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GLOSSARY OF LASER SENSOR TERMINOLOGY
TERMS EXPLAINING ACCURACY, RESOLTUION, TARGET CHARACTERISTICS
Several terms relating to optical sensing and distance measurement are used throughout Acuity’s application notes. Each of these is defined and described briefly here. This glossary is in alphabetical order.
Accuracy: The accuracy of a sensor is a measurement of the difference that can be expected between a sensor’s reading and the actual distance measured. The resolution is the smallest change in distance that a sensor can detect, and is typically a smaller value than the accuracy error. Accuracy may be affected by temperature, target reflectance or ambient light, which generally will not affect the resolution. The AR4000 laser rangefinder has a basic resolution limit of .0125 inch (0.30 mm)for a single sample, with accuracy of 0.1 inch (2.5 mm). For many applications, resolution is more important than absolute accuracy. The AR700 and AR200 laser displacement sensors have accuracies which are +/- 0.1% and +/- 0.2% of their measuring spans, respectively. In general, laser sensor manufacturers may interchangably use the terms Accuracy and Linearity although Linearity is a term most often associated with analog measuring devices.
Cooperative Target: A target that is designed to reflect light to the detector of a sensor. Cooperative targets include glass corner cube retroreflectors and retroreflective tape made by several manufacturers. In some applications, mirrors may also be used as cooperative targets.
Depth of Field: The span of distances over which a rangefinder can accurately measure distance. This may be limited by the focus of the light collection optics as well as the maximum distance at which enough light is reflected to the sensor. These two factors will determine how the sensor’s sensitivity changes with distance.
Diffuse Reflection: Diffuse reflection occurs when light strikes a target and is scattered over a wide angle. Plain white paper or flat (not glossy) wall paint are good diffuse materials. Diffuse targets are the best uncooperative targets, and may be measured to over a wide range of incident angles (up to 80 degrees for some materials).
Laser Power: The optical power level emitted by the laser in a sensor. The power may be specified as an average power, or as a peak power as well as an average if the sensor emits pulses or intermittent light output. All other factors being equal, the maximum range increase in proportion to the square root of the laser power: If the power is quadrupled, the maximum range will be doubled. Laser power is expressed in milliwatts (mW) or Watts.
Linearity: The largest deviation from a best-fit straight line over the measurement range, created by data from the sensor with reference taken from a true distance scale. Stated as +/- % of the full scale span.
Maximum Range: The maximum distance to which a sensor can pick up reflected light and obtain an accurate distance measurement. The maximum range may be limited by the power of the laser, the amount of light reflected by the target, and the sensitivity of the detector. It may also be limited by the measurement method used and the distance to which the sensor is accurately calibrated. Maximum range figures for Acuity sensors are specified to targets of 85% diffuse reflectance. Maximum range will typically increase with the square root of the reflectance.
Reflectance: The amount of light a target reflects, expressed as a percentage of the incident light. Diffuse reflectance refers to the amount of light scattered in all directions by a diffuse target. Specular reflectance refers to the amount of light reflected as by a mirror. Reflectance will depend on target color and composition and on the frequency of the light being reflected. Diffuse surfaces typically vary from 3% to 95% reflectance. Many surfaces such as glossy paint or glossy paper have both diffuse and specular components to their reflectance.
Repeatability: A measure of sensor stability over time. Typically, sample to sample repeatability will be lower for very fast sample rates, since less time is used to average the measurement. As the sample rate is lowered, repeatability will improve, but this does not continue indefinitely. Beyond some slower sample rate, repeatability will start to worsen as long term drift in the components and temperature changes cause changes in the sensor’s output.
Resolution: Smallest change in distance that a sensor can detect. Stated as +/- % of the full scale span. AR200 and AP620 sensors have CMOS detectors that permit a resolution of 0.03% of the span. The resolution of the AR4000 is governed by the internal clock used for time-of-flight
Response Time: The delay between the time of a change in the target position and the time the sensor’s output changes. This can be longer than one sample interval, if the sensor is processing or calibrating intermediate samples while transmitting the previous sample and taking the next measurement.
Retroreflection: The reflection of light off a target back in the direction from which it came, for a wide range of angles of incidence. Retroreflection is achieved through multiple reflections within a retroreflector. Retroreflectors include corner cubes and retroreflective tape. A high-quality corner cube retroreflector will return virtually all the light entering it to its source. Corner cubes may be used to extend range hundreds or thousands of times over ordinary surfaces. A corner cube array was left on the moon to allow accurate measurement of its distance from the earth. Retroreflective tape is useful in many applications. It typically consists of micospheres or cubes of glass or plastic which act like many tiny retroreflectors. It is inexpensive and easy to use, and some types will return more than 1000 times the light returned by uncooperative targets. For other applications it is unnecessary or impossible to use cooperative targets, as when measuring the position or level of materials in process. Acuity’s AR4000 sensors may be used on both cooperative and uncooperative targets.
Sample Rate: Rate that data samples are obtained from the sensor. The maximum attainable sample rate is determined by the selected operating mode and target reflectance. Sample Rate is the frequency with which a sensor updates its range output. The sample rate capability of distance sensors varies widely, depending on the measurement method and design of the device. Sample rates may be as low as one sample every few seconds and run up to millions of samples per second. The AR 4000 sensors may be set to any sample rate from 10 seconds/sample to 200,000 samples/second.
Sensitivity: A measure of the ability to obtain a reading on a dark target or with low laser power. Sensitivity decreases at long ranges.
Specular Reflection: Specular reflection occurs when light strikes a shiny or mirror-like surface and is reflected away at one angle. Glass, liquid surfaces, and polished metals are specular, and generally require a sensor configured specifically for specular surfaces. Acuity makes versions of the 4000 that can be used on specular surfaces, including liquids.
Target: The surface that a laser spot hits, from which light is reflected to the detector in an optical sensor. This may refer to any type of surface or material at which a sensor is pointed or to a specific object or material designed to reflect light. Target reflectance is the most important factor in determining the maximum range of a sensor.
Uncooperative Target: Uncooperative targets include any surfaces or material being measured that is not specifically designed to reflect light to the sensor. This includes shiny metal or painted surfaces, liquids, and loose or granular solid substances.