Hi Axel: Thank you very much for your explanation. I will follow your suggestions and redo the test. Best, Zhe
Hi Raquel: I will add illuminance measurement in the following test. I do need to measure vertical eye illuminance for my study. Thank you, Zhe On Wed, Mar 1, 2017 at 2:00 PM, <[email protected]> wrote: > Send HDRI mailing list submissions to > [email protected] > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.radiance-online.org/mailman/listinfo/hdri > or, via email, send a message with subject or body 'help' to > [email protected] > > You can reach the person managing the list at > [email protected] > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of HDRI digest..." > > > Today's Topics: > > 1. Re: Question about the overall correction factor for raw2hdr > (Raquel Viula) > 2. Re: Question about the overall correction factor for raw2hdr > (Axel Jacobs) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Wed, 01 Mar 2017 19:27:31 +0100 > From: Raquel Viula <[email protected]> > To: High Dynamic Range Imaging <[email protected]> > Subject: Re: [HDRI] Question about the overall correction factor for > raw2hdr > Message-ID: <d4dcd123.1ca46%[email protected]> > Content-Type: text/plain; charset="iso-8859-1" > > Hi Zhe, > I'm using a commercially calibrated camera (LMK mobile air) so my > experience > with calibration is limited. But I?ll try to answer your questions. > > 1) I can average the results (1.84) as the correction factor, but is it > correct? Some values have great difference (1.74 VS 1.93). > I don't think you should use an average. My understanding is that you > should > always use the calibration factor for the main light source in your scene. > The spectrum of the different light sources is different and that affects > the measurements. > > 2) I notice that I use the same model as Coltide, however, our results are > very different. Is this normal? (The production of each camera and lens > cannot be exactly the same?) > Apparently yes, they can be different. Not sure if it?s the production that > is different or it is simply the fact that mechanical systems do not always > behave in the same way. Even your own system will not always produce the > same result. That?s why you should also test the reproducibility of your > HDR > luminance value. You test that by doing not one but several captures and > using the average of those for calibration rather than the result of one > single capture. In that way you would be accounting for the camera's > mechanical uncertainties. > It looks like there are also sources of error relating to the luminance > meter measurement itself, as described in Inanici 2006, Evaluation of high > dynamic range photography as a luminance data acquisition system. So that > might add to the differences between yours and Clotilde?s calibration. > > 3) Does lighting source influence the accuracy of the factor? Focusing on > daylighting data collection, should I take images under consistent electric > lighting conditions or daylighting conditions? (Daylighting conditions have > dynamic changes, but electric situations offer lower results.) > Different light sources will provide different calibration factors. I was > told by the seller of my system that a measurement under white LED has > errors of 5 to 12% in relation to the measurement done with a halogen-based > calibrated camera. But LED has a lot of flicker, doesn?t it? I?m not sure > how people using your calibration method deal with fluctuations of the > lighting conditions during capture, in the case of daylight scenes from > variability of the sky and in the case of the electrically lit scenes due > to > flicker. Your LDR shots are done over a period of time and light can change > from one second to the next. I wonder when does your luminance spot > measurement take place in relation to the LDR captures and if that might > affect the result? Do you take illuminance measurements during luminance > collection to see how light varies? > In any case, I think you should always make you collection under very > stable > lighting conditions, whether daylight or electric light. One option is to > calibrate for the most stable electric light source that you can find and > then make the correction for the daylight spectrum independently, if you > have access to a spectrometer. > > Hope it helps. > > Best Regards > > Raquel > > > > Raquel Viula > > PhD candidate > > > > TU Delft | Faculty of Architecture and the Built Environment | > > Architectural Engineering and Technology > > Julianalaan 134, 2628 BL Delft, The Netherlands | P.O. Box 5043 2600 GA > Delft > M +31 (0)62 39 82942 | Email [email protected] > > > From: Zhe Kong <[email protected]> > Reply-To: High Dynamic Range Imaging <[email protected]> > Date: Monday 27 February 2017 22:25 > To: <[email protected]> > Subject: [HDRI] Question about the overall correction factor for raw2hdr > > Dear all: > Sorry to bring up this topic again, but I run into the same issue Coltide > has (Coltide's original post is here: > http://radiance-online.org/community/mailing-lists/archives/hdri). I also > use Canon 5D 2 with SIMGA Fisheye lens 8mm f/3.5. I want to figure out the > -C factor when using raw2hdr to combine LDR images. I took the images > (f/11) > under different circumstances: two HDR images were taken along a side > window, two images were taken under electric lighting condition, and the > last one were taken far away from a glazing. For each scene, 12 images were > taken and assembled. The luminance values from the HDR images and the > lighting meter are below (cd/m2): > Side window: 123.0(HDR) 225.0(measurement) 1.83(-C factor) > Side window: 101.0(HDR) 195.4(measurement) 1.93(-C factor) > Electric light: 1.9(HDR) 3.3(measurement) 1.74(-C factor) > Electric light: 1.4(HDR) 2.6(measurement) 1.79(-C factor) > Away from glazing: 9.8(HDR) 18.5(measurement) 1.88(-C factor) > > Questions: > 1) I can average the results (1.84) as the correction factor, but is it > correct? Some values have great difference (1.74 VS 1.93). > 2) I notice that I use the same model as Coltide, however, our results are > very different. Is this normal? (The production of each camera and lens > cannot be exactly the same?) > 3) Does lighting source influence the accuracy of the factor? Focusing on > daylighting data collection, should I take images under consistent electric > lighting conditions or daylighting conditions? (Daylighting conditions have > dynamic changes, but electric situations offer lower results.) > > Any suggestion or comment is appreciated. > > > -- > Zhe Kong > PhD Student > University of Wisconsin - Milwaukee > School of Architecture and Urban Planning > 2131 E. Hartford Ave, Milwaukee, WI 53211 > Office 327 > _______________________________________________ HDRI mailing list > [email protected]http://www.radiance-online.org/ > mailman/listinfo/hdri > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: <http://www.radiance-online.org/pipermail/hdri/ > attachments/20170301/80fcdb02/attachment-0001.html> > > ------------------------------ > > Message: 2 > Date: Wed, 1 Mar 2017 19:42:28 +0000 > From: Axel Jacobs <[email protected]> > To: [email protected] > Subject: Re: [HDRI] Question about the overall correction factor for > raw2hdr > Message-ID: <[email protected]> > Content-Type: text/plain; charset=windows-1252; format=flowed > > Hi Zhe and Raquel, > > my thought on this below... > > Best regards > > Axel > > On 01/03/17 18:27, Raquel Viula wrote: > > Hi Zhe, > > > > > 3) Does lighting source influence the accuracy of the factor? Focusing > > on daylighting data collection, should I take images under consistent > > electric lighting conditions or daylighting conditions? (Daylighting > > conditions have dynamic changes, but electric situations offer lower > > results.) > > > > Different light sources will provide different calibration factors. I > > was told by the seller of my system that a measurement under white LED > > has errors of 5 to 12% in relation to the measurement done with a > > halogen-based calibrated camera. But LED has a lot of flicker, doesn?t > > it? > > I looked into flicker some time ago, and presented my conclusions in a > presentation at the 2012 Radiance workshop: > > http://www.jaloxa.eu/mirrors/radiance_workshops/2012/Talks/ > Jacobs-AJ09-HDR_Radiance_WS-2012.pdf > > Skip to page 23 for flicker. > > In short: If you are seeing the actual artificial light sources in the > photograph, then you need to use short exposure times to capture them > accurately. In this case, it is safe to assume that the lights flicker > (simply because most do, particularly LEDs), and are not accurately > represented in the HDR. If HDR glare measurement is what you want to > do, this is obviously a problem. > > You probably won't have the equipment to measure flicker. What you can > do is take 5 to 10 photographs with the shortest exposure time you need > for your sequence. Keep all camera settings the same. If all > photographs look identical in their brightness, the light source is > flicker free, or flickers at such high frequency or with such a low > amplitude that you can ignore it. > > Hope this helps > > Best > > Axel > > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > HDRI mailing list > [email protected] > http://www.radiance-online.org/mailman/listinfo/hdri > > > ------------------------------ > > End of HDRI Digest, Vol 89, Issue 1 > *********************************** > -- *Zhe Kong* *PhD Student* *University of Wisconsin - Milwaukee * *School of Architecture and Urban Planning * *2131 E. Hartford Ave, Milwaukee, WI 53211 * *Office 327*
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