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
