Pasi Toivanen

A new optical method for high-accuracy determination of aperture area

We have developed a direct optical method for aperture area measurement, based on constant, known irradiance over the aperture. The method is described and results of test measurements are given, including wavelength dependence of the effective aperture area. Systematic uncertainty components are studied experimentally to reach a relative uncertainty of 10-4 for aperture sizes down to 3 mm in diameter. The most important advantages of the direct optical method are that it does not require any reference aperture, it can be applied to apertures of any shape, and the calibration setup is similar to the actual use of the apertures in filter radiometers.

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.

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.

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%.

Realizations of the units of luminance and spectral radiance at the HUT

Realizations of the units of luminance and spectral radiance at the Helsinki University of Technology (HUT) are presented. These realizations are linked to HUT units of luminous intensity and spectral irradiance using a characterized photometer, a spectroradiometer and an integrating-sphere light source. A new method for determining the spatial uniformity of the output of the integrating-sphere source is described. The uncertainty analysis indicates a relative expanded uncertainty of 3.6 × 10−3 (coverage factor k = 2) for the realization of the unit of luminance. The expanded uncertainty for the realization of the unit of spectral radiance varies between 6 × 10−3 and 2.5 × 10−2 in the wavelength region 360 nm to 830 nm.

Method for characterization of filter radiometers

We have developed a new method for characterizing the irradiance responsivity of filter radiometers. The method is based on a spatially uniform, known irradiance, generated by combining several identical laser beams. The measurement setup and the experimental demonstration at one wavelength are presented. The diffraction correction related to the generated irradiance is studied experimentally. The uncertainty analysis of the method indicates a relative standard uncertainty of 1 x 10(-3). The results with the new method are compared with the characterization measurements based on our present spectral-irradiance scale. The results have a relative deviation of 1 x 10(-3), which is well within the combined standard uncertainty of the comparison.

Realization of the luminous-flux unit using a LED scanner for the absolute integrating-sphere method

In the absolute integrating-sphere method, the total luminous flux of a lamp inside an integrating sphere is determined by comparing it with a known flux introduced into the sphere from an external light source. As the measurement geometry of the lamps to be compared is different, the spatial non-uniformity of the sphere surface may affect the results. In order to evaluate this effect, the spatial response must be measured. Miniature incandescent lamps have been used as scanning-beam sources in previous realizations, but these lamps are not widely available. In the present realization of the luminous-flux unit by the Helsinki University of Technology (HUT), light-emitting diodes (LEDs) were used as the light source in scanning the spatial response. Preliminary results confirm the applicability of the LED scanner and indicate moderate deviations of about 1 % from earlier luminous-flux calibrations.

Portable detector‐based primary scale of spectral irradiance

A method developed at the Helsinki University of Technology (HUT) for calibrating standard lamps of spectral irradiance in UV-A and UV-B regions is presented. The method is based on a compact filter radiometer that is characterized absolutely to measure spectral irradiance. The filter radiometer can be brought to laboratories where accurate calibrations are needed. The method overcomes some of the instability problems encountered when using lamps as transfer standards, which makes it also useful in intercomparison campaigns. Test measurements are presented where two standard lamps issued by the National Institute of Standards and Technology (NIST), USA, are compared with the HUT scale held by the radiometer. The results suggest that the agreement between the two scales (HUT and NIST) is approximately 1%-2% in the wavelength region from 300 nm to 400 nm.