Thanks, Greg Enjoy your well-deserved weekend.
Over and out. Axel On 3 February 2012 21:29, Gregory J. Ward <[email protected]> wrote: > Hi Axel, > >> From: Axel Jacobs <[email protected]> >> Date: February 3, 2012 1:21:43 PM PST > ... >> >> Will do. I shall report back to this thread. One last sub-question, if I may: >> >>> An overall calibration of the image luminance can be carried out (I >>> think) by measuring the vertical illuminance at the lens when the >>> exposure-bracketed sequence is taken, and then running findglare and >>> glarendx -t ver_illu on the HDR, which should give a calibration >>> factor that can then be used to fiddle with the EXPOSURE= line. This >>> is probably more accurate than calibrating against spot meter >>> readings. So far, so good. >> >> Does this make sense? Is this what the NYT trolley did? Spot luminance >> meter calibrations are a bit messy, because it's very difficult to >> match the target circle of the luminance meter against a pixel value >> ('L' in ximage), or against a box average >> ('drag-your-mouse-in-ximage', then hit 'L')? > > Yes, but you need to do it after all the spatial corrections have been > applied (solid angle + vignetting). > >> The problem with lux meters, on the other hand, is that cosine >> correction for near-the-horizon-angles is hopeless, even for 'proper' >> illuminance meters. Some manufacturers will not even give you a number >> for angles > 80deg, which is a problem almost identical to the HDR >> projection function. Perfect alignment appears to be critical for >> light sources close to the visible horizon of the meter, irrespective >> of the cos weighting. > > I guess this doesn't surprise me. Having the bright areas in front rather > than off to the side should improve things. You may be better off using a > patch calibration, which is what they did at LBNL in their advance glazings > test facility. I don't recall what we did on the NYT trolly as far as > calibration goes. > > Cheers, > -Greg > _______________________________________________ > HDRI mailing list > [email protected] > http://www.radiance-online.org/mailman/listinfo/hdri _______________________________________________ HDRI mailing list [email protected] http://www.radiance-online.org/mailman/listinfo/hdri
