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The document discusses calibration of radiometric detectors. It explains that metrology labs purchase calibrated transfer standard detectors from NIST that are then used to calibrate the lab's working standards. The working standards are calibrated using a monochromatic light source and are rotated, tracked, and recalibrated over time. Detectors are usually calibrated at the peak wavelength of the intended application using identical optics. An uncertainty of 10% or less is considered good for radiometry equipment, while NIST can achieve 1% uncertainty.
Calibrationfinal
- 1. Calibration “NIST-traceable” metrology labs purchase calibrated transfer standard detectors directly from the National Institute of Standards and Technology in Gaithersburg, MD. From 400 to 1100 nm, this transfer standard is a Hamamatsu S1337-1010BQ photodiode, a 10 x 10 mm planar silicon cell coated with synthetic quartz. The photodiode is mounted behind a precisely measured 7.98 mm diameter circular aperture, yielding an active area of 0.5 cm2. The responsivity is usually given every 5 nanometers. The calibration labs then use this transfer standard to calibrate their inter-comparison working standards using a monochromatic light source. These working standards are typically identical to the equipment that will be calibrated. The standards are rotated in the lab, tracked over time to monitor stability, and periodically recalibrated. Detectors are most often calibrated at the peak wavelength of the detector / filter / diffuser combination using identical optics for the intended application. The key to this calibration transfer is a reliable kinematic mount that allows exchangeability of detectors in the optical path, and a stable, power regulated light source. Complete spectroradiometric responsivity scans or calibration at an alternate wavelength may be preferred in certain circumstances. Although the working standard and the unknown detector are fixed in precise kinematic mounts in front of carefully regulated light sources, slight errors are expected due to transfer error and manufacturing tolerances. An overall uncertainty to absolute of 10% or less is considered very well for radiometry equipment, and is usually only achievable by certified metrology labs. An uncertainty of 1% is considered state of the art, and can only be achieved by NIST itself.
- 2. . Choosing Input Optics When selecting input optics for a measurement application, consider both the size of the source and the viewing angle of the intended real- world receiver. Suppose, for example, that you were measuring the erythe mal (sunburn) effect of the sun on human skin. While the sun may be considered very much a point source, skylight, refracted and reflected by the atmosphere, contributes significantly to the overall amount of light reaching the earth’s surface. Sunlight is a combination of a point source and a 2π steradian area source. The skin, since it is relatively flat and diffuse, is an effective cosine receiver. It absorbs radiation in proportion to the incident angle of the light. An appropriate measurement system should also have a cosine response. If you aimed the detector directly at the sun and tracked the sun's path, you would be measuring the maximum irradiance. If, however, you wanted to measure the effect on a person laying on the beach, you might want the detector to face straight up, regardless of the sun’s position. Different measurement geometries necessitate specialized input optics. Radiance and luminance measurements require a narrow viewing angle (< 4°) in order to satisfy the conditions underlying the measurement units. Power measurements, on the other hand, require a uniform response to radiation regardless of input angle to capture all light. There may also be occasions when the need for additional signal or the desire to exclude off-angle light affects the choice of input optics. A high gain lens, for example, is often used to amplify a distant point source. A detector can be calibrated to use any input optics as long as they reflect the overall goal of the measurement. Cosine Diffusers A bare silicon cell has a near perfect cosine response, as do all diffuse planar surfaces. As soon as you place
- 3. a filter in front of the detector, however, you change the spatial responsivity of the cell by restricting off-angle light. Fused silica or optical quartz with a ground (rough) internal hemisphere makes an excellent diffuser with adequate transmission in the ultraviolet. Teflon is an excellent alternative for UV and visible applications, but is not an effective diffuser for infrared light. Lastly, an integrating sphere coated with BaSO4 or PTFE powder is the ideal cosine receiver, since the planar sphere aperture defines the cosine relationship. Radiance Lens Barrels Radiance and luminance optics frequently employ a dual lens system that provides an effective viewing angle of less than 4°. The tradeoff of a restricted viewing angle is a reduction in signal. Radiance optics merely limit the viewing angle to less than the extent of a uniform area source. For very small sources, such as a single element of an LED display, microscopic optics are required to “underfill” the source. The Radiance barrel shown below has a viewing angle of 3°, but due to the dual lenses, the extent of the beam is the full diameter of the first lens; 25 mm. This provides increased signal at close distances, where a restricted viewing angle would limit the sampled area. Fiber Optics Fiber optics allow measurements in tight places or where irradiance levels and heat are very high. Fiber optics consist of a core fiber and a jacket with an index of refraction chosen to maximize total internal reflection. Glass fibers are suitable for use in the visible, but quartz or fused silica is required for transmission in the ultraviolet. Fibers are often used to continuously monitor UV curing ovens, due to the attenuation and heat protection they provide. Typical fiber optics restrict the field of view to about ±20° in the visible and ±10° in the ultraviolet.
- 4. Integrating Spheres An integrating sphere is a hollow sphere coated inside with Barium Sulfate, a diffuse white reflectance coating that offers greater than 97% reflectance between 450 and 900 nm. The sphere is baffled internally to block direct and first-bounce light. Integrating spheres are used as sources of uniform radiance and as input optics for measuring total power. Often, a lamp is place inside the sphere to capture light that is emitted in any direction. High Gain Lenses In situations with low irradiance from a point source, high gain input optics can be used to amplify the light by as much as 50 times while ignoring off angle ambient light. Flash sources such as tower beacons often employ Fresnel lenses, making near field measurements difficult. With a high gain lens, you can measure a flash source from a distance without compromising signal strength. High gain lenses restrict the field of view to ±8°, so cannot be used in full immersion applications where a cosine response is required.
- 5. Choosing a Radiometer Detectors translate light energy into an electrical current. Light striking a silicon photodiode causes a charge to build up between the internal "P" and "N" layers. When an external circuit is connected to the cell, an electrical current is produced. This current is linear with respect to incident light over a 10 decade dynamic range. A wide dynamic range is a prerequisite for most applications. The radiometer should be able to cover the entire dynamic range of any detector that will be plugged into it. This usually means that the instrument should be able to cover at least 7 decades of dynamic range with minimal linearity errors. The current or voltage measurement device should be the least significant source of error in the system. Billion-to-One Dynamic Range: Sunny Day 100,000. lux Office Lights 1000. lux Full Moon 0.1 lux Overcast Night 0.0001 lux The second thing to consider when choosing a radiometer is the type of features offered. Ambient zeroing, integration ability, and a “hold” button should be standard. The ability to multiplex several detectors to a single radiometer or control the instrument remotely may also be desired for certain applications. Synchronous detection capability may be required for low level signals. Lastly, poratbility and battery life may be an issue for measurements made in the field. Floating Current to Current Amplification International Light radiometers amplify current using a floating current to-current amplifier (FCCA), which mirrors and boosts the input current directly while “floating” completely isolated. The FCCA current amplifier covers an extremely large dynamic range without changing gain. This proprietary amplification technique is the key to our unique analog to digital conversion, which would be impossible without linear current preamplification.
- 6. We use continuous wave integration to integrate (or sum) the incoming amplified current as a charge, in a capacitor. When the charge in the capacitor reaches a threshold, a charge packet is released. This is analogous to releasing a drop from an eye dropper. Since each drop is an identical known volume, we can determine the total volume by counting the total number of drops. The microprocessor simply counts the number of charge packets that are released every 500 milliseconds. Since the clock speed of the computer is much faster than the release of charge packets, it can measure as many as 5 million, or as few as 1 charge packet, each 1/2 second. On the very low end, we use a rolling average to enhance the resolution by a factor of 4, averaging over a 2 second period. The instrument can cover 6 full decades without any physical gain change! In order to boost the dynamic range even further, we use a single gain change of 1024 to overlap two 6 decade ranges by three decades, producing a 10 decade dynamic range. This “range hysteresis” ensures that the user remains in the middle of one of the working ranges without the need to change gain. In addition, the two ranges are locked together at a single point, providing a step free transition between ranges. Even at a high signal level, the instrument is still sensitive to the smallest charge packet, for a resolution of 21 bits within each range! With the 10 bit gain change, we overlap two 21 bit ranges to achieve a 32 bit Analog to Digital conversion, yielding valid current measurements from a resolution of 100 femtoamps (10-13 A) to 2.0 milliamps (10-3 A). The linearity of the instrument over its entire dynamic range is guaranteed, since it is dependent only on the microprocessor's ability to keep track of time and count, both of which it does very well. Transimpedance Amplification Transimpedance amplification is the most common type of signal amplification, where an op-amp and feedback resistor are employed to amplify an instantaneous current. Transimpedance amplifiers are excellent for measuring within a fixed decade range, but must change gain by switching feedback resistors in order to handle higher or lower signal levels. This gain change introduces significant errors between ranges, and precludes the instrument from measuring continuous exposures. A graduated cylinder is a good analogy for describing some of the limitations of transimpedance amplification. The graduations on the side of the cylinder are the equivalent of bit depth in an A-D converter. The more graduating lines, the greater the resolution in the measurement. A beaker cannot measure volumes greater than itself, and lacks the resolution for smaller measurements. You must switch to a different size container to expand the measurement range - the equivalent of changing gain in an amplifier. In a simple light meter, incoming light induces a voltage, which is amplified and converted to digital using an analog-to -digital converter. A 10 bit A-D converter provides a total of 1024 graduations between 0 and 1 volt, allowing you to measure between 100 and 1000 to an accuracy of 3 significant digits. To measure between 10 and 100, however, you must boost the gain bya factor of 10, because the resolution of
- 7. the answer is only two digits. Similarly, to measure between 1 and 10 you must boost the gain by a factor of 100 to get three digit resolution again. In transimpedance systems, the 100% points for each range have tobe adjusted and set to an absolute standard. It is expected for a mismatch to occur between the 10% point of one range and the 100% point of the range below it. Any nonlinearity or zero offset error is magnified at this 10% point. Additionally, since voltage is sampled instantaneously, it suffers from a lower S/N ratio than an integrating amplifier. Transimpedance amplifiers simulate integration by taking multiple samples and calculating the average reading. This technique is sufficient if the sampling rate is at least double the frequency of the measured signal. Integration The ability to sum all of the incident light over a period of time is a very desirable feature. Photographic film is a good example of a simple integration. The image on the emulsion becomes more intense the longer the exposure time. An integrating radiometer sums the irradiance it measures continuously, providing an accurate measure of the total exposure despite possible changes in the actual irradiance level. The primary difficulty most radiometers have with integration is range changes. Any gain changes in the amplification circuitry mean a potential loss of data. For applications with relatively constant irradiance, this is not a concern. In flash integration, however, the change in irradiance is dramatic and requires specialized amplification circuitry for an accurate reading. Flash integration is preferable to measuring the peak irradiance, because the duration of a flash is as important as its peak. In addition, since the total power from a flash is low, an integration of 10 flashes or more will significantly improve the signal to noise ratio and give an accurate average flash. Since International Light radiometers can cover a large dynamic range (6 decades or more) without changing gain, the instruments can accurately subtract a continuous low level ambient signal while catching an instantaneous flash without saturating the detector. The greatest benefit of integration is that it cancels out noise. Both the signal and the noise vary at any instant in time, although they are presumably constant in the average. International Light radiometers integrate even in signal mode, averaging over a 0.5 second sampling period to provide a significant improvement in signal to noise ratio. Zero The ability to subtract ambient light and noise from readings is a necessary feature for any radiometer. Even in the darkest room, electrical “dark current” in the photodiode must be subtracted. Most radiometers offer a “Zero” button that samples the ambient scatter and electrical noise, subtracting it from subsequent readings. Integrated readings require ambient subtraction as well. In flash measurements especially, the total power of the DC ambient could be higher than the power from an actual flash. An integrated zero helps to overcome this signal to noise dilemma
- 8. Choosing a Filter Spectral Matching A detector’s overall spectral sensitivity is equal to the product of the responsivity of the sensor and the transmission of the filter. Given a desired overall sensitivity and a known detector responsivity, you can then solve for the ideal filter transmission curve. A filter’s bandwidth decreases with thickness, in accordance with Bouger’s law (see Chapter 3). So by varying filter thickness, you can selectively modify the spectral responsivity of a sensor to match a particular function. Multiple filters cemented in layers give a net transmission equal to the product of the individual transmissions. At International Light, we’ve written simple algorithms to iteratively adjust layer thicknesses of known glass melts and minimize the error to a desired curve. Filters operate by absorption or interference. Colored glass filters are doped with materials that selectively absorb light by wavelength, and obey Bouger’s law. The peak transmission is inherent to the additives, while bandwidth is dependent on thickness. Sharp-cut filters act as long pass filters, and are often used to subtract out long wavelength radiation in a secondary measurement. Interference filters rely on thin layers of dielectric to cause interference between wavefronts, providing very narrow bandwidths. Any of these filter types can be combined to form a composite filter that matches a particular photochemical or photobiological process.
- 12. Choosing a Detector Sensitivity Sensitivity to the band of interest is a primary consideration when choosing a detector. You can control the peak responsivity and bandwidth through the use of filters, but you must have adequate signal to start with. Filters can suppress out of band light but cannot boost signal. Another consideration is blindness to out of band radiation. If you are measuring solar ultraviolet in the presence of massive amounts of visible and infrared light, for example, you would select a detector that is insensitive to the long wavelength light that you intend to filter out. Lastly, linearity, stability and durability are considerations. Some detector types must be cooled or modulated to remain stable. required for other types. In addition, some can be burned out by excessive light, or have their windows permanently ruined by a fingerprint. Silicon Photodiodes Planar diffusion type silicon photodiodes are perhaps the most versatile and reliable sensors available. The P-layer material at the light sensitive surface and the N material at the substrate form a P-N junction which operates as a photoelectric converter, generating a current that is proportional to the incident light. Silicon cells operate linearly over a ten decade dynamic range, and remain true to their original calibration longer than any other type of sensor. For this reason, they are used as transfer
- 13. standards at NIST. Silicon photodiodes are best used in the short-circuit mode, with zero input impedance into an op-amp. The sensitivity of a light-sensitive circuit is limited by dark current, shot noise, and Johnson (thermal) noise. The practical limit of sensitivity occurs for an irradiance that produces a photocurrent equal to the dark current (Noise Equivalent Power, NEP = 1). Solar-Blind Vacuum Photodiodes The phototube is a light sensor that is based on the photoemissive effect. The phototube is a bipolar tube which consists of a photoemissive cathode surface that emits electrons in proportion to incident light, and an anode which collects the emitted electrons. The anode must be biased at a high voltage (50 to 90 V) in order to attract electrons to jump through the vacuum of the tube. Some phototubes use a forward bias of less than 15 volts, however. The cathode material determines the spectral sensitivity of the tube. Solar-blind vacuum photodiodes use Cs-Te cathodes to provide sensitivity only to ultraviolet light, providing as much as a million to one long wavelength rejection. A UV glass window is required for sensitivity in the UV down to 185 nm, with fused silica windows offering transmission down to 160 nm.
- 14. Multi-Junction Thermopiles The thermopile is a heat sensitive device that measures radiated heat. The sensor is usually sealed in a vacuum to prevent heat transfer except by radiation. A thermopile consists of a number of thermocouple junctions in series which convert energy into a voltage using the Peltier effect. Thermopiles are convenient sensor for measuring the infrared, because they offer adequate sensitivity and a flat spectral response in a small package. More sophisticated bolometers and pyroelectric detectors need to be chopped and are generally used only in calibration labs. Thermopiles suffer from temperature drift, since the reference portion of the detector is constantly absorbing heat. The best method of operating a thermal detector is by chopping incident radiation, so that drift is zeroed out by the modulated reading. The quartz window in most thermopiles is adequate for transmitting from 200 to 4200 nm, but for long wavelength sensitivity out to 40 microns, Potassium Bromide windows are used.
- 15. Graphing Data Line Sources Discharge sources emit large amounts of irradiance at particular atomic spectral lines, in addition to a constant, thermal based continuum. The most accurate way to portray both of these aspects on the same graph is with a dual axis plot, shown in figure 9.1. The spectral lines are graphed on an irradiance axis (W/cm2) and the continuum is graphed on a band irradiance (W/cm2/nm) axis. The spectral lines ride on top of the continuum. Another useful way to graph mixed sources is to plot spectral lines as a rectangle the width of the monochromator bandwidth. (see fig. 5.6) This provides a good visual indication of the relative amount of power contributed by the spectral lines in relation to the continuum, with the power being bandwidth times magnitude. The best way to represent the responsivity of a detector with respect to incident angle of light is by graphing it in Polar Coordinates. The polar plot in figure 9.2 shows three curves: A power response (such as a laser beam underfilling a detector), a cosine response (irradiance overfilling a detector), and a high gain response (the effect of using a telescopic lens). This method of graphing is desirable, because it is easy to understand visually. Angles are portrayed as angles, and responsivity is portrayed radially in linear graduations.
- 16. The power response curve clearly shows that the response between -60 and +60 degrees is uniform at 100 percent. This would be desirable if you were measuring a laser or focused beam of light, and underfilling a detector. The uniform response means that the detector will ignore angular misalignment. The cosine response is shown as a circle on the graph. An irradiance detector with a cosine spatial response will read 100 percent at 0 degrees (straight on), 70.7 percent at 45 degrees, and 50 percent at 60 degrees incident angle. (Note that the cosines of 0°, 45° and 60°, are 1.0, 0.707, and 0.5, respectively). The radiance response curve has a restricted field of view of ± 5°. Many radiance barrels restrict the field of view even further (± 1-2° is common). High gain lenses restrict the field of view in a similar fashion, providing additional gain at the expense of lost off angle measurement capability. The Cartesian graph in figure 9.3 contains the same data as the polar plot in figure 9.2 on the previous page. The power and high gain curves are fairly easy to interpret, but the cosine curve is more difficult to visually recognize. Many companies give their detector spatial responses in this format, because it masks errors in the cosine correction of the diffuser optics. In a polar plot the error is easier to recognize, since the ideal cosine response is a perfect circle. In full immersion applications such as phototherapy, where light is coming from all directions, a cosine spatial response is very important. The skin (as well as most diffuse, planar surfaces) has a cosine response. If a cosine response is important to your application, request spatial response data in polar format.
- 17. At the very least, the true cosine response should be superimposed over the Cartesian plot of spatial response to provide some measure of comparison. Note: Most graphing software packages do not provide for the creation of polar axes. Microsoft Excel™, for example, does have “radar” category charts, but does not support polar scatter plots yet. SigmaPlot™, an excellent scientific graphing package, supports polar plots, as well as custom axes such as log-log etc. Their web site is: http://www.spss.com/ A log plot portrays each 10 to 1 change as a fixed linear displacement. Logarithmically scaled plots are extremely useful at showing two important aspects of a data set. First, the log plot expands the resolution of the data at the lower end of the scale to portray data that would be difficult to see on a linear plot. The log scale never reaches zero, so data points that are 1 millionth of the peak still receive equal treatment. On a linear plot, points near zero simply disappear. The second advantage of the log plot is that percentage difference is represented by the same linear displacement everywhere on the graph. On a linear plot, 0.09 is much closer to 0.10 than 9 is to 10, although both sets of numbers differ by exactly 10 percent. On a log plot, 0.09 and 0.10 are the same distance apart as 9 and 10, 900 and 1000, and even 90 billion and 100 billion. This makes it much easier to determine a spectral match on a log plot than a linear plot.
- 18. As you can clearly see in figure 9.4, response B is within 10 percent of response A between 350 and 400 nm. Below 350 nm, however, they clearly mismatch. In fact, at 315 nm, response B is 10 times higher than response A. This mismatch is not evident in the linear plot, figure 9.5, which is plotted with the same data. One drawback of the log plot is that it compresses the data at the top end, giving the appearance that the bandwidth is wider than it actually is. Note that Figure 9.4 appears to approximate the UVA band. Most people are familiar with graphs that utilize linearly scaled axes. This type of graph is excellent at showing bandwidth, which is usually judged at the 50 percent power points. In figure 9.5, it is easy to see that response A has a bandwidth of about 58 nm (332 to 390 nm). It is readily apparent from this graph that neither response A nor response B would adequately cover the entire UVA band (315 to 400 nm), based on the location of the 50 percent power points. In the log plot of the same data (fig. 9.4), both curves appear to fit nicely within the UVA band.
- 19. This type of graph is poor at showing the effectiveness of a spectral match across an entire function. The two responses in the linear plot appear to match fairly well. Many companies, in an attempt to portray their products favorably, graph detector responses on a linear plot in order to make it seem as if their detector matches a particular photo- biological action spectrum, such as the Erythemal or Actinic functions. As you can clearly see in the logarithmic curve (fig. 9.4), response A matches response B fairly well above 350 nm, but is a gross mismatch below that. Both graphs were created from the same set of data, but convey a much different impression. As a rule of thumb - half power bandwidth comparisons and peak spectral response should be presented on a linear plot. Spectral matching should be evaluated on a log plot. The Diabatie scale (see fig. 9.7) is a log-log scale used by filter glass manufacturers to show internal transmission for any thickness. The Diabatie value, θ(λ), is defined as follows according to DIN 1349: θ(λ) = 1 - log(log(1/τ)) Linear transmission curves are only useful for a single thickness (fig. 9.6). Diabatie curves retain the same shape for every filter glass thickness, permitting the use of a transparent sliding scale axis overlay, usually provided by the glass manufacturer. You merely line up the key on the desired thickness and the transmission curve is valid.
- 21. Setting Up an Optical Bench A Baffled Light Track The best light measurement setup controls as many variables as possible. The idea is to prevent the measurement environment from influencing the measurement. Otherwise, the measurement will not be repeatable at a different time and place. Baffles, for example, greatly reduce the influence of stray light reflections. A baffle is simply a sharp edged hole in a piece of thin sheet metal that has been painted black. Light outside of the optical beam is blocked and absorbed without affecting the optic Multiple baffles are usually required in order to guarantee that light is trapped once it strikes a baffle. The best light trap of all, however, is empty space. It is a good idea to leave as much space between the optical path and walls or ceilings as is practical. Far away objects make weak reflective sources because of the Inverse Square Law. Objects that are near to the detector, however, have a significant effect, and should be painted with “black velvet” paint or moved out of view. A shutter, door, or light trap in one of the baffles allows you to measure the background scatter component and subtract it from future readings. The “zero” reading should be made with the source ON, to maintain the operating temperature of the lamp as well as measure light that has defeated your baffling scheme.
- 22. Kinematic Mounts Accurate distance measurements and repeatable positioning in the optical path are the most important considerations when setting up an otical bench. The goal of an optical bench is to provide repeatability. It is not enough to merely control the distance to the source, since many sources have non-uniform beams. A proper detector mounting system provides for adjustment of position and angle in 3-D space, as well as interchangeability into a calibrated position in the optical path. To make a kinematic fixture, cut a cone and a conical slot into a piece of metal using a 45° conical end mill (see fig. 8.2). A kinematic mount is a three point fixture, with the third point being any planar face. The three mounting points can be large bolts that have been machined into a ball on one end, or commercially available 1/4-80 screws with ball bearing tips (from Thorlabs, Inc.) for small fixtures. The first leg rests in the cone hole, fixing the position of that leg as an X-Y point. The ball tip ensures that it makes reliable, repeatable contact with the cone surface. The second leg sits in the conical slot, fixed only in Yaw, or angle in the horizontal plane. The use of a slot prevents the Yaw leg from competing with the X-Y leg for control. The third leg rests on any flat horizontal surface, fixing the Pitch, or forward tilt, of the assembly. A three legged detector carrier sitting on a kinematic mounting plate is the most accurate way to interchange detectors into the optical path, allowing intercomparisons between two or more detectors. Bibliography http://www.intl-lighttech.com/support/light-measurement-calibration-chapter-14-light-measurement- tutorial
- 23. RIZAL TECHNOLOGICAL UNIVERSITY BONI MANDALUYONG COLLEGE OF ENGINEERING AND INDUSTRIAL TECHNOLOGY BACHELOR OF SCIENCE IN ELECTRICAL ENGINEERING (Illuminationg Engineering Design) Aljon Garcia Kevin Dimaandal Melaiza Jean Lacson Dandrift Figueroa John Royces Valencia Isaac Delos santos Arjenly Valiente Engr. Arnold Ilagas September 6, 2015