Measurement of Light. Photosensors

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1 Measurement of Light Photosensors Detection of light is a basic need for everything from devices to plants and animals. In the case of animals, light detection systems are very highly specialized, and often operate very near to thermodynamic limits to detection. Device researchers have worked on techniques for light detection for many years, and have developed devices which offer excellent performance as well. Light is a quantum-mechanical phenomena. It comes in discrete particles called photons. Photons have a wavelength, a velocity, a frequency, energy, and even a momentum. Among all of this, it is important to remember the relationship between energy and wavelength. In all cases, the energy of the photon determines how we detect it. Light detectors essentially may be broken into two categories. The so-called Quantum detectors all convert incoming radiation directly into an electron in a semiconductor device, and process the resulting current with electronic circuitry. The Thermal detectors simply absorb the energy and operate by measuring the change in temperature with a thermometer. We will start be examining the Quantum detectors, since they offer the best performance for detection of optical radiation. In all of the quantum detectors, the photon is absorbed and an electron is liberated in the structure with the energy of the photon. This process is very complicated, and we will not examine it in detail. It is important to recognize that semiconductors feature the basic property that electrons are allowed to exist only at certain energy levels. If the device being used to detect the radiation does not allow electrons with the energy of the incident photon, the photon will not be absorbed, and there will be no signal. On the other hand, if the photon carries an amount of energy which is `allowed' for an electron in the semiconductor, it can be absorbed. Once it is absorbed, the electron moves freely within the device, subject to electric fields (due to applied voltages) and other effects. Many such devices have a complicated `band structure' in which the allowed energies in the structure change with location in the device. One example of such a `band structure' is that offered by a p-n diode, In a diode, the p-n junction produces a step in the allowed energy levels, resulting in a direction in which currents flow easily and the opposite direction in which current flow is greatly reduced. A photo-diode is simply a diode, biased against its easy flow direction (`reverse-biased') so that the current is very low. In a photon is absorbed and an electron is freed, it may pass over E507 Instrumentation Engineering 1

2 the energy barrier if it possesses enough energy. In this respect, the photodiode only produces a current if the absorbed photon has more energy than that needed to traverse the p-n junction. Because of this effect, the p-n photodiode is said to have a `cutoff wavelength' - photons with wavelength less than the cutoff produce current and are detected, while photons with wavelength greater than the cutoff do not produce current and are not detected. Fig. 1: Connection of a photodiode in a photovoltaic mode Photodiodes may be biased and operated in two basic modes : photovoltaic and photoconductive. In the photovoltaic mode, the diode is attached to a virtual ground preamplifier as shown in Fig. 1, and the arrival of photons cause the generation of a voltage which is amplified by the op-amp. The primary feature of this approach is that there is no dcbias across the diode, and so there is no basic leakage current across the diode aside from thermally-generated currents. This configuration does suffer from slower response because the charge generated must charge the capacitance of the diode, causing a R-C delay. Fig. 2: Photoconductive operating mode In the photoconductive mode, the diode is biased, and the current flowing across the diode is converted to a voltage (by a resistor), and amplified. Aphotoconductive circuit is shown in Fig. 2. The primary advantage of this approach is that the applied bias decreases the effective capacitance of the diode (by widening the depletion region), and allows for faster response. Unfortunately, the dc bias also causes some leakage current, so detection of very small signals is compromised. E507 Instrumentation Engineering 2

3 In addition to making optical detectors from diodes, it is also possible to construct them from transistors. In this case, the `photocurrent' is deposited in the base of a bipolar junction transistor. When subjected to a collector-emitter bias (for npn), the current generated by the photons flows from the base to the emitter, and a larger current is caused to flow from the collector to the emitter. For an average transistor, the collector-emitter current is between 10 and 100x larger than the photocurrent, so the phototransistor is fundamentally more sensitive than the diode. Photodiodes and phototransistors are very widely available. These devices are also available in packages which are designed for particular applications. For example, it is common to use a light emitting diode and a detector mounted in a pair so that passing objects can interrupt the optical beam between them. "Opto-interruptors " which consist of such emitter-detector pairs are available in a wide variety of configurations. "Proximity E507 Instrumentation Engineering 3

4 detectors " which are situated side-by-side sense the presence of a reflecting surface by causing reflected light to strike the detector. Fig. 3: Incremental Encoders Other applications of optical detector-emitter pairs include measurement of the rotation rate of electric motors. In this case, a disk is mounted on the shaft of the motor with a large number of slits cut through it. The detector emitter pair is mounted so that the slits cause an oscillation in the signal - and the rotary position can be determined by counting the peaks in signal. This is called an optical encoder, and it is widely used in electric motors, as shown in the Fig. 3. E507 Instrumentation Engineering 4

5 ? Optical shaft Encoders produce information about angular displacement in digital form this is useful because a digital output in compatible with computers and other digital electronic systems.? An optical encoder is a traducer in which linear or angular displacement varies the transmission of light from a source to a detector? There are two main types of encoder device 1. Incremental Encoder? It produces an output signal showing that some displacement of a shaft has taken place. Further output signals are counted and from these the angular displacement of a shaft can be measured relative to some arbitrary datum. The following figure a typical shaft Encoder? The incremental shift encoder consists of a disc rigidly attached to the shaft whose displacement is to be measured. 2. Absolute Encoder? Absolute encoder produces an output signal which shows the total displacement of a shaft from a null position.? Absolute encoder differs from the incremental encoder in that the output signal it produces is in the coded form; this produces an absolute displacement of the shaft. Exercise on Optical Encoder E507 Instrumentation Engineering 5

6 Fig. 4: Spectral Response of an Infrared Photodiode Most phototransistors and photodiodes have their peak sensitivity in the near infrared (see Fig. 4;. The peak sensitivity occurs near the cutoff wavelength (near 1 um) and extends to shorter wavelengths. The location of this peak sensitivity is due to the energy of the `band gap' in silicon, and is not easily adjusted. Table 1: Band Gaps and Longest Wavelengths Photosensors can be made from other electronic materials with different band-gaps, as shown in Table 1. There is another important consideration to keep in mind when selecting photo sensors. In addition to the photo carriers in the device, thermally-generated carriers can be produced. The distribution of energies generated by thermal processes is dependent on the thermodynamics of the device, and on the temperature. Because of this relationship, increasing the temperature causes an increase in the number of thermally generated carriers. Conversely, reducing the band gap of a room-temperature device will also cause an increase E507 Instrumentation Engineering 6

7 in the number of thermally-generated carriers. Silicon detectors work well at room temperature, but heating to more than 100C starts to cause substantial increases in `dark current'. Detectors made from materials other than silicon may offer increased cutoff wavelength, but may also require cooling below room temperature. In general, there is a nearly linear relationship between the maximum operating temperature and the cutoff energy for the detector. By selecting a material with a cutoff energy 1/5 that of silicon (such as InSb), it is necessary to cool the device to about 1/5 of the maximum operating temperature of silicon ( cooling to 77K is optimal for InSb). This tradeoff between cutoff and operating temperature imposes severe cost issues for operation of devices at fairly long wavelengths. Fig. 5: Operating Ranges for Some Infrared Detectors There is a simple relationship between the temperature of an infrared source and the peak wavelength of the blackbody spectrum. where the wavelength is in microns, and the temperature is in Kelvin. So, for room temperature, the max. wavelength is near 10 microns. E507 Instrumentation Engineering 7

8 Photo Conductors are light dependent resistors. These sensors have resistance variation dependent on illumination. They are similar to thermistors in that they have negative temperature coefficient. Applications of Light Sensors are? Measurement of Light levels (Lux meters)? Light control in some camera? Measurement and control of shaft encoder? etc Phototransistor Demo Fig. 1: Phototransistor Circuit In the last lecture, we described the phototransistor as a device which operates by converting incoming photons to electrons in the base of a bipolar transistor. As for any such transistor, E507 Instrumentation Engineering 8

9 the base current causes a larger collector-emitter current to flow, which is detected by a circuit. The easiest way to detect a current is to use a resistor to convert it to a voltage, and we have built such a circuit, as shown in Fig. 1. In this case, there is an oscillator circuit that is powering a light-emitting diode, causing a light oscillation at 1 khz. The phototransistor is pointed at the LED, and detects the oscillation in the incident light. The circuit converts the current to a voltage with a pull-down resistor, buffers the signal, high-pass filters the signal, and then converts it to a square wave with a comparator. This circuit is one of many possible such circuits, and may be considered typical. We can see that the signal at the beginning of the circuit reflects the oscillation as well as the background illumination (dc and 60 Hz components). Some of the filter design is intended to reduce the sensitivity to these `noise' components while preserving the sensitivity to the signal at 1 khz. The variable resistor at the front of the circuit is an important degree of freedom. The current which flows through the transistor cannot exceed the saturation current : where Vbias is the total collector-emitter voltage, and Rpd is the value of the pull-down resistor. If the background is very bright, the current flowing in the device may already be very close to the saturation value, and any additional signal illumination will not produce much additional signal. Depending on the amount of background illumination, it is possible to saturate the detector, thereby reducing the sensitivity to signals. We use a variable resistor here to allow adjustment so that the detector is biased at a point of good performance. It is possible to obtain such detectors in side-by-side emitter-detector pairs, which cause a signal only if a reflective object is nearby. Depending on the biasing and the background illumination, it is possible to detect objects at range of more than 1 cm. 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. E507 Instrumentation Engineering 9

10 Fig. 2: Microbolometer A drawing of the microbolometer is shown in Fig. 2. 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 current 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. E507 Instrumentation Engineering 10

11 Fig. 3: 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. The same PVDF we are using in the accelerometer lab is also a decent thermometer. As for piezoelectricity, pyroelectricity is a phenomena in which a change in temperature causes thermal expansion, which causes the appearance of charge (through the piezoelectric effect) (see Fig. 3; Fig in the book). 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, E507 Instrumentation Engineering 11

12 radiation is occasionally focused on the detector, causing a transient in signal which is detected. Fig. 4: A Facet Lens A good illustration of this concept is shown in Fig. 4 (Fig in 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 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. E507 Instrumentation Engineering 12