Making light work of light measurement. Th e G u i d e To P O S I T I O N S E N S I N G

Transcription

1 Th e G u i d e To P O S I T I O N S E N S I N G

2 Table of Contents Table of Contents About UDT Instruments History Service Quality Technology Publications Professional Societies Warranty Application Information Introduction Important Terms Typical Applications Aero Drive, San Diego, CA 913 (858)

3 About UDT Instruments History The early beginnings of UDT Instruments can be traced to 1967 when a small group of inventors at United Detector Technology (UDT) began manufacturing the first commercially available transimpedance amplifiers for planar-diffused and Schottky barrier silicon photosensors. Over the next several years, this same group of people went on to pioneer leading-edge technological innovations for photometers, radiometers, fiber-optic power meters and optical position-sensing instruments. By the early 1980 s, this highly skilled and successful group grew into an autonomous entity known as UDT Instruments. Drawing on the momentum generated by UDT s precision photometric instruments,the company developed an inventive handheld colorimeter for the growing television and computer peripherals markets. The development of UDT s SS9400 colorimeter promises to strengthen our company s position as a leader in precision electro-optics instrumentation, while meeting the stringent demands of a multitude of CRT calibration requirements. UDT is poised and ready to excel to greater technological excellence with only one goal in mind: to meet and exceed the ever-changing needs of its customers worldwide. Service We at UDT Instruments stand behind our products and the companies who use them. For this reason, we continue to service those same light-measuring instruments that we built twenty years ago. By offering these services to our customers, both new and established, we stay involved with our products and extend a personal touch to our business relationships. We know of no other company in our industry that hires more qualified sales engineers, people who really understand light measurement principles and practices. By hiring such knowledgeable engineers, we ensure you that you will get the best electro-optic instruments to fit your application and budget. Quality The instrument you receive is certain to be reliable and accurate. We maintain a Quality program that affects every indicator module, sensor head, and optical accessory we sell. And when it comes time for re-calibration, upgrades, or repairs, you ll discover that our service and metrology departments reflect this same commitment to quality and personalized service Aero Drive, San Diego, CA 913 (858)

4 About UDT Instruments Technology Publications Professional Societies Warranty UDT Instruments has always been and continues to be at the forefront of light measurement technology. We hold U.S. and worldwide patents on our QED products, which are absolute radiometric reference standards in the visible and near IR spectrum. Our QED-00 product won a prestigious IR-100 award as one of the 100 most significant U.S. inventions in These products were developed in conjunction with the National Institute of Standards & Technology (NIST) and the National Physical aboratory (NP). UDT Instruments continues to work with the NIST under Cooperative Research And Development Agreements (CRADA) in order to develop even more state-of-the-art products into the 1st Century. In addition to our comprehensive "Guide To" tutorial series, UDT regularly publishes articles in trade journals and other scientific literature which we've made available as application notes to explain subtle details and applications of our technology. UDT is committed to supporting the industry through its professional society affiliates. We are proud to be sustaining members of: Society of Photo Optical Instrumentation Engineers (SPIE) Optical Society of America (OSA) National Association of Broadcasters (NAB) aser Institute of America (IA) Illuminating Engineering Society of America (IES) Society For Information Display (SID) UDT also actively participates in the Council for Optical Radiation Measurement (CORM) and the Commission Internationale l'eclairage (CIE). UDT Instruments warrants that its products are free from defects in material and workmanship under normal use and service for a period of one year from the date of shipment from our factory. UDT Instruments s obligation under this warranty is limited to the replacement or repair of any product determined to be defective during the warranty period,. provided the product is returned to the factory pre-paid. This warranty does not apply to any equipment that has been repaired or altered, except by UDT Instruments, or which has been subject to misuse, negligence,. or accidents. It is expressly agreed that this warranty will be in lieu of all warranty of merchantability. No other warranty is expressed or implied. UDT Instruments is not liable for consequential damages Aero Drive, San Diego, CA 913 (858)

5 Introduction Photodetectors that measure the brightness of light sources find use all around us. They are contained within such everyday items as cameras, bar-code readers, and laser printers. They are a mainstay in most scientific instruments used to measure light intensity or color. But there is another class of optical detectors unrelated to light source brightness measurements, per se. And though these devices find use in a wide range of applications, many technologists are unfamiliar with them. For example, medical researchers use such detectors to track the high-speed patterns of human eye movement, and to perform. 3-D modeling of human motion. Optical scientists rely on such devices to align lasers, light sources, and mirrors to within fractions of microns. This technology is incorporated into ultra-fast, accurate auto focusing schemes for a variety of optical systems, such as microscopes. And such industrial measurements as machine tool alignment and vibration analysis are performed with these detectors. Rather than quantifying the brightness of a light source, these types of measurements are concerned with finding its position in space. As such, this category is generally referred to as optical position sensing, and includes measurements of: angle straightness location height centering surface uniformity distance movement vibration UDT Instruments has integrated this detector technology into a number of specialized optical instruments, which, as we shall see, are being creatively applied to a wide range of measurements in science and industry Aero Drive, San Diego, CA 913 (858)

6 Important Terms Basic Detector Theory It s important to understand that optical position sensing instruments aren t imaging systems like cameras and arrays. The latter require scanning and processing electronics. To calculate the location of an optical event on their surface with high resolution, a computer and image analysis software are needed. As such, using imaging systems for spatial measurements is relatively slow. Instead, position sensors know the position of a light spot on their surface, i.e., the photocurrent they produce is directly proportional to position. So optical position sensing instruments are used where speed, resolution, and simplicity are needed - cameras and arrays, where image detection and analysis are required. To understand optical position sensing instruments, it s important to understand the sensors they make use of. These form the heart of the systems, and fall into two basic categories: segmented and continuous. Quadrant detector electrical connections and formulas. Segmented Position Sensing Detectors Also known as quadrant and bi-cell detectors, these devices have two or four distinct photosensitive elements separated by a minuscule gap. A light spot illuminating just one element only produces photocurrent in that element. When the spot is translated across the surface of the detector, the energy becomes distributed between adjacent elements. The ratio between the photocurrent outputs from these elements determines the relative position of the spot on the surface. It s important to note that the detector only provides position information over a linear distance of the spot diameter. Elsewhere, it is known to be in a specific segment, but not exactly where. Because of this, when working with lasers, defocusing may be required in order to obtain maximum range. With a segmented device, another spatial consideration is key: the response to movement of a circular spot is non-linear. This is because the ratio of the spot s movement to the percentage of its area that shifts between adjacent segments is non-linear. For these reasons, segmented detectors are best used as nulling and centering devices. And for such applications, their performance is unparalleled. In fact, a repeatability of 0.lµm has been routinely demonstrated. This high resolution stems from the almost perfect response uniformity between elements. Also, with light-level sensitivities approaching one picowatt, segmented devices will work with far dimmer sources than will continuous position sensing detectors Aero Drive, San Diego, CA 913 (858)

7 Important Terms Conversion Formulas The position of a light spot with respect to center on a quadrant detector is found using: X=((A+D)-(B+C))/(A+B+C+D) Y=((A+B)-(C+D))/(A+B+C+D) where A,B,C, and D are the photocurrent produced in each segment. The difference signal is divided by the sum in order to cancel out the effects of light level variation. For continuous position sensors the formulae are also simple. For a one-dimensional device: x/=(x -X 1 )/(X 1 +X ) Where X and X1 are the photocurrent signals from each contact, and x is the position along the axis. Similarly, position is calculated for dualaxis tetra-lateral or duo-lateral devices using: x/=(x -X 1 )/(X 1 +X ) y/=(y -Y 1 )/(Y 1 +Y ) The formulae are just slightly more complex for pin-cushion types: x/=((x +Y 1 )-(X 1 +Y ))/(X 1 +X +Y 1 +Y ) y/=((x +Y )-(X 1 +Y 1 ))/(X 1 +X +Y 1 +Y ) High-performance electronic circuits that perform these simple arithmetic functions are incorporated into UDT Instruments position sensing instruments such as the Model 531, 431 and 301-DIV Differential Amplifier. Continuous Position Sensing Detectors When position sensing applications require measurement over a wide spatial range, continuous detectors are the right choice. The primary difference between segmented detectors and continuous ones is that the latter is a single photodiode. There is no gap or dead region between cells. Continuous position sensing detectors derive position by dividing photogenerated electrons within their substrate, not by profiling intensity distribution on the surface as segmented detectors do. Therefore, a -axis continuous sensor acts as a pair of light-controlled variable resistors that measure the X and Y position of an incident light spot. Compared to segmented detectors, the primary advantage of continuous position sensors is their wide dynamic range: They measure the position of a light spot right out to their edge. It s also important to note that these sensors determine the centroid of a light spot. This gives them the advantage of being indifferent to a spot s shape or intensity distribution. For nulling or centering applications, the spatial resolution of a continuous device is inferior to that of a comparable segmented device. This stems from the lower signal-to-noise ratio of continuous devices. So continuous position sensors work best for measuring a light spot s movement over a wide range. Continuous position sensors are available in one-and two-dimensional configurations, and come in four types: duo lateral, tetra-lateral,. pin-cushion and transparent duo-lateral. The duo-lateral type has electrodes on both its front and rear. surfaces. From the equivalent circuit it can be seen that each position signal is divided into just two parts...but by two separate resistive. layers. This approach produces minimal position sensing error and very high resolution. Tetra-lateral types have four electrodes on the front surface of the photodiode. As such, the total induced photocurrent is divided into. 4 parts by the same resistive layer. Compared with the duo-lateral type, tetra-lateral devices are more non-linear for positions further from their mechanical centers. However, the tetra-lateral devices. produce less dark current and have a faster response time. And they are somewhat easier to operate since minimal, or even zero, bias. voltage is required. Pin-cushion devices are basically an improved tetra-lateral, with reduced signal non-linearity at the edges. This is achieved by increasing the photodiode s surface sensitivity and modifying its electrodes. The pin-cushion device offers all the advantages of the tetra-lateral type, namely, low dark 8581 Aero Drive, San Diego, CA 913 (858)

8 Important Terms Transparent duo-lateral detectors are essentially the same in principle as duo-lateral. However, they are constructed by depositing amorphous silicon on a transparent substrate. Thus, an incident beam can pass right through the detector after experiencing a small amount of attenuation and diffusion. ateral-effect detector electrical connections and formulas. Calculating Position Resolution Definition: Resolution is defined as the minimum displacement that can be resolved by a position sensor in a given electro-optical system. Consider a single-axis, lateral-effect position sensor which produces current x and x. Position is given by. 1 P= ( x1 x ) x1+x However, there is uncertainty in the values of x and x due to noise 1 currents n and n. Therefore, the measured position is: 1 P meas = [ (x 1 +n 1 ) (x +n ) ] (x 1 +n 1 ) + (x +n ) inear transfer function of lateral-effect diodes. P meas (max) = [ x 1 x ] x 1 +x -n Maximum error occurs when both noise signals are negative and approximately equal in value. The maximum measured position is: S/N = Since the signal-to noise ratio is: x 1 +x n Aero Drive, San Diego, CA 913 (858)

9 Important Terms 1 We can solve for n and substitute into Equation, to obtain a maximum error value in terms of the signal-to-noise ratio which we can readily estimate and control. Thus: Pmeas (max) =[ x 1 x ] x 1 +x x 1 +x ( ) S/N S/N x 1 x x 1 +x = S/N 1 ( ) is the worst case erroneous measurement. The Modulus of Error is: 1 δp= ( S/N+1 ) If S/N>>1 then: δp= S / N For a typical 10mm x 10mm tetra-lateral type photodiode, one can expect a noise equivalent current of about 40nA, and a maximum signal current of about 00µA. As such, in spatial terms, the noise equals 1 micron. Resolution of a position sensor should not be confused with accuracy or linearity. The resolution is independent of these properties which are intrinsic to the type of detector and not to the signal-to-noise ratio of the system. It is interesting to note that this formula for resolution equally applies to a bi-cell when one considers as the spot diameter: Up to this point we have discussed how to resolve the position of a light spot on the surface of a detector. Now let us define how this relates to resolving the position of a test object or light source Aero Drive, San Diego, CA 913 (858)

10 ateral-effect Photodetector Typical Applications Test Object δ d φ θ Measuring inear Displacement. δ o Sensing Displacements of Specular or Diffuse Surfaces In sin φ = δ this example, the UDT Instruments Model 531 is used to determine the δ o position d of a flat surface. The same approach works to detect the level of a liquid or the position of almost any object, depending on the light source and the object s reflectivity. With the laser beam incident at angle φ the relationship between movement of the light spot, and object movement is: HeNe aser δ d = δ o sinφ when the total angle φ+θ is 90º. For the required position resolution of the test object and the. predicted resolution of the detector: δ d = δ o (min) sinφ Therefore, φ = sin -1 δ o δ d For example, with a detector resolution of 1.5 microns and an object resolution of 1 micron, φ=41. δ d(max) δ o (max) Similarly, the angle of incidence and the maximum sinφ movement of the test object determine the minimum active length of the detector: δ tan δ = d φ δ d ateral-effect Photodetector Test Object δ φ / Test object rotation measurement. δ φ For movement of 100 microns by the object and an angle of. 49 degrees, the detector must have a length of at least δ d 13 microns. = tanδφ Measuring rotation about a fixed point HeNe aser In this case, we know that: δ d(max) tan δ φ(max) δ d (min) tanδφ (min) So the detector s position resolution is related to angular. resolution in the object plane by: Aero Drive, San Diego, CA 913 (858)

11 Typical Applications Error may occur if linear displacement and rotation occur at the same time, since the position sensor cannot distinguish between the object s linear and angular movements. However, the error may be corrected by employing a photodetector to measure the combined effect, and an autocollimator to measure rotation alone. Remote Angle Sensing 6" One of the primary uses of a position sensing detector is for. measuring angles, usually of mirrors, but sometimes of relatively. diffuse surfaces. UDT Instruments, manufactures a number of electronic autocollimators that connect directly to the Models 531 and 431. However, under special situations where the distance to the mirror is large, or where the mirror itself is small, the configuration shown here works well. Mirror Model 531 HeNe aser Remote angular sensing. The mw helium-neon laser is positioned approximately two feet from the mirror under test. The receiving lens and the working distance are chosen to provide the desired mirror angular coverage. In this example, a -inch diameter 135 mm focal length lens would cover a mirror tilt of 38 milliradians. The Model 531 should be set to bias the detector to operate in the photoconductive mode when a relatively high power light source such as HeNe laser is used. Otherwise, the detector will saturate and become nonlinear. Varying the lens focal distance varies the angular sensitivity. A bare detector may also be used; however, it will limit the angular coverage. 8' 6" Model 531 Sensing the position of macroscopic objects. HeNe aser Model 1716 Model Sensing the Position of Moving Objects Here, two cases are considered, both using the same detector configuration. Real-world applications include sensing the position of machine tools and robot arms, or monitoring movement in bioengineering. Case I: The light source is a 0 mw helium-neon laser shining onto a white, diffuse, lambertian surface. Using a 8 mm focal length f/.8 camera lens, the field of view of the lens system at 6 inches is.1 inches (given a detector with a 0.4 x 0.4 inch active area). A narrow-band 633 nm filter will reduce detector sensitivity to ambient light. A faster lens and shorter working distance will utilize the Model 531 s accuracy to an even greater extent. Case II: Using a 1000 W tungsten lamp, working distances up to. or greater than 00 feet are possible. For instance, for the Model 531 set on low gain, at 8 feet with an f/1.4 lens, the signal level is 8% of full scale. Using a higher gain on the Model 531 adds another factor of 1000 to the signal level. Therefore, on high gain the same performance can be achieved Aero Drive, San Diego, CA 913 (858)

12 Typical Applications Model Model /16" A 5/16" 15" Model 1715 Sensing the small-scale displacement of a wire. S 531 HeNe aser Sensing Displacement of a Wire Position sensing detectors are often used in measurement situations where the sample is too delicate, small, or unsuited to contact measurements. In this example, the position of a inch diameter wire is determined in one dimension. The location information may be useful to servo-control equipment such as wire take-up reels. The light source is a mw helium-neon laser, and its beam is expanded into a fan of light by a 5-mm focal length cylindrical lens. This fan is 5/16-inch wide by the time it arrives 15 inches away at the sample. A higher intensity light source can be used or the light source area coverage can be reduced to increase the signal level and the resolution. Though the sample in this case is a wire, anything similar in shape and size will produce about the same signal. For instance, the sample may be glass fiber from a fiber furnace, a thread, liquid stream, or a stream of solid particles. The collection system consists of two short focal length lenses, and a 633-nm narrowband filter, which decreases interference from ambient light. Profilometry Model Microtube. Adapters. (UDT) 8" Model 1714 Model X Microscope Objectives Profilometry HeNe. aser This is essentially a variation on the surface displacement measurement scheme discussed earlier. A helium-neon laser or collimated laser diode is focused onto the test surface with a microscope objective. This provides the requisite small measurement spot. Then, another microscope objective images the reflected spot onto the position sensing detector. The working distance, lens magnification, and test surface reflectivity all influence the system s overall profiling sensitivity Aero Drive, San Diego, CA 913 (858)