Valéry Ann Jacobs

HERMES High-Resolution Spectroscopy of HD 149382-Where Did the Planet Go?

A close substellar companion has been claimed to orbit the bright sdB star HD 149382 with a period of 2.391 d. In order to check this important discovery we gathered 26 high resolution spectra over 55 days with the HERMES spectrograph on the 1.2 m Mercator telescope on La Palma, and analyzed the resulting radial velocities. Our data show no sign of any significant radial‐velocity periodicities, and from the high precision of our measurements we rule out any RV variations with amplitudes higher than 0.79 km/s on periods shorter than 50 days.

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.

Opinion: An upper limit to lighting?

Lighting standards conventionally specify the minimum illuminances that need to be met in a given situation. But how about the other end of the scale: Should lighting standards set maximum illuminances as well as minima? Every week, I am reminded of this issue, when my students ask me: ‘‘What is the maximum illuminance that is allowed in this situation?’’ Each time, I find that question very difficult to answer. I first encountered the idea of a maximum illuminance when studying surgical lights in the operating theatre. These light sources should be able to deliver between 40,000 lx and 160,000 lx at the centre of the beam. The upper limit is intended to prevent an undesirable temperature rise in the operating field, so the maximum total irradiance and the ratio of this irradiance to illuminance are constrained. Specifically, the maximum total irradiance is limited to 1000W/m in the lit area at 1 m from the light source. The ratio of this irradiance to illuminance is constrained to a maximum of 6mW/m/lx. The problem that may arise here is subtle. In principle, the idea behind the maximum illuminance is not to constrain the illuminance but to constrain the ratio between irradiance and illuminance so as to prevent burn wounds to patients. Given this ratio, the maximum illuminance depends on the spectrum of the light source, and at the time, only halogen spectra were considered. Imagine the consequences of adopting the same threshold using LED technology: the illuminance could then become a multiple of the current value. Clearly, this would not be beneficial for the visual comfort, nor the performance of the medical staff. As another example, photobiological risks to the eye depend on the irradiance at eye level and the time it is exposed to that light source. This topic is most obvious in the context of the ‘‘blue light hazard’’ which is often associated to LED technology in the media, although the risk applies to every light source, whether it be natural or artificial lighting. Although recommendations have been put forward, the issue of maximum illuminance has not yet been implemented. Similarly, the illumination of artworks forces us to consider the concept of maximum illuminance. On the one hand, museum lighting should be adequate for spectators to see the artwork and for colours to be accurately rendered and discriminated but on the other hand photochemical reactions and heating may damage the artwork and change its colours. An optimal balance between both should be sought, thus limiting the maximum illuminance. Finally, turning our attention to the consequences of lighting, setting maximum illuminances could reduce energy consumption and might reduce light pollution. When delivering the right amount of light at the right place and at the right time is considered to be one of the main aims of the lighting designer, more research on maximum illuminance would be very welcome.

Color sensitivity of the multi-exposure HDR imaging process

Abstract Multi-exposure high dynamic range(HDR) imaging builds HDR radiance maps by stitching together different views of a same scene with varying exposures. Practically, this process involves converting raw sensor data into low dynamic range (LDR) images, estimate the camera response curves, and use them in order to recover the irradiance for every pixel. During the export, applying white balance settings and image stitching, which both have an influence on the color balance in the final image. In this paper, we use a calibrated quasi-monochromatic light source, an integrating sphere, and a spectrograph in order to evaluate and compare the average spectral response of the image sensor. We finally draw some conclusion about the color consistency of HDR imaging and the additional steps necessary to use multi-exposure HDR imaging as a tool to measure the physical quantities such as radiance and luminance.