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.

An optical method for calibration of the aperture area is described and studied both theoretically and experimentally. A spatially uniform, known irradiance is formed over the aperture by overlapping identical, parallel laser beams centred at constant spacing in an orthogonal lattice. The ratio of the throughput power and irradiance gives the area of the aperture. The method has several advantages compared with previous methods: it measures the area of the aperture directly, the shape of the aperture is not limited to a circle, it is relatively inexpensive to establish, it does not damage the edges of the aperture and the calibration set-up is similar to that for the actual use of the aperture. It is estimated that the relative standard uncertainty is 1.6 × 10 −4 in calibration of a circular 3 mm diameter aperture. The results that the present method gave for one aperture have been compared with the result of a mechanical calibration at the National Physical Laboratory (UK). The relative difference between the results was 2.4 × 10 −4 , with a combined standard uncertainty of 2.5 × 10 −4 .

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.

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 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 1 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 .

A description of a detector-based realization of the unit of luminous flux (lumen) at the Helsinki University of Technology (HUT) is presented. The realization is based on the absolute integrating-sphere method developed at the National Institute of Standards and Technology (NIST), with some modifications. The measurement setup consists of a 1.65 m integrating sphere, two photometers, a precision aperture and an external luminous-flux source. The characterization and maintenance of the measurement system are described and the uncertainty budget of the realization is presented. The uncertainty analysis indicates a relative expanded uncertainty (k = 2) of 4.7 × 10 −3 for the realization. According to the results of an earlier bilateral comparison between the HUT and the NIST, the ratio of the measured luminous flux value of HUT to that of NIST was 1.0006 with an expanded uncertainty (k = 2) of 10 × 10 −3 , including uncertainties due to realization of the units. Another indirect test measurement indicated a corresponding ratio of 0.9984 with the luminous flux measurements of BIPM with an expanded uncertainty (k = 2) of 11 × 10 −3 , including uncertainties due to realization of the units.