Stray light performance of a combined monochromator-spectrograph UV irradiance measuring instrument

Critical UV applications such as disinfections and dermatological treatments call for accurate UV spectral irradiation measurements. Single monochromators and spectrographs are not able to provide satisfying accuracy due to poor stray light rejection. The main drawbacks of a double monochromator scanning system are the need for motion synchronization, the inconvenience in performing field measurements and the time-consuming scanning approach. In this paper, a combination of a monochromator with a large bandpass and a spectrograph is proposed, which allows for accurate UV spectral irradiance measurements with short measurement times of less than 1 s, and yet with acceptable stray light rejection. Stray light levels are determined for both monochromatic and heterochromatic primary radiation using slit functions and cut-off filters. These experiments revealed a considerable UV stray light response reduction for the monochromator-spectrograph combination compared to single spectrograph instruments. The stray light performance in the 250-400 nm range matches with results obtained with double monochromator instruments. With this setup, the quality of a double dispersing instrument is combined with the quick data acquisition of a spectrograph.status: publishe

Spectral UV Irradiance Measurements with a Double Monochromator System and with a Combined Monochromator-Spectrograph Measuring Instrument

Critical UV applications such as disinfection and dermatological treatments call for the need for accurate UV spectral irradiation measurements. Due to poor stray light rejection, single monochromators and spectrographs are not able to provide a satisfying accuracy. For critical measurements, a scanning double monochromator offers a much better solution. At the Light&Lighting Laboratory, a combination of a single monochromator with a large bandpass and a spectrograph has been developed and it has been shown that this apparatus has stray light rejection characteristics even better than those obtained with a commercially available high-accuracy scanning UV-VIS double monochromator system. In this paper, the UV irradiance of several light sources is determined using both instruments. The experimental spectra and the stray light contributions are discussed.status: publishe

A comparison between near field and far field goniophotometer measurements

Near field goniophotometry is a recent promising measurement technique to determine the photometric characteristics of light sources. At the Light & Lighting Laboratory the recently installed goniophotometers will be used to determine luminous intensity distributions of a wide variety light sources of various dimensions. In this work results obtained with the near field goniophotometer are compared with photometric data obtained with a far field goniophotometer. The near field goniophotometers allow to obtain information about both the luminous flux and the luminous intensity of light sources. In this paper these parameters were investigated for a calibrated incandescent source and for a remote phosphor LED source. The luminous flux and luminous intensities as obtained from both lamps are comparable for both near and far field goniophotometers. Small deviations appear during far field goniophotometry due to the lamp motion in this equipment.status: publishe

Near-field and far-field goniophotometry of narrow-beam LED arrays

In lighting calculations and simulations, the emission of a light source is conventionally modeled using the far-field luminous intensity distribution. However, the advent of luminaires including large arrays of LEDs with focusing optics creating narrow beams has made the traditional limiting photometric distance to reach far-field conditions less easy to determine. Furthermore, even correct far-field data can lead to erroneous predictions when illuminances are determined on a task surface which is positioned within the near-field region. A near-field representation could overcome these problems, but experimental validation for such LED arrays is lacking. This paper reports on near-field and far-field laboratory experiments using an array of two and five narrow-beam LEDs. A near-field approach makes discussions to determine the far-field photometric distance superfluous and leads to correct illuminances at any location with respect to the array, irrespective of the dimensions of the array and the beam angle of the individual components. Introducing the near-field representation of light sources in lighting design offers more accurate predictions when luminaires based on LED arrays with focusing optics are involved.status: publishe

BRDF and gloss measurements

Gloss is an important aspect of our visual perception of objects. Many developments of gloss measurement have been carried out as part of the technical work of the ASTM, resulting in the ASTM method D523. Different application domains, manufacturers and gloss meters related to different standards can be found, and instruments from different manufacturers show wide variation in aperture size and beam geometry. The spectral Bidirectional Reflection Distribution Function (BRDF) is a fundamental quantity of the sample, and any other quantity can be calculated from the BRDF. In this paper, the relationship between BRDF characteristics and the standard gloss measurements on different NCS gloss scale samples is studied. Due to the limited dynamic range of the industrial instruments, it is rather difficult to characterize matt samples accurately. On the contrary, BRDF measurements provide much more information. Integration of the BRDF over the cone angle of the receptor correlates with standard gloss measurement results, but only at certain geometries. A full description and understanding of the complete optical design of a gloss meter seems to be necessary to improve this correlation.status: publishe

The effect of Schottky metal thickness on barrier height inhomogeneity in identically prepared Au/n-GaAs Schottky diodes

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A new integrating sphere design for spectral radiant flux determination of light-emitting diodes (LEDs)

Light-emitting diode (LED) technology is developing very quickly and may be considered an alternative for traditional light sources. However, at this moment, manufacturers and end users of LEDs are facing a rather basic but major problem. The lack of standardization regarding optical and electrical characterization of LEDs appears to compromise a successful implementation. In particular, numbers quoted for the luminous flux, and consequently for the efficacy of LEDs, are very sensitive data because they are used to impress and push the LED market. In this paper, the most was made of the typical hemispherical radiation of high-power LEDs to increase the accuracy of the flux determination using a custom-made integrating sphere. Recently developed measurement techniques such as the use of an external spectral irradiance standard and an optimized spectral irradiance detection head are combined with a very particular port geometry and a minimized baffle area. This results in a uniform spatial response distribution function (SRDF), which guarantees an accurate radiant and luminous flux determination, irrespective of the spatial intensity distribution of the LED package or luminaire. The effect of the directional response of the detector head on the SRDF has been explored. Measurements on LED devices with and without external optics are presented, illustrating the possibilities of the measurement setup.status: publishe