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 (LLTFs) 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.

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.

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.

We present a practical implementation of the wavelength-modulation spectroscopy technique with second-harmonic detection for selective detection and concentration measurements of polycyclic aromatic hydrocarbons (PAHs)-common fossil fuel pollutants. The method is based on excitation light wavelength modulation around the maximum of a narrow peak in the absorption spectrum of a PAH and lock-in detection of fluorescence at the second harmonic of the excitation wavelength modulation frequency. A violet LED is used as the excitation source and wavelength modulation is performed via narrowband interference filter tilting. Feasibility is demonstrated with a carcinogenic PAH, benzo(k)fluoranthene. Model computations are presented to estimate the influence of the background signal on the sensitivity of the method.

We present a new method for differential temperature measurement based on thermal sensitivity of the fluorescence lifetime of thermographic phosphors (TP). Pairs of thermographic phosphors are excited with intensity modulated light at frequencies ω and ω + Δω. The phase shift Δθ of the summary fluorescence intensity beat signal envelope is measured. A prototype of fluorometric differential temperature sensor is developed and feasibility of the method is experimentally demonstrated with a Sm2+:SrFCl crystal and the 5D1–>7F0 transition for high thermal sensitivity. The observed linear dependence between envelope phase shift Δθ and temperature difference ΔT agrees with the theoretical prediction. Sensitivity of S = −0.97°/°C was achieved. This method could also be applied to differential measurements of any parameter affecting fluorescence lifetime.