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Funding Information: The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This project, 19NRM02 RevStdLED, has received funding from the EMPIR programme co-financed by the participating counties and from the European Union’s Horizon 2020 research and innovation programme. VM acknowledges partial support by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167. The general V(λ) mismatch index f1′ specifies the mismatch between the relative spectral responsivity of a photometer, and the spectral luminous efficiency function for photopic vision, V(λ). A short review of its historical development explains the reasons for the current definition and which adjustments may be helpful in the future. The properties of the current definition are described in detail. It is very likely that photometers will be calibrated with a white light-emitting diode (LED) light source as the reference in the future. This might involve the need for a more appropriate definition of the general V(λ) mismatch index, either by using a different normalisation in f1′ for the relative spectral responsivity of the photometer or by introducing a different type of function for assessing the mismatch. On the other hand, the measurement of coloured LEDs is also becoming increasingly important. So, is a single general mismatch index for white and coloured light sources sufficient?

Funding Information: Academy of Finland (320167, 314364, 326444). Publisher Copyright: © 2022 OSA - The Optical Society. All rights reserved. We report the photoacoustic (PA) response in the terahertz (THz) range by employing a detection process actuated with a silicon cantilever pressure sensor and a carbon-based radiation absorber. The detection relies on the mechanical response of the cantilever, when the volume of the carrier gas inside the PA cell expands with the heat produced by the radiation absorber. The detector interferometrically monitors the movement of the cantilever sensor to generate the PA signal. We selected the absorber material with the highest THz responsivity for detailed studies at 1.4 THz (214 µm wavelength). The observed responsivities of two different radiation absorbers are nearly the same at 1.4 THz and agree within 10% with responsivity values at 0.633 µm wavelength. The results demonstrate the potential of covering with a single PA detector a broad spectral range with approximately constant responsivity, large dynamic range, and high damage threshold.

Funding Information: This project (18SIB10 chipS.CALe) has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. Authors of Aalto University acknowledge the support by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167. Publisher Copyright: © 2022 BIPM & IOP Publishing Ltd. We present a method to determine the internal quantum deficiency (IQD) of a predictable quantum efficient detector (PQED) based on measured photocurrent dependence on bias voltage and a 3D simulation model of charge carrier recombination losses. The simulation model of silicon photodiodes includes wafer doping concentration, fixed charge of SiO2 layer, bulk lifetime of charge carriers and surface recombination velocity as the fitted parameters. With only one set of physical photodiode defining parameters, the simulation shows excellent agreement with experimental data at power levels from 100 μW to 1000 μW with variation in illumination beam size. We could also predict the dependence of IQD on bias voltage at the wavelength of 476 nm using photodiode parameters determined independently at 647 nm wavelength. The fitted values of doping concentration and fixed charge extracted from the simulation model are in close agreement with the expected parameter values determined earlier. At bias voltages larger than 5 Vat the wavelength of 476 nm, the internal quantum efficiency of one of the tested PQEDs is measured to be 0.999 970 ± 0.000 027, where the relative expanded uncertainty of 0.000 027 is one of the lowest values ever achieved in spectral responsivity measurement of optical detectors.

Lisää oa-julkaisu, kun saatavilla. A sensitive photoacoustic detection approach employing a silicon cantilever is investigated for power measurement of electromagnetic radiation. The technique which is actuated by pressure waves generated through radiation-induced heat depicts high sensitivity for a considerably large spectral range from 325 nm to 1523 nm. The implemented method shows linear response in the measurement of radiation power from 15 nW to 6 mW demonstrating a dynamic range of almost six orders of magnitude. A numerical model has been developed to analyze and optimize the measurement sensitivity. The model allows studying different dimensions of the cantilever which is one of the key components of the radiation detection process. The numerical results are in good agreement with experimental results. The electromagnetic power detection technique shows future potential for industrial applications and scientific studies.

Funding Information: This work has been done in the frame of the Metrology for Earth Observation and Climate project (MetEOC-3), Grant No. 16ENV03 within the European Metrology Programme for Innovation and Research (EMPIR). EMPIR is jointly funded by the EMPIR participating countries within EURAMET and the European Union. The work is also supported by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167. Publisher Copyright: © 2022 BIPM & IOP Publishing Ltd. Present article describes a 3D gonioreflectometer developed at Aalto University for the measurement of spectral bidirectional reflectance distribution function (sBRDF) of horizontally aligned samples, needed to study sBRDF of sand from satellite calibration test sites. The new set-up incorporates three motorized arms that allow to measure in- and out-of-plane sBRDF. Illumination of the sample is provided by a supercontinuum laser coupled with laser line tunable filters and detection is performed with two types of photodiodes (Si and InGaAs) enabling collection of sBRDF in the spectral range from 450 nm to 1700 nm. The instrument is operated in the relative mode and its traceability to SI is achieved with the help of a 2D absolute reference gonioreflectometer available at Aalto University. The calibration procedure as well as system's uncertainty estimation are described. The performance of the 3D gonioreflectometer is validated by measurements of a ceramic diffuse reflectance sample.

The predictable quantum efficient detector (PQED) is a primary standard of optical power, which utilizes two custom-made induced-junction photodiodes that are mounted in a wedged trap configuration for the reduction of reflectance losses. PQED photodiodes of p-type and n-type were characterized for their dark current dependence on reverse bias voltage at room temperature. As simulations predict that the dark current will decrease exponentially with temperature, the temperature dependence of dark current for the n-type photodiodes was also measured. Two n-type induced-junction photodiodes were assembled inside a liquid nitrogen cryostat. The results from the dark current measurements indicate that the cooled n-type photodiodes are suitable for measuring optical fluxes in the few photon regime. A photon flux of approximately 7,000,000 photons per second was measured using the PQED at a cryogenic temperature with a relative standard uncertainty of 0.15%. The results support the utilization of the PQED as a primary standard of optical power in single and few photon applications.

Funding Information: The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Aalto University Energy Efficiency Research Program (AEF) and by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167. Publisher Copyright: © The Chartered Institution of Building Services Engineers 2021. Copyright: Copyright 2021 Elsevier B.V., All rights reserved. We report for the first time the effect of ageing due to adaptive control of lighting on the lifecycle costs of luminaire installations. Two types of LED street luminaire were aged (1) as the luminaire lit for 9 hours and then switched off for 3 hours and (2) under intensive ageing using adaptive control conditions mimicking presence of vehicles, where the luminaires were switched with 30-second interval between 100% and 20% power within the cycle of (1). The test continued for 32,000 hours. Luminaire lifetime was found to decrease by 10% due to the intensive adaptive control, which exaggerates the ageing as compared to real traffic conditions. We estimate that a realistic rate of dimming cycles would decrease the luminaire lifetime by 0–2%, increasing the lifecycle costs of a lighting installation by 0–0.7%. With a traffic-aware control system, capable of 50% energy saving in lighting, the lifecycle costs are estimated to reduce by 25% as compared to a similar LED luminaire installation without traffic-awarecontrol, outweighing the costs due to the reduced lifetime. The smart controlling of lighting is considered as a useful way to meet future energy saving targets.

Funding Information: This work has been done in the frame of the projects 16NRM08 BiRD and 18SIB03 BxDiff, that have received funding from the EMPIR programme co‐financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. The work has also been supported by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167. Publisher Copyright: © 2022 The Authors. Color Research and Application published by Wiley Periodicals LLC. Modern studies of bidirectional reflectance distribution function (BRDF) and its applications using data and machine-driven science require formatting of BRDF data according to Findable, accessible, interoperable and reusable (FAIR) data principles. As a solution a FAIR universal BRDF file-format based on Java Script Object Notation (JSON) is proposed. JSON principles as well as file structure are explained and examples are given. Automatic validation of universal BRDF file format is realized with the help of JSON schema. Furthermore, the source code and accompanying documentation are presented in dedicated supporting material files. It is expected that after its wide adoption, the proposed BRDF file format will enhance collaboration between different research groups and benefit machine-driven science. The uptake is facilitated by introducing a BiRDview—a modern open-source web-based application for BRDF visualization.

Funding Information: Hannu Ronkainen is acknowledged for useful discussions. We thank financial support from Business Finland co-innovation project RaPtor (Project No. 6030/31/2018). This work is partly funded by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), Decision No. 320167. Publisher Copyright: © 2021 IOP Publishing Ltd. We investigate the predictable quantum efficient detector (PQED) in the visible and near-infrared wavelength range. The PQED consists of two n-type induced junction photodiodes with Al2O3 entrance window. Measurements are performed at the wavelengths of 488 nm and 785 nm with incident power levels ranging from 100 μW to 1000 μW. A new way of presenting the normalized photocurrents on a logarithmic scale as a function of bias voltage reveals two distinct negative slope regions and allows direct comparison of charge carrier losses at different wavelengths. The comparison indicates mechanisms that can be understood on the basis of different penetration depths at different wavelengths (0.77 μm at 488 nm and 10.2 μm at 785 nm). The difference in the penetration depths leads also to larger difference in the charge-carrier losses at low bias voltages than at high voltages due to the voltage dependence of the depletion region.

A photoacoustic detection of tritiated water (HTO) is presented. The method uses cantilever-enhanced photoacoustic spectroscopy (CEPAS) to reach sub-ppb sensitivity for HTO in the gas phase. A noise equivalent concentration of 0.88 ppb is reached with a sampling time of 1 min. The high performance and small sample volume of CEPAS allows sensitive detection of HTO from a sample with low total activity.

Funding Information: Funding: This project (18SIB10 chipS.CALe) received funding from the EMPIR program co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation program. This research was funded, in part, by The Research Council of Norway (Tildeling av grunnbevilgning for 2021—prosjektnr. 194068/F40). A CC BY or equivalent license is applied to any author accepted manuscript (AAM) version arising from this submission, in accordance with the grant’s open access conditions. Authors of Aalto University acknowledge the funding by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167. Publisher Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. We performed a systematic study involving simulation and experimental techniques to develop induced-junction silicon photodetectors passivated with thermally grown SiO2 and plasma-enhanced chemical vapor deposited (PECVD) SiNx thin films that show a record high quantum efficiency. We investigated PECVD SiNx passivation and optimized the film deposition conditions to minimize the recombination losses at the silicon-dielectric interface as well as optical losses. Depositions with varied process parameters were carried out on test samples, followed by measurements of minority carrier lifetime, fixed charge density, and optical absorbance and reflectance. Subsequently, the surface recombination velocity, which is the limiting factor for internal quantum deficiency (IQD), was obtained for different film depositions via 2D simulations where the measured effective lifetime, fixed charge density, and substrate parameters were used as input. The quantum deficiency of induced-junction photodiodes that would be fabricated with a surface passivation of given characteristics was then estimated using improved 3D simulation models. A batch of induced-junction photodiodes was fabricated based on the passivation optimizations performed on test samples and predictions of simulations. Photodiodes passivated with PECVD SiNx film as well as with a stack of thermally grown SiO2 and PECVD SiNx films were fabricated. The photodiodes were assembled as light-trap detector with 7-reflections and their efficiency was tested with respect to a reference Predictable Quantum Efficient Detector (PQED) of known external quantum deficiency. The preliminary measurement results show that PQEDs based on our improved photodiodes passivated with stack of SiO2/SiNx have negligible quantum deficiencies with IQDs down to 1 ppm within 30 ppm measurement uncertainty.

Publisher Copyright: © 2021 BIPM & IOP Publishing Ltd. The characteristics of a predictable quantum efficient detector (PQED) in terms of optical non-linearity in the visible to near-infrared range were investigated under zero-bias and reverse-bias voltage conditions. In the zero-bias condition, linear behavior was observed in the wavelength from 405 nm to 1060 nm in the photocurrent range of 1 nA to 10 mu A, and saturation occurred for photocurrents over 10 mu A for all wavelengths. In the reverse-bias voltage of 10 V, the linear behavior was observed in the photocurrent range of 64 nA to 1 mA except for the wavelength of 1060 nm, and the saturation photocurrent increased up to 1 mA. The supralinearity value at 1060 nm increased sharply from the photocurrent of 100 mu A, and its maximum value reached up to 0.34% at the photocurrent of 1 mA because of the back surface recombination and the longer absorption length. The spectral linearity results of the PQED help us to understand the charge-carrier loss mechanism in the PQED, and would lead to more accurate optical measurement with it.

According to our experimental results, a nitrogen flow used to prevent dust and moisture entering a detector may influence measurements performed with trap detectors in overfilled conditions. A stable light source was measured with a wedged trap detector with 4 mm aperture, and the nitrogen flow rate was varied. The nitrogen flow was found to have the largest effect of up to 0.8% on the responsivity of the detector at around 1.0 l min−1 flow rate. The effect of nitrogen flow can be removed down to 0.02% by an added crossflow which removes the nitrogen out of the optical axis. In another experiment, the effect was removed almost completely by changing the flowing gas from nitrogen to synthetic dry air. We also present measurement results that indicate the responsivity changes with nitrogen to be smaller than 0.05% with underfilled beam geometry, even without the added crossflow. Based on simulations, the nitrogen flow through the detector forms a gradient-index type gas lens in front of the detector increasing the effective aperture area and thus the responsivity. In the underfilled measurement geometry there is no light close to the aperture edge which could be refracted inside the detector. Finally, we consider methods to ensure that the responsivity changes due to the gas flow remain below 0.05% in overfilled measurement geometry, without compromising the cleanliness of the detector with too small gas flow rate.

Funding Information: The work was supported by the Academy of Finland (Project Numbers 326444 and 314364) and by the Academy of Finland’s Flagship Programme, Photonics Research and Innovation (PREIN), Decision Number: 320167. Publisher Copyright: © 2021 The Authors. Published by American Chemical Society. A high-sensitivity light-emitting diode (LED)-based photoacoustic NO2 sensor is demonstrated. Sensitive photoacoustic gas sensors based on incoherent light sources are typically limited by background noise and drifts due to a strong signal generated by light absorbed at the photoacoustic cell walls. Here, we reach a sub-ppb detection limit and excellent stability using cantilever-enhanced photoacoustic detection and perform a two-channel relative measurement. A white-light LED is used as a light source, and the spectrum is divided into two wavelength channels with a dichroic filter. The photoacoustic signals generated by the two wavelength channels are measured simultaneously and used to solve the NO2 concentration. The background signal is highly correlated between the two channels, and its variations are suppressed in the relative measurement. A noise level below 1 ppb is reached with an averaging time of 70 s. This is, to the best of our knowledge, the first time a sub-ppb detection limit is demonstrated with an LED-based photoacoustic NO2 sensor. As LEDs are available at a wide selection of emission wavelengths, the results show great potential for development of cost-effective and sensitive detectors for a variety of other trace gasses as well.

Funding Information: The work was funded by the Academy of Finland (Project numbers 326444 and 314364) and by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167 . This work made use of Tampere Microscopy Center facilities at Tampere University. Publisher Copyright: © 2021 The Author(s) We present an experimental comparison of photoacoustic responsivities of common highly absorbing carbon-based materials. The comparison was carried out with parameters relevant for photoacoustic power detectors and Fourier-transform infrared (FTIR) spectroscopy: we covered a broad wavelength range from the visible red to far infrared (633 nm to 25 μm) and the regime of low acoustic frequencies (< 1 kHz). The investigated materials include a candle soot-based coating, a black paint coating and two different carbon nanotube coatings. Of these, the low-cost soot absorber produced clearly the highest photoacoustic response over the entire measurement range.

Photoacoustic detection is a sensitive method for measurement of light-absorbing particles directly in the aerosol phase. In this article, we demonstrate a new sensitive technique for photoacoustic aerosol absorption measurements using a cantilever microphone for the detection of the photoacoustic signal. Compared to conventional diaphragm microphones, a cantilever offers increased sensitivity by up to two orders of magnitude. Here we reached a noise level of 0.013 Mm$^{-1}$ (one standard deviation) with a sampling time of 20 s, using a simple single-pass design without a need for a resonant acoustic cell. We demonstrate the method in measurements of size-selected nigrosin particles and ambient black carbon. Due to the exceptional sensitivity, the technique shows great potential for applications where low detection limits are required, for example size-selected absorption measurements and black carbon detection in ultra clean environments., Comment: 18 pages, 6 figures

We have developed a setup for measuring differential spectral responsivities of unifacial and bifacial solar cells under bias light conditions. The setup uses 30 high-brightness LEDs for generating a quasi-monochromatic light source covering the wavelength range 290–1300 nm. Halogen lamps are used to generate bias-lighting conditions up to the irradiance level of 1000 W/m2. The setup has been fully characterized for spectral irradiances and spatial uniformities of all light sources. Validation measurements carried out using a reference cell of 2 × 2 cm2 area from Fraunhofer ISE demonstrated an agreement better than 2% over the wavelength range of 425–940 nm, with an expanded uncertainty of 2.6%. In the UV and IR regions, the discrepancies are higher but still within estimated uncertainties. The setup is also tested in measuring spectral responsivities of large 15 × 15 cm2 bifacial solar cells. The associated problems are discussed.

This paper presents the spectral responsivity calibrations of two indium gallium arsenide (InGaAs) and one germanium based near-infrared photovoltaic detectors using a wavelength tunable laser source based on a supercontinuum laser developed at the Metrology Research Institute, Aalto University. The setup consists of a supercontinuum laser based on a photonic crystal fiber as the light source, a laser line tunable filter, and coupling optics. These responsivity calibrations are performed against a pyroelectric radiometer over a wide spectral range of 800–2000 nm. Our wavelength tunable laser source has a high spectral power up to 2.5 mW with a narrow spectral full-width-at-half-maximum of 3 nm at a wavelength of 1100 nm. Despite the sharp spectral intensity variations, no artifacts are observed in the spectral responsivities of the detectors. Comparison of the spectral responsivities of the InGaAs detectors measured using the wavelength tunable laser and the earlier calibrations performed at the Metrology Research Institute in 2010 and 2016, shows that the higher spectral power of wavelength tunable light source decreases the expanded uncertainty from approximately 4% to 2.2–2.6% over the spectral range of 820–1600 nm. Temperature dependence of the spectral responsivities near the band gap edges are also measured and analysed.

Foils made of different materials are often used as reflective insulators. Many manufacturers aim to accurately measure their optical properties to estimate and improve their performance. However, flat reflectance reference samples used in measurements do not correctly represent reflective insulators and cause discrepancies between different measurement techniques. The current work presents a method for modelling surface shape of appropriate reflectance reference samples that could be produced by additive manufacturing. The method is based on studying the reflection distribution of reflective insulators and is described with an example of aluminium foil. The method's performance is validated using Monte-Carlo simulations.

We introduce a cryostat setup for measuring fundamental optical and electrical properties of light-emitting diodes (LEDs). With the setup, the cryostat pressure and the LED properties of the forward voltage, junction temperature, and electroluminescence spectrum are monitored with temperature steps less than 1.5 K, over the junction temperature range of 81-297 K. We applied the setup to commercial yellow AlGaInP and blue InGaN LEDs. At cryogenic temperatures, the fine structure of the electroluminescence spectra became resolved. For the yellow LED, we observed the phonon replica at 2.094 eV that was located 87 meV below the peak energy at the junction temperature of 81 K. For the blue LED, we observed the cascade phonon replicas at 2.599 eV, 2.510 eV, and 2.422 eV with the energy interval of 89 meV. For both LED types, the forward voltage increased sharply toward the lower temperatures due to the increased resistivity of materials in the LED components. We found significant differences between the temperaturedependent behaviors of the forward voltages, spectral peak energies, and bandgap energies of LEDs obtained from the Varshni formula. We also noted a sharp pressure peak at 180-185 K arising from the solid-vapor phase transition of water when the base level of the cryostat pressure was approximately 0.4 mPa.

37th European Photovoltaic Solar Energy Conference and Exhibition; 811-815, The IEC 61853 standard series “Photovoltaic (PV) module performance testing and energy rating” aims to provide a standardized measure for PV module performance, namely the Climate Specific Energy Rating (CSER). An algorithm to calculate CSER is specified in part 3 based on laboratory measurements defined in parts 1 and 2 as well as the climate data set given in part 4. To test the comparability and clarity of the algorithm in part 3, we share the same input data, obtained by measuring a standard photovoltaic module, among different research organizations. Each participant then uses their individual implementations of the algorithm to calculate the resulting CSER values. The initial blind comparison reveals differences of 0.133 (14.7%) in CSER between the ten different implementations of the algorithm. Despite the differences in CSER, an analysis of intermediate results revealed differences of less than 1% at each step of the calculation chain among at least three participants. Thereby, we identify the extrapolation of the power table, the handling of the differences in the wavelength bands between measurement and climate data set, and several coding errors as the three biggest sources for the differences. After discussing the results and comparing different approaches, all participants rework their implementations individually and compare the results two more times. In the third intercomparison, the differences are less than 0.029 (3.2%) in CSER. When excluding the remaining three outliers, the largest absolute difference between the other seven participants is 0.0037 (0.38%). Based on our findings we identified four recommendations for improvement of the standard series.

Hyperspectral cameras capture the spectral power distribution of the objects in the imaged view via dozens of narrow-band spectral channels of the camera. Knowledge of the spectral responsivity of the channels is essential when interpreting the acquired hyperspectral data and assessing its reliability. The spectral responsivity of the camera channels may vary within the image area. This paper presents a measurement setup and data analysis routine for characterising the spectral responsivity of a hyperspectral camera. This method was used to characterise the spectral responsivity of a Fabry-Perot-interferometer-based hyperspectral camera. The characterisation method implemented in this study was able to reveal several channel leaks in the measured wavelength range. In the image area there is an approximately 1.5 nm shift in the channel wavelengths, and up to 10% variation in the channel bandwidths. The expanded uncertainties (k = 2) for the measured channel bandwidths, sensitivities and wavelengths were 7.9%, 9.5% and 0.64 nm, respectively.

This paper presents a method for characterising spatial responsivity of hyperspectral cameras. Knowing the responsivity of the camera as a function of pixel coordinates allows applying a flat-field correction on image data. The method is based on scanning the field of view of the camera with a broadband radiance source, based on an integrating sphere, and combining the captured frames to synthesise a uniform radiance source filling the whole field of view of the camera at the focus distance. The method was compared with a traditional approach where the aperture of an integrating sphere is imaged from a close distance, filling the entire field of view of the camera. The measurement setup was tested with a hyperspectral camera, based on a tunable Fabry–Pérot interferometer. Without the flat-field correction, the average standard deviation of the pixel responsivities across all the spectral channels of the camera was 3.78%. After the correction, the average standard deviation was reduced to 0.40% and 3.25% for the aperture-scanning method and the close-distance method, respectively. The expanded uncertainty (k = 2) for the flat-field correction obtained using the scanning method was 0.68%–0.78%, depending on the spectral channel of the camera.

10 pags., 8 figs., 4 tabs. -- Open Access funded by Creative Commons Atribution Licence 3.0, In this paper, the fisheye camera method is validated for spatial non-uniformity corrections in luminous flux measurements with integrating spheres. The method was tested in eight integrating spheres with six LED lamps, and the determined angular intensity distributions and spatial non-uniformity correction factors were compared with the results of five goniophotometers. The average closeness score, describing the similarity between any two distributions, was 94.6 out of 100 for the distributions obtained using the fisheye camera method when compared with the goniophotometric results. The average closeness score for the five goniophotometers, when each goniophotometer was compared with the other four, was 96.6. On average, the relative deviation between the two methods was 0.05% when calculating the spatial corrections. The most significant sources of uncertainty for the fisheye camera method were large, view-obstructing sphere elements residing close to the camera port., The work leading to this study is partly funded by the European Metrology Programme for Innovation and Research (EMPIR) Project 15SIB07 PhotoLED ‘Future Photometry Based on Solid State Lighting Products’. The EMPIR initiative is cofunded by the European Union’s Horizon 2020 research and innovation programme and the EMPIR Participating States. Acknowledges the funded position of Aalto ELEC doctoral school and the support of the Tutkijat Maailmalle program.

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We present a simple method for fully-stabilized mid-infrared optical frequency comb generation based on single-pass femtosecond optical parametric generation that is seeded by a continuous-wave laser. We have implemented the method in a periodically poled lithium niobate crystal that produces a frequency comb tunable across 3325 - 4000 nm (2380 - 3030 cm-1). The method generates the mid-infrared (idler) comb with known and stabilized Carrier-Envelope Offset (CEO) frequency without the need to directly detect it. The idler CEO is continuously tunable for almost half of the repetition rate and can be modulated, while maintaining its central frequency. Together with the high output power (up to 700 mW) and low intensity noise (0.018% integrated in 10 Hz - 2 MHz bandwidth) this makes the demonstrated mid-infrared frequency comb promising for many applications such as high-precision molecular spectroscopy, frequency metrology and high harmonic generation., Comment: Corrected typos, added references

We have developed a spectral model for describing the shape of the emission spectrum of InGaAlP-based red light-emitting diodes (LEDs) with quantum-well structure. The model is based on Maxwell-Boltzmann distribution with junction temperature Tj and an experimental two-dimensional joint density of states (DOS). We model the DOS with a sum of two exponentially broadened step functions describing the two lowest sub-bands in semiconductor quantum well. The relative locations ΔE1 = 0 meV and ΔE2 = 112.7 meV above the band gap energy Eg = 1.983 eV and the ratio 2.13 of the step heights were fixed using an experimental DOS extracted from a LED spectrum measured at known Tj and driving current I. The model can then be fitted to other spectra of other LED samples at varied Tj and I by varying the fitting parameters Eg, Tj, and the broadening of the sub-band edges. The model was tested for three LED samples over I = 200–370 mA and Tj = 303–398 K. Junction temperatures obtained by modeling were compared with calibrated Tj obtained by the forward voltage method. The mean absolute difference was about 2.9 K (0.8%) over the whole region studied and the maximum difference was 8.5 K. The thermal coefficient measured for Eg was −0.509 meV K−1. For the first and second sub-band edges, the thermal broadening coefficients were 18 μeV K−1 and 37 μeV K−1, respectively

The Predictable Quantum Efficient Detector (PQED) is evaluated as a new primary standard of optical power. Design and characterization results are presented for a new compact room temperature PQED that consists of two custom-made induced junction photodiodes mounted in a wedged trap configuration. The detector assembly includes a window aligned in Brewster angle in front of the photodiodes for high transmission of p polarized light. The detector can also be operated without the window, in which case a dry nitrogen flow system is utilized to prevent dust contamination of the photodiodes. Measurements of individual detectors at the wavelength of 488 nm indicate that reflectance and internal quantum efficiency are consistent within 14 ppm and 10 ppm (ppm = part per million), respectively, and agree with the predicted values. The measured photocurrent ratio of the two photodiodes confirms the predicted value for s and p polarized light, and the spatial variation in the photocurrent ratio can be used to estimate the uniformity in the thickness of the silicon dioxide layer on the surface of the photodiodes. In addition, the spatial non-uniformity of the responsivity of the PQED is an order of magnitude lower than that of single photodiodes. Such data provide evidence that the room temperature PQED may replace the cryogenic radiometer as a primary standard of optical power in the visible wavelength range.

Polystyrene and many other materials turn yellow when exposed to ultraviolet (UV) radiation. All photodegradation mechanisms including photoyellowing are functions of the exposure wavelength, which can be described with an action spectrum. In this work, a new high-resolution transmittance measurement setup based on lasers has been developed for measuring color changes, such as the photoyellowing of translucent materials aged with a spectrograph. The measurement setup includes 14 power-stabilized laser lines between 325 nm and 933 nm wavelengths, of which one at a time is directed on to the aged sample. The power transmitted through the sample is measured with a silicon detector utilizing an integrating sphere. The sample is mounted on a high-resolution XY translation stage. Measurement at various locations aged with different wavelengths of exposure radiation gives the transmittance data required for acquiring the action spectrum. The combination of a UV spectrograph and the new high-resolution transmittance measurement setup enables a novel method for studying the UV-induced ageing of translucent materials with a spectral resolution of 3-8 nm, limited by the adjustable spectral bandwidth range of the spectrograph. These achievements form a significant improvement over earlier methods.

openaire: EC/H2020/771049/EU//FREEDLES Leaf reflectance and transmittance spectra are essential information in many applications such as developing remote sensing methods, computing shortwave energy balance (albedo) of forest canopies, and monitoring health or stress of trees. Measurement of coniferous needle spectra has usually been carried out with single integrating spheres, which has involved a lot of tedious manual work. A small double integrating sphere would make the measurements considerably faster, because of its ease of operation and small sample sizes required. Here we applied a compact double integrating sphere setup, used previously for measurement of broad leaves, for measurement of coniferous needles. Test measurements with the double integrating sphere showed relative underestimation of needle albedo by 5–39% compared to a well-established single integrating sphere setup. A small part of the bias can be explained by the bias of the single sphere. Yet the observed bias is quite significant if absolute accuracy of measurements is required. For relative measurements, e.g. for monitoring development of needle spectra over time, the double sphere system provides notable improvement. Furthermore, it might be possible to reduce the bias by building an optimized measurement setup that minimizes absorption losses in the sample port. Our study indicates that double spheres, after some technical improvement, may provide a new and fast way to collect extensive spectral libraries of tree species.