Farshid Manoochehri

High-accuracy spectrometer for measurement of regular spectral transmittance

A high-accuracy spectrometer has been developed for measuring regular spectral transmittance. The spectrometer is an automated, single-beam instrument that is based on a grating monochromator, reflecting optics, and an averaging sphere detector unit with a silicon photodiode. The uncertainties related to wavelength calibration, detector nonlinearity, system instability, beam displacement, polarization, stray light, interreflections, and beam uniformity are determined for the visible spectral range from 380 to 780 nm. A total uncertainty of 3 × 10(-4) (1σ) is estimated for transmittance measurements of homogeneous neutral-density filters. The uncertainty of the wavelength scale is 0.06 nm. As a specific application, calibration of V(λ)-correction filters is studied. To verify the accuracy of the transmittance measurements, a comparison of the measured and predicted transmittances of a sample of high-purity fused silica is made, revealing agreement at the 5 × 10(-4) level.

Development of a detector-based absolute spectral irradiance scale in the 380–900-nm spectral range

A detector-based absolute scale for spectral irradiance in the 380-900-nm wavelength region has been developed and tested at the Helsinki University of Technology (HUT). Derivation of the scale and its use for photometric and colorimetric measurements are described. A thorough characterization of a filter radiometer, constructed from a reflection trap detector, a precision aperture, and a set of seven temperature-controlled bandpass filters, is presented. A detailed uncertainty analysis of the scale indicates a relative standard uncertainty of approximately 0.2% throughout most of the wavelength region. The standard uncertainties obtained in measurements of correlated color temperature and luminous intensity of three Osram Wi41/G tungsten-halogen lamps are 2 K and 0.3%, respectively. The spectral irradiance scale is compared with the HUT luminous intensity scale. The agreement of the results at the 0.1% level is well within the combined standard uncertainty of the two scales.

Realization of the unit of luminous intensity at the HUT

A description is presented of an upgraded trap-detector-based realization of the units of luminous intensity (candela) and illuminance (lux) at the Helsinki University of Technology (HUT). The realization is accomplished using a reference photometer, a light source and a distance-measurement system. A thorough characterization is presented of the reference photometer, consisting of a reflection trap detector, a temperature-controlled V(λ) filter and a high-precision aperture. The maintenance of the units is described. An updated uncertainty budget of the realization is given. Two of the three main uncertainty components of our earlier realizations have been significantly decreased. The uncertainty analysis indicates a relative expanded uncertainty of 2.2 × 10-3 for the realization of the candela and 1.8 × 10-3 for that of the lux. The HUT has participated in three international measurement comparisons, whose results are reviewed. According to the results, the HUT candela deviates by +4.0 × 10-3 from the candela of the Swedish National Testing and Research Institute with an expanded uncertainty of 10-2, -2.7 × 10-3 from that of the National Physical Laboratory (UK) with an expanded uncertainty of 5.6 × 10-3 and -3.3 × 10-3 from the world mean with an expanded uncertainty of 5.9 × 10-3.

Characterisation of optical detectors using high-accuracy instruments

The facilities of the Metrology Research Institute at the Helsinki University of Technology, and methods for characterisation of optical detectors for spectral radiant intensity and irradiance responsivity, are described. The instrumentation for such characterisations includes a reference spectrometer with a number of auxiliary set-ups, and equipment for the spectral irradiance measurements with a filter radiometer based on a trap detector. The methods of realising the spectral responsivity scales based on an absolute cryogenic radiometer in house are addressed. The procedures and results of characterisation of a multipoint measuring system of photosynthetically active radiation, by employing the available facilities, are briefly described. The absolute irradiance responsivity of the device is determined by using a photometric lamp, whose spectral irradiance has been measured with the filter radiometer. The combined standard uncertainty of this set of calibrations is 3.6% at the 1σ level. The uncertainty is caused almost completely by the multipoint measuring system.

Optical power and transmittance measurements and their use in detector-based realization of the luminous intensity scale

Responsivity calibrations of trap detectors with a cryogenic absolute radiometer and a power-stabilized 543.51 6-nm He-Ne laser are described. The trap detectors are used as transfer standards of optical power at visible wavelengths because they have a predictable spectral responsivity. New results on transmittance measurements of PRC Krochmann V(λ) filters are presented, especially the angle and temperature dependence of the transmittance. The characterized trap detector and the V(λ) filter are used for a novel realization of the SI base unit of luminous intensity, the candela. Test measurements indicate a good agreement with previous photometric scales.

Filter radiometry based on direct utilization of trap detectors

A new design for a filter radiometer based on a trap detector, a set of temperature-controlled filters, and a precision aperture is described. This filter radiometer can be used to realize high-accuracy scales for illuminance, luminous intensity, and spectral irradiance. The new filter radiometer is an improved version of our earlier designs. It has been improved in such a way that the filter can be easily and reliably changed. This makes it more suitable for spectral irradiance measurements, where lamps usually have to be measured at several wavelengths. The results of the characterization of the filter radiometer equipped with a lambda filter using two different methods are presented. The results are in agreement at the level of 0.3%.

High-accuracy measurement of specular spectral reflectance and transmittance

The realization of spectrophotometric quantities at the Helsinki University of Technology is based on our reference spectrometer. The reference spectrometer is a high-accuracy instrument developed for measuring spectral specular transmittance and reflectance in a wavelength range extending from ultraviolet to near-infrared. The relative uncertainty estimates for transmittance measurements of neutral-density filters are ca. 0.05%. For spectral reflectance the estimated uncertainties are between 0.14% and 0.34% depending on the sample reflectance and the measurement geometry. We have derived and verified equations that enable both the reflectance and transmittance of various samples to be predicted. Utilizing these equations, the reflectance and transmittance can be accurately calculated for samples with known refractive index. For precise calculations, the characteristics of the measurement beam must be taken into account.

Precision spectrometer for measurement of specular reflectance

The HUT reference spectrometer was modified for measuring specular reflectance in the wavelength range of 300 to 850 nm. The instrument is based on a diffraction-grating monochromator, reflecting optics, sample control mechanics and detection systems with linear responsitivities. Relative standard uncertainties between 0.14% and 0.32% were estimated for the reflectance measurements. For spectral reflectance between 1.5% and 15%, the results of test measurements using samples with known reflectances confirm that for all geometries the relative deviations are less than 0.36%. A set of ultraviolet (UV)-interference filters was measured in the UVB wavelength range, and the results are used as a part of filter radiometer characterization.

Characterization of a multipoint measuring system for photosynthetically active radiation

Abstract The principles for characterization of a multipoint measuring system of photosynthetically active radiation are described. The absolute irradiance responsivity, at a wavelength of 550 nm, and the relative spectral responsivity of the device are determined so that a calibration factor can be provided to convert the readings of the device to the units of photosynthetically active radiation. A high-accuracy spectrometer has been utilized for measuring the relative spectral responsivity of the system sensor. A reflection trap detector whose spectral responsivity is traceable to a cryogenic radiometer has been used as the reference detector. The absolute irradiance responsivity of the device has been determined by using a lamp with known spectral irradiance. The combined standard uncertainty of this set of calibrations is 3.6% at 1[sgrave] level. The uncertainty is caused almost completely by the multipoint measuring system.