Book in Focus
The Properties of Optical Radiation Detectors and Radiometers"/>

29th June 2022

Book in Focus
The Properties of Optical Radiation Detectors and Radiometers

By Dr George P. Eppeldauer


Recently Developed Optical Detectors/Radiometers, Their Calibrations and Applications

Keywords: optical detectors; optical radiometers; light measurement; photometry; radiometry; colorimetry; radiation temperature measurement; SI units; measurement uncertainties

Summary

Improved detector technology in the past two decades has opened a new era in the field of optical radiation measurements. Optical radiation is detected in light measurement, photometry, colorimetry, optical radiometry, and high temperature measurement. The properties of the improved detectors must be known in order to perform uniform measurements with low uncertainty. The detector needs to be mounted in a radiometer-housing which must be equipped with input optics to measure the required physical quantity (such as radiant power, irradiance, or radiance etc.). These optical radiometers, according to the new International System of Units (SI), can be used as standards to realize and propagate a radiometric or photometric scale (a unit of the SI). All scales should be traceable (through a National Metrological Institute) to the new SI redefined in 2019. Since the detectors convert the physical quantities to be measured into photocurrents, the detector output current is an SI traceable sub-scale.

An increased number of calibration and measurement facilities and procedures could be developed with lower measurement uncertainties using the newly developed detector/radiometer standards instead of traditionally applied source standards (such as blackbodies and lamps). The shrinking of traditional source-based calibrations and measurements and the large increase in optical detector-based calibrations and applications motivated the writing of this book series.

This book series provides a comprehensive description of optical detector-based radiometric practices. Instead of giving the traditional, lexical-type tutorial information, it systematically organizes and describes research-based material. The large number of examples provided cover the applications of modern optical radiation detectors in the field of radiometry, photometry, colorimetry, and radiation temperature measurements. All the discussed devices and applications have been implemented, realized, tested, verified, and evaluated. The book series includes many hundreds of designs, drawings, and measurement schemes, as well as numerous detector-based measurement and calibration setups, measurement equations and results, calibration-transfer and measurement procedures, all tested in practical applications. The measurements were performed in both spectral (versus wavelength increments) and broadband (integrated) measurement modes. The goal was to obtain uniform measurement results with low uncertainties using simple (modern) devices, improved facilities, methods, and procedures that are described with enough detail for the reader to successfully repeat them. The applications and evaluations follow the recommendations of international standardization. The discussed subjects are organized, detailed, and distributed into five volumes. The name of the book series is Optical Radiation Detectors and Radiometers.

Book 1: The Properties of Optical Radiation Detectors and Radiometers

Properties of radiometric quality detectors are to be known in order to select the right detector type or model for the measurement of a particular quantity, such as radiometric, photometric, color, etc. Such properties are spectral, spatial, angular, and temperature-dependent responsivities. As an example, Fig. 1 shows the spectral responsivity of widely used Si, Si-light-trap, Ge, InGaAs, extended-InGaAs, InSb, photovoltaic (PV) HgCdTe (MCT), and pyroelectric detectors. These detectors can be used to cover the ultraviolet to infrared wavelength range from 200 nm to 14 mm. The maximum-to-minimum non-uniformity of spatial responsivity, within the active area of a detector, could change between 0.2 % and 25 % for these different single-element detectors. With a proper detector arrangement, this non-uniformity can be decreased when Si-light-trap radiometers are used. For a properly constructed and measured Si trap radiometer, a peak-to-peak change of 0.016 % was measured within the overall active area. Minimizing these changes is important when radiant power (total beam power) is to be measured and the beam position may change along the active area of the detector.

Noise performance, radiometric sensitivity and responsivity-linearity also should be known for the selected detector. Electronic characteristics of the detectors, including resistance, drift, settling time, and stability are to be known or tested. In addition to photodiodes (which are the workhorses of measurements), other detectors like photoconductors, optical bolometers, pyroelectric detectors, thermopiles, photomultiplier tubes, cryogenic thermal detectors and radiometers are described in this book. Non-linearity issues of detectors and detector-applied systems, including non-linearity test methods, are discussed as well. Detector characteristics of different types and models are compared to help in making the right selection for a given measurement task.

This is the first published book to investigate the improved performance of optical radiation detectors developed from the ultraviolet to the far-infrared in the past two decades. These are the new detectors that opened up a new era in radiometric, photometric, colorimetric, and radiation-temperature measurements, where earlier blackbody sources and lamps were used with lower performance and in limited application areas. This book can help practicing scientists, researchers, engineers, technicians, instrument/detector manufacturers, and students to learn, compare, and select the proper detectors for building, using, and calibrating electro-optical instruments with System International (SI) traceability and lowered measurement uncertainty in extended application areas.


Fig. 1. Spectral power responsivities of widely used radiometric-quality detectors from UV to IR.

Book 2: Optical Detector Applications for Radiometric Measurements

Useful applications of optical detectors for radiometric, photometric, and colorimetric calibrations and measurements can be done only if the detector properties are known. Detector applications include design considerations for the optical radiometers where the modern radiometric quality detectors are used. The applications involve photovoltaic and photoconductive detector operations and their electrical output signal measurements. Design of radiometer housings with input optics depends on the type of the quantities to be measured. The most frequently measured (input) radiometric quantities are radiant power, irradiance, and radiance. Modifications to the input optics of the radiometers make it possible to measure equivalent photometric quantities, such as luminous flux, illuminance, and luminance.

As an example, Fig. 2 shows a versatile radiometer housing for a cooled photodiode and a thermoelectric-cooler heated filter-holder. The filter or filter package (not shown in the drawing), located inside of the filter holder, needs temperature-controlled heating to avoid condensation and changes in the spectral transmittance. Photodiodes typically need cooling to increase their (shunt) resistance. The preamplifier is located in the back of the cylindrical housing where the electronic circuits are mounted on two printed circuit boards. The electronic unit can be detached from the front unit which is important in order to perform gain-calibrations for the preamplifier.

The measurement mode can be direct current (DC), alternating current (AC), or pulse, depending on the specific measurement task. The detectors and their output-signal measuring circuits are to be matched (to each other) to obtain optimum performance for a given measurement. A well-designed detector–preamplifier (matched) pair should produce an optimized first-stage for the radiometer amplifier, resulting in a high signal-to-noise ratio, a wide signal range with linear operation, and a low uncertainty in the performed measurements.

Fig. 2. Radiometer housing for a cooled photodiode and a heated filter-holder.

To achieve the above goals, the frequency dependent fundamental gain equations, such as signal gain, loop gain, and closed-loop voltage gain of photodiode transimpedance-type (operational amplifier-based) photocurrent meters are analyzed. For output signal measurements, the use of sinewave measuring lock-in amplifiers and the calibration of charge measuring amplifiers are discussed. Optimization of noise in photocurrent measurements is discussed and noise performances obtained using DC and AC dark noise measurements are compared. For flash-light and radiation-pulse measurements, voltage- and current-integrators are discussed.

Detector-impedance multiplying bootstrap amplifiers are modeled and evaluated for low (shunt) resistance photovoltaic detectors to decrease noise amplification from the radiometer input to its output. Biasing current-amplifier design for photoconductive detectors is described and evaluated. Frequently needed temperature monitor or control for input color-filters, detectors, and preamplifiers to further improve measurement uncertainty and long-term stability are also discussed.

AC radiation measurements using choppers and lock-in amplifiers are considered, as is the calibration of the sinewave measuring lock-in amplifier.

Book 3: Optical Detector and Radiometer Standards

The standard devices for optical radiometric calibrations and measurements are either sources or detectors. Two to three decades ago, all radiometric calibrations were performed against source standards. The primary source standards are blackbody sources of known temperature (fixed-point blackbody radiators). The International Temperature Scale of 1990 (ITS-90) standardized the freezing-point temperature of molten metals utilized in black-body radiators where the spectral power distribution could be calculated from Planck’s Radiation Law. When radiometric devices (lamps or detectors) were calibrated against a blackbody source, a long calibration (scale derivation) chain was used. One problem here, however, was that the spectral power distribution of tungsten lamps is different than that of the Planckian radiators they are calibrated against. In source-based scale derivations, radiometric standard lamps with known spectral power or irradiance distributions were used to calibrate sources, detectors, devices, and instruments for radiometric, photometric, pyrometric, and color measurements. The measurement uncertainties obtained using standard lamps are significantly higher than one percent. The poor uncertainty and reproducibility that are wavelength- and burning time-dependent were mostly produced by the long chain in the scale derivation.

Traditionally, synchrotron radiation sources (emitting under vacuum conditions) are also used as primary radiometric standards. The spectral radiant power of an electron storage ring can be calculated from the electron energy, the magnetic induction, and the electron current stored in the ring. A synchrotron source covers a very wide wavelength range including IR, VIS, UV, VUV, and soft X-ray regions. Utilizing synchrotron radiation for standardization is much less user-friendly than using detector standards and the uncertainty for the calculation of the spectral radiant power is at least a decade higher in the near IR and VIS ranges than in the absolute radiometers discussed in this book.

In the last two decades, because of optical detector improvements, detector-based calibrations increased, and source-based calibrations decreased. It was necessary to develop modern radiometers using the improved detectors for realizing, holding, and disseminating the new detector-based scales.

In detector-based calibrations, the primary standard is an absolute (traceable to the watt) radiometer. The responsivity of an absolute radiometer for radiant power is known with uncertainties between 0.1 % and 0.01 % (coverage factor k=2). The radiant power is a derived SI unit. Absolute radiometers (frequently called absolute detectors) that are either electrical substitution radiometers or quantum efficient (self-calibrated) detectors are discussed. Transfer and working standards, including Si light-trap detectors, UV damage-free Si photodiodes, IR-enhanced Si detectors, low-noise pyroelectric radiometers, filter-radiometers, and near-IR and short-wave-IR radiometers, are described in detail. Input spheres are mounted in front of spatially non-uniform detectors to improve their uniformity, and to increase the size of a small detector by mounting a reasonable size aperture upon the sphere input.

Radiant power responsivity may be called an ivory-tower unit because it is mostly used (except laser power measurements) for scale realizations and propagations. For most field applications, irradiance responsivity calibration of the radiometers is needed. The discussed sphere-detectors can be used as power-to-irradiance responsivity converters.

Radiance and luminance meters are equipped with imaging input optics. The difference between them is found in the shape of the spectral responsivity function which can be modified using optical (photometric) filters. The input optics can convert an illuminance meter into a luminance meter. The luminance measurement angles may be changed by choosing one of the several different hole-sizes in a mirror-aperture wheel. The design of an illuminance meter with an attachable input (luminance measuring) optics is shown in Fig. 3. A second imaging optics (lens) is used in this input optics to produce efficient blocking for radiation coming from outside of the target area and to maintain a constant luminance responsivity throughout the focus range of the camera lens located at the front of the input optics. This device can also measure radiant power, irradiance, and radiance when the photometric filter is removed. The silicon photodiode also can be replaced with other (for example, near-IR) detectors to cover the near-IR range as well.

Fig. 3. Front design of an illuminance/luminance measuring photometer.

The detector- and radiometer-based calibrations can produce significantly lower uncertainty in the scale /propagations compared to source-based calibrations and they are simpler and more user friendly. The higher stability of the detectors (compared to lamps) results in better long-term stability and reproducibility in radiometric, photometric, and color measurements.

Book 4: Detector-Based Reference Calibrations for Electro-Optical Instruments

Spectral calibrations and measurements utilizing improved calibration setups developed using new radiometer standards are described in this book, while these detector-based reference-level spectral calibrations for the measurement of the most important radiometric quantities (used in secondary laboratories and field applications) are also discussed. In addition, the book described definitions, methods, and transfer of the reference spectral radiant power, irradiance, and radiance responsivity. Responsivity is the ratio of the output electrical signal to the input radiometric or photometric quantity. When the output signal of a standard (of known responsivity) is measured, using the known responsivity, the input quantity can be determined. These spectral measurements are performed at equal wavelength increments, using monochromatic radiation. Relative spatial, angular, and spectral variations of responsivities and absolute responsivities are determined for detectors, radiometers, and photometers. Measurement geometry, measurement setups, typical types and properties of different detectors, radiometers, and photometers, are discussed. The measurement methods include procedures to obtain traceability to National Measurement Institutes and guidance for selecting detector and radiometer standards.

The book also describes responsivity scale realizations and extensions from reference silicon standards for the ultraviolet (UV) to infrared (IR) ranges, while directional errors when using the detector and radiometer standards are analyzed. The spectral responsivity calibrations are discussed for photometers and tristimulus colorimeters and for electronic imaging devices, while evaluation of measurement uncertainty and state of the art measurement uncertainty values are provided. Responsivity uncertainties are derived from principal measurement equations. Common problems in the above measurements, including spectral issues, DC and AC measuring methods and instruments are also discussed.

As an example, AC calibration/measurement mode for the IR is discussed. At wavelengths longer than about 2000 nm, a significant DC signal will be produced by the background radiation. This background signal shows up only in infrared detectors that respond to optical radiation longer than 2000 nm. An IR detector cannot be calibrated using a DC measuring setup because the background-produced DC signal (photocurrent) is high compared to the useful DC signal (to be measured). Since the high DC background radiation is not stable, the DC (background) signal will drift. Because of the drift, the DC background signal cannot be subtracted from the useful DC signal. Therefore, measurement of IR detectors in DC mode would result in drift-dominated signal measurements. To avoid drift-dominated measurements, IR detectors must be measured in AC mode.

For short-wave IR calibrations, either photovoltaic (PV) HgCdTe (MCT) detectors or extended-InGaAs (E-IGA) detectors are used. Both can have high detector (shunt) resistance, which is needed for high sensitivity applications. E-IGA detectors with small active area (such as 1 mm2) and high shunt-resistance (about 15 MW) have low noise equivalent power (NEP), where the NEP is the ratio of the output noise to the responsivity. As discussed in Optical Detector and Radiometer Standards, a noise equivalent current, NEC=7 fA could be measured in AC mode, with a 1 mm diameter E-IGA detector (using a cold FOV limiter) at an electrical bandwidth of 0.16 Hz. This corresponds to an AC-mode NEP = 5.4 fW which is close to the 3 fW typical DC-mode NEP of silicon photodiodes.

As an example, Fig. 4 shows a setup where E-IGA-1 and -2 working standards are calibrated for spectral power responsivity against sphere-input E-IGA and pyroelectric transfer standards.

Fig. 4. Calibration of extended InGaAs (EIGA-1 and EIGA-2) working standards against sphere-input (sphere-EIGA) and pyroelectric (cylinder-shielded) transfer-standards.

Pyroelectric transfer standard detectors are used to extend the spectral responsivity measurements from the silicon wavelength range to a wider range. For pyroelectric detector use, the optical radiation must be chopped. Either voltage mode or current mode preamplifiers can be used. In the voltage mode preamplifiers, Field Effect Transistors (FETs) are used to amplify the voltage drop on the detector. Here, the input stray capacitance is 10 to 100 times the stray feedback capacitance of a current mode preamplifier. Therefore, the frequency response of the current mode preamplifiers (current-to-voltage converters) is much broader. The output of the preamplifier is measured by a synchronized (to the modulation/chopped frequency) lock-in amplifier and the filtered (DC) lock-in output signals are converted into digital signals using analog-to-digital converters or digital voltmeters (DVMs). When the pyroelectric detector is calibrated against a standard detector, or working standard detectors are calibrated against a pyroelectric detector, all the detectors should be operated in AC (chopped) measurement mode. The responsivity of pyroelectric detectors is low (about six orders of magnitude lower than silicon photodiodes); therefore, their NEP is high. High NEP needs high beam power from the monochromatic source. Using traditional pyroelectric detectors with NEP ~ 40 nW/Hz^1/2, 4 µW or higher beam-power is needed to achieve a relative standard measurement uncertainty of 1 % (which needs a signal-to-noise ratio of 100). Recently, the NEP of pyroelectric detectors has been decreased by an order of magnitude using frequency compensations performed by organic black paint-coating and by decreasing the LiTaO3 crystal thickness. As discussed here, traditional monochromators can be used with recently developed low-NEP pyroelectric detectors for spectral power responsivity measurements between 250 nm and 30 mm.  

Since the responsivity of a square-wave measuring lock-in amplifiers may depend on the shape of the chopped input signal, use and calibration of sinewave measuring lock-in amplifiers that measure the fundamental frequency component of the input signal are also discussed in the book.

Book 5: Broadband UV-, VIS-, and IR-Radiometric, Photometric, Color, and Temperature Measurements

Traditionally, radiometric, photometric, and color calibrations came from the source standard. Color temperature, irradiance, and luminous intensity standard lamps were calibrated against black-body radiators through several derivation steps. Because of the multiple-step calibration procedures, and the significant change of source characteristics during and between calibrations, long-term reproducibility of only close to 1 % (k=2) could happen in addition to the close to 1 % (k=2) spectral irradiance uncertainty (with respect to SI units) on the test irradiance lamps in the visible range. Traditionally, color temperature scales are based on the spectral irradiance scale with the additional application of the CIE standardized weighting functions. Color temperature is a light source parameter related to spectral quality (relative spectral power or irradiance distribution).  

Recently, improved photometric and radiometric scales have been realized and maintained using detector-based standards. The reference spectral responsivity scales discussed in Detector-Based Reference Calibrations for Electro-Optical Instruments were utilized here to perform broadband photometric and radiometric calibrations and measurements in practical applications with low uncertainties.

In most practical applications, broadband measurements are used where the spectral product of the source distribution and the meter’s spectral responsivity are measured. While in photometry, the standardized spectral response function of the meter covers only the visible interval, measurements outside the visible spectral interval (such as ultraviolet or infrared) require different standards. Using a traditional detector- or source-standard, the differences in the spectral distributions of test sources (like LEDs) and in the spectral responsivities of the source measuring detectors produce different broadband measurement results with large errors. To perform uniform broadband measurements, a signal measurement procedure had to be developed and introduced. This new procedure, described here in detail, should be introduced as an international standard to solve long-existing measurement uncertainty problems, especially in the ultraviolet range. The new procedure can be applied for sources (like LEDs) and for detectors and radiometers. A simple version of the procedure can be utilized by using a flat-response pyroelectric detector (similarly to an illuminance measuring lux meter) to measure integrated irradiance, the radiometric equivalent of illuminance, from UV to IR. The calibration procedure of the optical radiation measuring detector-preamplifier (radiometer) is also discussed here. This book will help the reader to switch from single standard-based broadband calibrations to signal measurement procedure-based broadband calibrations to obtain uniform measurement results with low uncertainty.

Utilizing the new generation optical detectors and radiometers and applying the signal-measurement-based broadband radiometric measurement procedure makes it possible to determine the integrated irradiance from different UV and IR sources (like LEDs and IR background radiation), and to make radiance transfer to VIS and IR LEDs. As an example, the first time-determined integrated irradiance values (the radiometric equivalent of illuminance which is widely measured in photometry) from four UV LEDs as measured with a flat-response organic-black coated pyroelectric detector at a source-to-detector separation of 40 cm are shown in the Table below. The signal-range of the measured integrated irradiance values was close to five orders of magnitude. The manufacturers reported typical output flux values are shown in parenthesis. The 365-nm peak LED, with the 1 W typical output flux (in the last row of the Table), is a second-generation LED cluster that did not require a collimating lens (nor a beam-homogenizer) to produce the high, 48.6 mW/cm2 integrated irradiance with a +/- 0.5 % spatial nonuniformity within a -5 mm to +5 mm beam (target) spot.

Table: Integrated irradiance from four UV LED sources measured by a flat-response pyroelectric detector at a distance of 40 cm. The measurement uncertainties are less than 1 % (k=2).

The SI traceable broadband measurements (through National Measurement Institutes or the discussed intrinsic detector standards) were performed from UV to IR, using recently developed optical detector or radiometer standards. The uncertainty issues are also discussed in the book.

Linear and traceable measurement of detector DC output currents to the 0.01 % (k=2) uncertainty level, and AC photocurrent calibrations to the 0.04 % (k=2) level made it possible to decrease uncertainties in radiometric, photometric, tristimulus-color, and radiation thermometry measurements. The detector-amplifier gain-calibrations, and gain-linearity tests are also discussed in Book 5.


Dr George P. Eppeldauer retired from the US National Institute of Standards and Technology (NIST) in October 2019, having previously served as Project Leader of Detector Metrology. During his career, he developed standard optical radiometers, photometers, colorimeters, and radiation thermometers, and realized detector responsivity based scales. He was the lead-author and editor of the NIST Technical Notes #1438 and #1621, and he received the Gold Medal Award from the US Department of Commerce in 2010 for developing SIRCUS, the highest accuracy reference spectral-responsivity calibration facility of NIST. He is the author of 198 articles and book-chapters.


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