Farshid Manoocheri

Gonioreflectometer for measuring spectral diffuse reflectance

Gonioreflectometric determination of reflectance factors that involves hemispherical collection of reflected flux, which is an alternative to integrating sphere-based methods, is discussed. A detailed description of a gonioreflectometer built at the Helsinki University of Technology is presented. The instrument is used to establish an absolute scale of total diffuse reflectance factors throughout the spectral range 360-830 nm. The hemispherical reflectance factors are obtained through integration of the gonioreflectometric measurement results. The reflectance factors of white high-quality artifacts can be determined with a combined standard uncertainty of 0.20%. Results of test measurements were found to be in agreement with values traceable to other absolute scales based on integrating-sphere methods.

Characterization of filter radiometers with a wavelength-tunable laser source

Results of filter radiometer characterization with a wavelength-tunable Ti : sapphire laser in the wavelength band around 900 nm are presented. The effect of interference between the reflections from filter surfaces in the case of coherent laser light was studied and reduced with a special filter design with antireflection coatings. Measuring the responsivity as a function of wavelength over a very narrow band was used to reveal the remaining interference effects. Uncertainty analysis and test results indicate that filter radiometers can be characterized with a relative standard uncertainty of 9×10−4 using the scanning method. The results agree well with more conventional monochromator-based measurements.

Characterization of thin films based on reflectance and transmittance measurements at oblique angles of incidence

The optical parameters of a SiO2 thin-film coating determined from the spectral reflectance and transmittance measurements at various incidence angles, including the normal incidence and the Brewsters angle, are compared in this paper. The high-accuracy measurements were carried out through visible-near-infrared spectral regions by using our purpose-built instruments. The optical parameters obtained from the reflectance and the transmittance data are consistent over the angles of incidence and agree within 0.2%. The effect of important systematic factors in the oblique-incidence spectrophotometric measurements is also discussed.

Intrinsic Wavelength Standard Absorption Bands in Holmium Oxide Solution for UV/visible Molecular Absorption Spectrophotometry

The transmittance minima of 18 absorption bands of a solution of 40 g/L holmium oxide in 10% (volume fraction) perchloric acid are certified as intrinsic traceable wavelength standards, by means of a multicenter measurement on material from a single source coupled with comparisons of a variety of preparations of the material evaluated on a single instrument. Fit-for-purpose artifact standards for the experimental calibration or validation of wavelength scales of chemical spectrophotometers can be carefully produced by end users themselves or by commercial standards producers. The intrinsic (data) standard confers traceability to the SI unit of length in place of costly transfer artifacts and repetitive calibration procedures. Certified values are provided for instrumental spectral bandwidths of 0.1–3.0 nm in 0.1 nm intervals, and information values are provided to a spectral bandwidth of 10 nm at wider intervals. Expanded uncertainties are typically less than ±0.1u2009nm for certified band positions.

Determination of thin-film parameters from high accuracy measurements of spectral regular transmittance

The importance of reliable spectral transmittance measurements for reliable thin-film characterization is demonstrated. By using a model of the spectral transmittance of thin-film samples, the effect of various uncertainty components in transmittance measurements on the determined thin-film parameters is analysed. The experimental results for aluminium dioxide and tantalum pentoxide thin-film samples show good agreement with the model.

Goniofluorometer for characterization of fluorescent materials

A goniofluorometer has been built that is capable of measuring in various viewing angles ranging from 10 degrees to 90 degrees . The incident angle can be varied from 0 degrees to 8 degrees . The goniofluorometer can measure bispectral luminescent radiance factors in the wavelength range of 250-800 nm. To our knowledge, there are no other reported results of similar devices capable of spectral measurements in various measurement geometries.

Comparison measurements of 0∶45 radiance factor and goniometrically determined diffuse reflectance

A comparison between the absolute gonioreflectometric scales at the Helsinki University of Technology (TKK) and the Physikalisch-Technische Bundesanstalt (PTB) has been accomplished. Six different reflection standards were measured for their 0:45 spectral radiance factor between 250 and 1650 nm in 10 nm intervals. Also, the 0:d reflectance factor between 400 and 1600 nm in 100 nm intervals was determined from goniometric reflectance measurements over polar angles with subsequent integration within the hemisphere above the sample. Goniometric comparisons covering such an extensive wavelength range and also several different sample materials are rarely implemented. For all but one sample, the difference between the results obtained at the TKK and the PTB was, with the exception of a couple of measurement points, within the expanded uncertainty (k = 2) of the comparison at least up to a wavelength of 1400 nm. All differences between the measurement results can be understood, except for one translucent sample in the visible wavelength range. The effect of sample translucency was found to be significant in the NIR wavelength region. Also, a general tendency of an increase of the TKK values relative to the PTB values in the UV region was observed. Possible causes for this phenomenon are discussed.

Characterization of germanium photodiodes and trap detector

We have developed and characterized new detectors based on germanium (Ge) photodiodes to be used in the wavelength region between 900 and 1650 nm. The effects of spatial uniformity, temperature and low shunt resistance on the spectral responsivity measurements are studied. Our results for the spatial uniformities show improvements as compared with earlier studies. The spectral reflectances of a Ge photodiode and trap detector are also studied, and the trap reflectance is found to be less than 10−4 at wavelengths longer than 900 nm. The results of this study show that with careful use, the large area Ge photodiodes can offer a cost-effective alternative as compared with InGaAs photodiodes of similar diameters.

Non-Lambertian behaviour of fluorescence emission from solid amorphous material

The commonly made assumption of Lambertian behaviour of fluorescence emission has been studied in solid amorphous material. A goniofluorometer capable of measurements in measurement geometries from 0 : 10 to 0 : 85 has been used for this purpose. The measurements indicate that the fluorescence emission from solid amorphous material is clearly non-Lambertian.

Uncertainty analysis of photometer quality factor f_1

The applicability of biased and random error models for determining the uncertainty of photometer quality factor using Monte Carlo simulations is investigated. In the biased error model the contribution to the measured value has the same sign and almost the same magnitude at neighbouring wavelengths, whereas in the random error model contributions of varying signs appear at the neighbouring wavelengths. Both error models are used with real spectral responsivity data and uncertainty budgets of two photometers. Analytical considerations supported by simulation results show that the random error model alone may easily underestimate the uncertainty of . The biased error model is recommended as the basis of the uncertainty evaluation of . It is also demonstrated by real photometer data that the uncertainty of can be highly sensitive to fine details of the photometer spectral responsivity when the biased error model is used.