A new device for dynamic luminance mapping and glare risk assessment in buildings

A new device for dynamic luminance mapping and glare risk assessment in buildings

Apiparn Borisuit*, Mirjam Münch, Laurent Deschamps, Jérôme Kämpf, Jean-Louis Scartezzini

Solar Energy and Building Physics Laboratory (LESO-PB) Ecole Polytechnique Fédérale de Lausanne (EPFL)

CH – 1015 Lausanne, Switzerland

ABSTRACT

High dynamic range imaging has been shown to be a reliable tool to assess luminance maps and glare risk probability in buildings. However, there are some limitations of image capturing time, especially when dealing with highly dynamic and contrasted daylight situations. We used a newly developed prototype of a digital camera which contains a high dynamic range pixel array chip, with a logarithmic scale for encoding. This type of camera allows to effectively assessing luminance, contrast and contrast directions, by taking only a single image or by performing real time recordings. The device was equipped with a fisheye lens and V-lambda filters to adapt the camera’s spectral sensitivity to the human eye. After spectral as well as photometric calibration and vignetting correction, the device was tested to perform luminance mapping of real scenes. The results showed that luminance maps of a room can be efficiently assessed under dynamic daylight and mixed day- and electric lighting conditions in a very short time (i.e. 100 ms), when compared to classical HDR imaging techniques. This allows us to calculate glare indexes of a scene simultaneously. The camera opens a variety of new applications as a useful tool for architects, building designers and lighting experts. The device can be used to easily monitor daylight availability and glare indexes in existing buildings and further developments for advanced (day-) lighting control can be envisaged. Keywords: High dynamic range (HDR), CCD camera, photopic filters, luminance mapping, glare, dynamic daylight

1. INTRODUCTION Besides horizontal illuminance on a given task area, visual comfort is a key element for lighting design. Visual comfort at workplaces has been extensively investigated with respect to perceived glare, luminance distribution and visual performance1. High visual comfort requires that the luminance ratio between the surrounding vertical planes and the work planesbe controlled, particularly in an office room equipped with video display terminals. Moreover, high contrast between light sources and background luminance can cause glare sensations. In order to investigate luminance and glare risks in indoor lighting conditions, a recent advanced imaging technique, the so-called high dynamic range (HDR) with a charge coupled device (CCD) camera, can be used. By applying this technique, a digital image taken with a single aperture exposure does not cover the full dynamic range of daylight due to the limitations of the CCD sensors. Therefore, to create an HDR imagewe need to take several low dynamic range (LDR) images with multiple exposures2. In a next step, we can assess luminance distribution3-5 from an HDR image by using adhoc software such as Photosphere6. From an HDR image, glare indexes7-9can be calculated withopen-source software, such as Radiance10 or Evalglare11. It is not only time consuming to collect several LDR images (30-90s) by a conventional CCD to produce a single HDR picture, but the luminance distribution might also change while taking those images; we therefore need to capture images more rapidly, especially under dynamic daylight conditions. To achieve this goal, we aimed at testing a newly developed CCD camera to produce faster HDR images and to simultaneously assess glare risksunder different lighting conditions.

*[email protected]; phone +41 216936265

Nonimaging Optics: Efficient Design for Illumination and Solar Concentration IX,edited by Roland Winston, Jeffrey M. Gordon, Proc. of SPIE Vol. 8485, 84850M

© 2012 SPIE · CCC code: 0277-786/12/$18 · doi: 10.1117/12.929845

Proc. of SPIE Vol. 8485 84850M-1

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2. METHODOLOGY

2.1 The camera-like light sensor ‘IcyCAM’

The small camera-like light sensor ‘IcyCAM’ has been developed atthe Centre Suisse d’Electroniqueet de Microtechnique (CSEM; Neuchâtel, Switzerland). The essential elements of this device are the intra-scene dynamic range of the optical front-end and a data representation that is independent from the illumination level. It contains a ‘System-on-Chip’ (SoC), by combining an optical front-end and a processor on the same device. The SoC enables a single chip to perform image acquisition, analysis and decision-making12. Each pixel achieves a 132 dB intra-scene dynamic range due to a logarithmic compression13. The logarithmic encoding allows one single HDR image to be real-time recorded in 780μs to 2s exposure duration14.According to its intra-scene dynamic range function, the IcyCAM provides a real-time recording of images. It can record HDR images with muchgreater speed performance than the classical HDR technique with a conventional CCD camera. A fisheye lens fromFujinon (Fujifilm, Tokyo, Japan) was installed on the IcyCAM in order to mimic the approximate field of view of a human eye (≈120°)18. Since the aim of this project was to use the IcyCAM device for dynamic luminance mapping and glare risk assessments, we calibrated first the device and corrected it for vignetting effects.

2.2 Spectral sensitivity calibration

Narrow bandwidth monochromatic light beams (generated by a 1000 Watt Xenon lamp and a grating monochromator) were used as a reference light source for the spectral sensitivity calibration of the IcyCAM. The camera and a calibrated spectrophotometer (Specbos 1201, JETI, Jena, Germany) measured the emitted radiance (at 555 nm)at different light intensities. The camera provided one value per pixel on a grayscale (in arbitrary units from 0 to 1024 digits) and the spectrophotometer measured the respective radiance (μW.m2.sr-1)15. The functiony = 1E-13x5- 6E-11x4 + 1E-08x3 - 1E-06x2 + 7E-05x + 0.0002describes the relation between grayscale values and measured radiance (R² = 0.9994). In a next step, light intensity was kept constant and the same measures with the IcyCAM and the spectrophotometer were taken every5 nm between 380 and780 nm. The relative raw spectral sensitivity of IcyCAM is shown on Figure 1a) together with the standard CIE 1931 photopic luminosity function (V (λ))16. Next, three customized colored glasses were selected as filters. The optimal thickness was calculated to correct the spectral response of IcyCAM. After implementation of the filters in IcyCAM, the same steps as described above were repeated to obtain a sensitivity curve which corresponds to the photopic sensitivity function (V(λ); Figure 1b). In order to assess the error between the relative spectral sensitivity from IcyCAM and the photopic curve,the CIE standard error f’, equal to 8.3%, was calculated using the formula17:

%)()(93584.0'0

λλλ dVsf rel −= ∫∞

(1)

Figure1: Relative spectral sensitivity assessment of IcyCAM; 1a) Relative raw spectral sensitivity of IcyCAM (filled circles) shown with the CIE 1931 photopic sensitivity function (V (λ)16; open triangles). 1b) Corrected spectral sensitivity response of IcyCAM, equipped with filters (filled circles) shown with the CIE 1931 photopic sensitivity function (V (λ)16; open triangles).



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2.3 Photometric calibration

To perform the photometric calibration, a total of 138 measurements (from 0.01 cd/m2to 15’560 cd/m2) were made by simultaneously using the IcyCam together with a luminance meter (Minolta LS-110) as a reference sensor. Polychromatic white light from a 1000 W Xenon lamp and a 1200 W metal halide spotlight were used as reference light sources for the photometric calibration. The camera and the calibrated spectrometer monitored the emitted radiance values: the camera provided the associated pixels on a greyscale (in arbitrary units from 0 to 1024 digits) and the luminance meter the corresponding luminance (cd/m2). The relation between the pixel grayscale values and their associated luminance has been determined using an exponential fitting function15: y = e0.017x (R2=0.97; Figure 2). The function was also implemented in the “ImageCAM” software

Figure 2: Correlation between the pixel grayscale values and the associated luminance Theblack crosses indicate the measurements and the solid line indicates the fitted exponential function (R2=0.97).



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2.4 Vignetting correction

A fisheye lens normally exhibits noticeable light falloff (so-called vignetting effect) for image pixels located far from the optical axis. This vignetting effect had to be corrected to compensate the luminance loss at an image’s edges. To assess potential vignetting effect, several pictures were taken from a constant light source. The camera was set on an orthogonal axis to the light source and was then rotated every 5° in both horizontal and vertical directions andpictures were taken at each position. The luminance of the light source at different positions was then plotted (Figure 3). Mathematical corrections for the vignettingeffectwere determined and implemented in the “ImageCAM” software. Measurements taken with the IcyCAM at different angles werethen fitted witha polynomial function: y =2E-12x5 + 2E-10x4 - 6E-09x3 + 3E-06x2 + 3E-06x + 0.999 (R2=0.97).

Figure 3:Measurement of the IcyCAMwith fisheye lens. The crosses indicate the measurements for different angles from the center and the solid line shows the fitted polynomial function (R2=0.97).

3. RESULTS 3.1 Luminance mapping

After thecalibration steps, the IcyCAM device was tested by performing luminance mapping of real scenes. The luminance maps were validated with a Minolta LS110 luminance meter and compared with luminance maps, obtained by a Nikon Coolpix 5400 CCD camera, equipped with a fisheye Nikon FCE-9 lens. The comparisons were made in a real office room at the Solar Energy and Building Physics Laboratory (LESO-PB) on the EPFL campus (EcolePolytechniqueFédérale de Lausanne, Switzerland). Macbeth color checker® and different room elements were used as targets for luminance mapping8.A set of pictures was taken i) under dynamic daylight conditions and ii) under electric lighting conditions (2 x 36W fluorescent tubes; 3000K). Luminance values of a Macbeth color checker® and the room elements were extracted in two different ways: i) by the implemented “ImageCAM” software for IcyCAM images and ii) with the software Photosphere6 for images obtained from the ‘classical HDR’ technique (CCD Camera Nikon Coolpix); both data sets were then compared with the luminance meter values, as shown on Figure 4. For the office with electric lighting, the coefficients of determination (R2) from luminance mapping by IcyCAM and the ‘classical HDR’ in comparison with the luminance meter were 99.44% and 99.74%, respectively. For the office with daylight, the R2for luminance mapping by IcyCAM and the ‘classical HDR’ in comparison with the luminance meter were 97.31% and 96.88%, respectively.



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3.3 Assessments under different lighting conditions

In order to assess the results from the HDR images derived fromIcyCAM and the CCD camera Nikon Coolpix under different lighting conditions, the experiment was repeated in an office room of the LESO-PB building. A set of LDR images was taken with the CCD camera Nikon Coolpix (duration:~60s) and a sequence of real-time images was simultaneously recorded with the IcyCAM under electric (for 10s) and daylight conditions (for 60s). In a next step, three glare indexes (DGP, DGI, UGR) and background luminance were calculated. The obtained values for the three glare indexes and the background luminance are shown together with false color HDR images on Table 2a (for electric light) and 2b (for daylight).

Electrical light: Results from Icycam at10s and from the conventional CCD camera were compared with the data obtained with IcyCAM at 0s. The differences were lower than 3% under electric lighting conditions for both devices, except with the conventional camera for the DGI (5.5% lower than IcyCAM) and the background illuminance (13% greater than IcyCAM; Table 2a).

Daylight condition: Table 2b illustrates the glare index results measured with IcyCAM (three different recording times) and the CCD camera Nikon Coolpix with the recording time (~60s). For the dynamic daylighting conditions, there was no modification for the DGP across the different recording times or between the two devices. The changes for the DGI, UGR and the background illumination were in the range from 0% to 58% (mean ± SD: 20±17.2%). The largest difference for background illumination (58%) was found in the HDR images derived from the CCD camera Nikon Coolpix; Figure 5).

Table 2: Glare indexes (DGP, DGI and UGR) and background luminance assessed at the same position in electric lighting conditions and dynamic daylighting conditions with the IcyCAM (between 0 and 25s) and the CCD camera Nikon Coolpix (‘classical’ HDR; during ~60s).

LightingConditions Glare Indexes DGP DGI UGR Background Luminance

a) Electric

Lighting Condition

IcyCAM<1s 0.18 9.21 11.74 48.0

IcyCAM~10s 0.18 9.17 11.69 48.04

‘Classical’ HDR

From 14 LDR images during~60s

0.18 8.7 11.34 54.4

b) Dynamic

Daylighting Condition

IcyCAM<1s 0.23 8.75 12.14 135.79

IcyCAM~10s 0.23 9.58 13.02 104.12

IcyCAM~25s 0.23 8.31 11.83 169.78

Classical HDR

From 14 LDR images during~60s

0.23 10.79 15.21 215.14



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Icycam (lOs) Icycam (25s) 'Classical' HDR (60s)

Figure 5: Relative differences in dynamic daylight conditions between Icycam (at 10s and 25s) and the CCD camera Nikon Coolpix (after 60s) regarding the glare indexes and the background luminance at 0s (with Icycam). Black circles=DGP; white circles=DGI; black triangles =UGR and white triangles=background illumination.

4. DISCUSSION We successfully calibrated and tested the new device IcyCAM in real scenes. We also compared different luminance maps and background luminance with those obtained by conventional HDR techniques. Different glare risk assessments under constant and varying lighting conditions showed that the new device has the advantage to capture HDR images more rapidly which is of great benefit in dynamic daylight situations.

The calibration steps and vignetting correction resulted in high correlations with the reference devices: IcyCAM can thus be reliably used to perform measurements of real scenes. The results for luminance maps between IcyCAM and the conventional CCD camera were similarly high under both tested lighting conditions.We found that under constant electric light conditions the differences of glare indexes between both devices were rather small (less than 6%), except for the background luminance (13%). Under dynamic daylight, the differences of glare indexes between both devices were larger, compared to the electric lighting condition, and background luminance changed by maximally 58%. Sincethe main parameters taken into account for the calculation of glare indexes were background luminance, luminance of potential glare sources, and solid angles19,22, the differences of glare indexes weremost likely caused by the changes ofbackground luminance

Small distortions of the two different fisheye lenses might also have indirectly contributed to the differences of glare indexes, since they can impact the solid angles whichare used in the glare index formula. However,the differences between both deviceswere smaller for the DGP than the DGI and UGR. This could be due to the vertical illuminance, which is an additional parameter of the DGP9. Since we (manually) measured the same vertical illuminance during the image captures with IcyCAM and the conventional CCD camera, the DGP resulted insmaller differences than the other two glare indexes. Apart from that, background luminance was also used to calculate the luminance ratio in order to assess visual comfort of an environmental lighting scene. When background luminance changed within short time periods as it was the case in dynamic daylight conditions, the assessments for visual comfort and/or other subjective measures might also have changed.

The main advantage of the IcyCAM luminance mapping is its high speed performance. Besides the benefits of time savings, a more accurate assessment is achieved when compared to classical HDR imaging techniques, particularly, under variable dynamic daylightingconditions..The high speed of IcyCAM may allow using it also as a sky scannerin the



future – to assess sky luminance distributions even with very dynamic sky conditions. Since the IcyCAM is equipped with photopic filters which are based on human cone sensitivity, it can be used to assess lighting situations in the visible range of light. An additional potential application of IcyCAM might be to use a second sensitivity curve (which is blue-shifted, also known as C-lambda curve23-24), to monitor more biological, non-image forming functions such as the circadian rhythms in humans. Lastly, further developments for advanced (day-) lighting control may be envisaged.

5. CONCLUSION The new device IcyCAM was calibrated and adapted for our purposes. It is fast and reliably giving information about any indoor luminance distribution and glare risks, even under highly dynamic daylighting conditions. The IcyCAM camera has the potential to become a new and useful application tool for architects, building designers and lighting experts.

ACKNOWLEDGEMENTS

The first and the second author received financial support from the VELUX Foundation (Switzerland). The authors wish to thank the Centre Suisse d’Electronique et de Microtechnique (CSEM) for this fruitful collaboration as well as Pierre Loesch, Chantal Basurto and LenkaMaierova for their excellent support.

REFERENCES

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A new device for dynamic luminance mapping and glare risk assessment in buildings

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