
In
recent years, the DOD has invested a great deal of R+D funds into detection
techniques which allow long-wave detection from uncooled platforms. An
additional focus of this work has been techniques which are compatible with the
formation of dense arrays. One interesting device which has emerged due to this
investment has been the Uncooled Detector arrays made by Honeywell.
These detectors are based on the simplest thermal design - a resistance thermometer. What is novel about this device is that it combines the best microfabrication technology with good thermometer technology and electronics integration.
Fig. 1: Microbolometer
A drawing of the microbolometer is shown in Fig. 1. The basic idea is to use silicon microfabrication techniques (like those in the ADXL50 accelerometer) to make an isolated thermal structure with very little heat capacity. As we saw in the thermometer lecture, the thermal infrared detector is improved by minimizing the heat capacity.
In the final device, a flake of silicon nitride with dimensions of 50 um x 50 um x 0.5 um is floated above a silicon substrate. This flake is supported by a pair of legs, and is coated with a resistive material with a good thermal coefficient of resistance. Underneath the flake is a transistor which is used to connect the current-measuring circuit to the device using a conventional row-column addressing technique. The device currents are passed out to a processing circuit on the perimeter of the device by the x and y metal leads.
In this device, much research went into developing a technique for depositing the nitride on top of a transistor, for releasing the devices with very high yield, and for obtaining a sensitive thermometer in the form of a deposited metal film. This resistor is made from vanadium oxide, which offers a TCR of about 1% near room temperature. The resistance change is a result of a structural phase transition in vanadium oxide above room temperature, so this device must be held near room temperature to allow operation with good sensitivity.
Having developed this technology, Honeywell has gone on to make dense arrays (200x200), and to continue optimizing the performance of the devices. In the last couple of years, a complete camera system has been demonstrated. This base technology has been offered for licensing, and is presently being commercialized by several manufacturers of infrared imaging systems.
This device does not out-perform the MCT imager, but it does enable operation at room temperature, and might be available at low cost with further development.
Fig. 2: Simplified Model of a Pyroelectric Effect
Another very important technology for low-cost uncooled infrared detectors has emerged in recent years in the form of pyroelectric plastic material. PVDF is a pyroelectric material that is a decent thermometer. Analogous to piezoelectricity and strain, pyroelectricity is a phenomena in which a change in temperature causes thermal expansion, which causes the appearance of charge (through the piezoelectric effect).
Infrared detectors have been available for many years based on other specialized piezoelectric materials. The best of them is Deuterated Tri-Glycine Culfide (DTGS). This very expensive material offers the best pyroelectric coefficients, and is commonly used for IR detection in laboratory spectrometers, and in early motion detection systems.
A variety of other pyroelectric materials are also available - it is generally true that any material which is piezoelectric is also pyroelectric. There are many applications which need good performance (lab spectroscopy, for example), and these applications generally justify use of the best material available.
On the other hand, there are other applications in which the best detector performance is not required. In these applications, PVDF film has become the best choice available - primarily due to the tremendously low cost of the device material.
A good example of a low-performance application is an infrared motion detector. Nowadays, it is common to offer backyard lighting systems or door opening systems which detect the presence of a moving object with elevated infrared emission. If you wave your hand about, the infrared scene that can be detected features a variation in the infrared signal to some pixel of an imaging system. So what is needed is an array of detector elements and some sort of focused optics. Without the focused optics, moving your hand about does not produce a change in the total illumination- and would not produce a variable signal.
Remember that the pyroelectric detectors do not detect heat - only changes in heat.
So, it has become common to package a PVDF detector array in a low-cost optical package which uses a Teflon lens to focus the light. Teflon lens material is also inexpensive, and is transmissive enough in the IR that is does a decent job.
Typical Teflon lenses used in motion detection systems are made with a surface texture that includes several circular bumps. These bumps act as focusing lenses, and will bring light from a particular part of the scene to the detector. As a warm object moves through the scene, radiation is occasionally focused on the detector, causing a transient in signal which is detected.
Fig. 3: A Facet Lens
A good illustration of this concept is shown in Fig. 3 (from the book). As the `person' moves across the scene, the array of lenses produces an oscillating illumination on the detector. The device itself is a small (1 mm) piece of PVDF mounted in a transistor can. A thin metal electrode on the upper surface of the film is grounded to the can, and a thick electrode on the lower surface is connected to an external charge amplification circuit.
A typical motion detector allows the setting of a `threshold', which is simply an electrical threshold in the detection circuit, and an output voltage which indicates the threshold has been crossed recently. Usually, you can also set the duration of illumination after a detection event.
Many such products are now available on the market. I bought a motion detection light fixture at home depot recently which included the detector and circuit, light mount bracket, sockets for two bulbs, and 2 bulbs, all for 24.99. Clearly, this detector is inexpensive!
This system is set up for demonstration during the lecture, and we can see that, after a brief warm-up period, it is very difficult to approach the sensor without triggering the circuit, yet the circuit does not false-trigger.
So, there has been a recent substantial improvement in the availability of crummy, but inexpensive IR sensors, and a family of decent devices for imaging systems are emerging. Both of these devices will represent opportunities for new products, and should be watched closely.