Jari Hovila

Method for analysing luminous intensity of light-emitting diodes

Light-emitting diodes (LEDs) have built-in lenses that enable spatially limited light beams. The use of lenses increases the luminous intensity levels, but complicates accurate LED intensity measurements. A novel method for determining the luminous intensity of the LED is proposed. The method based on a modified inverse-square law describes the behaviour of an LED in terms of its luminous intensity, the radius of the virtual source and the location of the virtual source. The applicability of the method was tested for 17 LED types with different packages, angular intensity distributions and power levels. When applying the new method to the measurement data, instead of the inverse-square law of the point source, the distance dependence of apparent LED luminous intensity of up to 47% reduced to statistical variation of less than 1%.

Determination of the diffuser reference plane for accurate illuminance responsivity calibrations

It is difficult to predict where the effective measurement plane is situated with dome-shaped diffusers often used in commercial photometers and radiometers. Insufficient knowledge of this plane could lead to large systematic errors in calibration of the illuminance responsivity of photometers. We propose a method that can be used to determine this reference plane accurately, based on the inverse-square law between the measured signal and the distance from the source. The method is demonstrated with three commercial photometers with dome-shaped diffusers of different geometries. By taking into account the measured shifts of the reference planes (5.0 +/- 0.5 mm, 7.8 +/- 0.3 mm, and 8.5 +/- 0.7 mm), we reduced the systematic measurement errors up to 2% to statistical uncertainty components at the level of 0.2%.

Realization of the unit of luminous flux at the HUT using the absolute integrating-sphere method

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 set-up 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.

Determination of distance offsets of diffusers for accurate radiometric measurements

A method for the determination of the effective measurement plane of spectroradiometer diffusers at various wavelength regions is described. The method is based on the inverse-square law of the distance dependence of the measured signal. The scheme is tested with three planar and one dome-shaped spectroradiometer diffuser at four wavelength bands. The distance offsets of the diffusers determined in the UVA region are from 0 mm to 2.1 mm for the planar diffusers and 6.4 mm for the dome diffuser, whereas the corresponding values in the NIR region are from 0 mm to 7.7 mm and 8.2 mm. The uncertainties of the measured reference plane positions of the diffusers are estimated to be 0.3 mm. If the reference plane position is not properly taken into account in calibration measurements with lamps, large systematic errors may appear when measuring radiation from distant sources. We also investigate the wavelength dependence of the angular responsivity of the diffusers. A clear correlation appears between the wavelength dependences of the distance offsets and the angular responsivity curves.

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.

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.

International comparison of the illuminance responsivity scales and units of luminous flux maintained at the HUT (Finland) and the NIST (USA)

An international comparison has been conducted to compare the illuminance responsivity scales (A/lx) and the units of luminous flux (lm) maintained at the National Institute of Standards and Technology (NIST, USA) and the Helsinki University of Technology (HUT, Finland). Both laboratories realize the illuminance unit by absolutely calibrated photometers and the luminous flux unit by the absolute integrating-sphere method. Standard photometers were used as transfer standards for the illuminance responsivity comparison, and standard lamps in the luminous flux comparison. The ratio of the measured illuminance responsivity values (HUT/NIST) was 0.9992 with an expanded uncertainty (k = 2) of 0.0013, and the ratio of the measured luminous flux values was 1.0006 with an expanded uncertainty (k = 2) of 0.0018. The relative expanded uncertainties of the agreement of the units, including the uncertainties of the realizations of the units as well as the uncertainty of the comparison, were 0.0047 and 0.0101, respectively.

Final Report of CCPR-K3.b.2-2004: Bilateral comparison of illuminance responsivity scales between the KRISS (Korea) and the HUT (Finland)

A bilateral comparison of illuminance responsivity scales between the KRISS and the HUT was carried out, where the HUT acted as the pilot and link to the key comparison CCPR-K3.b. The ratio of the measured illuminance responsivities (KRISS/HUT) was 0.9982, with expanded uncertainty of 0.0060 (k = 2) including the uncertainty of the comparison and the uncertainties of the realization of the scales. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the Mutual Recognition Arrangement (MRA).

Evaluation of calibration methods of a photometer measuring maritime light-emitting diode buoy lanterns

A photometer used for on-line product testing of light-emitting diode (LED) buoy lanterns is calibrated for illuminance responsivity using two different methods. The first method is based on absolute calibration of the photometer with the CIE standard illuminant A light source, combined with spectral correction factors (SCFs) calculated from the measured spectral responsivity of the photometer and relative spectra of the LED lanterns. The second method is based on direct comparison with a characterized reference photometer using the LED lanterns as light sources. Comparison of the resulting correction factors shows that both methods agree within 1%. However, the second method includes geometrical aspects and LED characteristics that caused problems. These problems are discussed and the reasons for recommending the first method are given.