Fisheye camera method for spatial non-uniformity corrections in luminous flux measurements with integrating spheres

Abstract

This paper presents a fisheye camera method for determining spatial non-uniformity corrections in luminous flux measurements with integrating spheres. Using a fisheye camera installed into a port of an integrating sphere, the relative angular intensity distribution of the lamp under test is determined. This angular distribution is used for calculating the spatial non-uniformity correction for the lamp when combined with the spatial responsivity data of the sphere. The method was validated by comparing it to a traditional goniophotometric approach when determining spatial correction factors for 13 LED lamps with different angular spreads. The deviations between the spatial correction factors obtained using the two methods ranged from −0.15% to 0.15%. The mean magnitude of the deviations was 0.06%. For a typical LED lamp, the expanded uncertainty (k = 2) for the spatial non-uniformity correction factor was evaluated to be 0.28%. The fisheye camera method removes the need for goniophotometric measurements in determining spatial non-uniformity corrections, thus resulting in considerable system simplification. Generally, no permanent modifications to existing integrating spheres are required.

Future Photometry Based on Solid-State Lighting Products

“…On average, the relative deviation between the fisheye camera method and goniophotometer method in determining spatial corrections for the artefacts tested was only 0.05 %. The fisheye camera method has been described in [Kokka 2017] and the results of the validation with several measurement facilities in [Kokka 2019]. Figure 5 shows a photograph taken with a fisheye camera through a port of an integrating sphere illuminated by an LED lamp under test and the resolved angular intensity distribution of that lamp.…”

Recent advances and perspectives in photometry in the era of LED lighting

“…Knowing the geometric relation between the system components, it is possible to calculate the intensity distribution within the angular range of the screen or construct the full intensity distribution by rotating a light source with wide angular pattern. The extreme case of this method is to use a 2π camera (fisheye) on the wall aperture of an integrating sphere [49].…”

“…The fisheye method was originally developed for spatial non-uniformity corrections in luminous flux measurements with integrating spheres, but it can be also used for fast measurement of light source angular distributions [42,49]. The method needs to be calibrated for the angular responsivity of the sphere with a light source of known angular distribution, determined by the goniometric measurement (see figure 3).…”

Validation of the fisheye camera method for spatial non-uniformity corrections in luminous flux measurements with integrating spheres

“…To avoid the laborious goniophotometric measurements, alternative methods have been proposed to take into account spatial non-uniformities of integrating spheres. These methods include the use of a six-port integrating sphere [11,12], a directional luminous flux standard lamp [13], or fisheye-lens cameras [14,15].…”

“…In this paper, the fisheye camera method [15] for determining spatial non-uniformity corrections is validated by measuring the relative angular intensity distributions of six LED lamps in eight integrating spheres using the camera method, and comparing the correction factors with those obtained using five goniophotometers of different types. In addition to studying the spatial correction factors for luminous flux measurements, the angular intensity distributions obtained using the two methods are also directly compared.…”