Hi Zhe,
I'm using a commercially calibrated camera (LMK mobile air) so my experience
with calibration is limited. But I¹ll 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 it¹s 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. That¹s 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 Clotilde¹s 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, doesn¹t it? I¹m 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
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