I suggest to use two parallel approaches, one with your LED, one with an incandescent low voltage. The problem with LED luminance measurements from HDRI is that a) the color gamut is small and b) the photopic sensitivity curve V(lambda) currently used is incorrect. It underestimates blue wavelengths quite a bit. Therefore, start with an incandescent first, because LEDs have a strong blue peak around 440 or 460 nm, depending on the manufacturer. You are dealing with the liumitations of V(lambda) and then with tristimulus values which are just not good enough for narrowband LEDs, including white ones with a blue peak.
When you use the incandescent instead use it in a black lab. Place a few white paper samples on the walls at different known locations. Place your incandescent at a defined location. Derive the geometry, all distances and all angles between those white papers and the lamp. Turn the incandescent on and the room lights off. Measure the luminance of the white papers at a location that you marked with a soft pencil. Make HDR images and determine the luminance and scale. Based on the luminance values of the papers, you can now derive the luminance of the incandescent filament. Make HDR images of the filament as seen from those papers. Check if the corresponding illuminance values of the white paper at reflectance around 85% corresponds to the luminance values of the filament in that direction. Repeat several times to give you confidence. Once you are heading in the right direction, you can repeat the whole procedure with an LED. But this time your values will be around 20-30% off. You should really use a CCD camera with a photopic filter to get better values, and then try to come up with a procedure to minimize errors for the HDR approach. Regards Martin Moeck Osram ________________________________ From: Tyukhova, Yulia [mailto:[email protected]] Sent: Thursday, March 01, 2012 9:46 PM To: [email protected] Subject: [HDRI] HDRI capture of LED (histograms and range) Hello! I've been experimenting with the number of photos to include in final HDRI. I took a sequence of photos of a single LED with reflectance standards included in the scene. EOS7D 28-105mm lens at 28mm F16 1/8000-1/15'' F4 1/30-5 mins With the ND filter t=0.0094 Images are fused with raw2hdr. They were calibrated at white reflectance standard 215 cd/m2. I've noticed the following tendency: If I fuse different number of photos (cut the number of photos on the shortest end), after calibrating at white reflectance standard, I get different luminance values for the LED. Shortest exposure to fuse L,cd/m2 1/125'' f16 4.5*106 1/250'' f16 9.06*106 1/500'' f16 18*106 I've seen interesting discussion between Axel and Greg on photos to include. But it seems like there are many uncertainties. In HDRI second edition book it says "The darkest exposure should have no RGB values greater than 200 or so, and the lightest exposure should have no RGB values less than 20 or so. Do NOT include an excess of exposures beyond this range, as it will do nothing to help with the response recovery and may hurt." I assume it is the same for any HDRI sequence, not only for response curve. I have plenty of photos of dark exposures that have no values greater than 200, same with the light exposures and 20.If somebody can clarify what photos should be included or have any other suggestions that would be great Thank you, Yulia
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