Hello everybody!
I’ll provide the summary of my research and have questions within the
summary.I would appreciate any of your help!
My research investigates if HDRI technique can precisely capture
luminances of small bright light sources (e.g. LED garage fixtures) with narrow
light distributions.
I was able to figure out luminance values for a single LED, which can be
compared to the ones from HDR images. But I have a couple of questions/concerns
on HDRI technique and Photosphere.
At first, I’ve used “regular” scene to retrieve response
curve of the camera (large smooth gradients with very dark and bright areas,
and had reflectance standards for the absolute calibration).
Camera: EOS T1i Rebel with 28-105mm lens, at 28mm
Calibrated at the grey reflectance sample 186.45 cd/m2
CF=0.957
I’ve got the following RC for RGB:
red(r) = -6.434199e-03+ 4.518039e-01*r + 1.291426e+00*r^2 + 1.802896e+00*r^3;
green(g) = -5.804720e-03+ 4.175837e-01*g + 1.176582e+00*g^2 + 1.721643e+00*g^3;
blue(b) = -4.376831e-03+ 3.784418e-01*b + 1.075695e+00*b^2 + 1.658471e+00*b^3
If I look at the histogram of the scene, maximum
luminance within the scene is 60,291 cd/m2.
Then I use this RC to analyze HDRI of a captured LED.
The value is 230,000 cd/m2 for a single LED, which is low (it’s has
to be around 7*106 cd/m2). So, it underestimates the
luminance.
It seems like calibration point is critical here. I’ve
decided to try to capture a different scene for deriving RC with a wider range.
It would make sense that camera has to see higher luminance values in order to
accurately measure them later. The dynamic range has to cover measured values.
1. 1. How does
Photosphere deals/approximates/calculates the upper end of the curve? I assume
it gives more weight to mid tone values? But what happens with high luminance
values?
So, the new brighter scene was picked with the direct
sun! But in order to avoid the damage of the camera’s sensor, measurements were
taken before the sunset.
In the new brighter captured scene without the
calibration all values for reflectance standards were overestimated, while the
value for the sun underestimated. Then I decided to calibrate my scene at the
sun!
But when I apply absolute calibration, it simply
multiplies CF to all values.
2. 2. I assumed when CF
is applied, it does not equally change all values, but does it proportionally
to RC (since it is not linear). Why does
it do it equally for the whole range?
Lsun=80*106 cd/m2. And of course
CF is very big 391.
New RC:
red(r) = 3.219064e+00+ -2.655078e+01*r + 9.351069e+02*r^2 +
-2.115052e+03*r^3+1.594538e+03*r^4;
green(g) = 2.094164e+00+ -1.468109e+00*g + 7.306838e+02*g^2 +
-1.720743e+03*g^3+1.380693e+03*g^4;
blue(b) = 1.049078e+00+ 1.591820e+01*b + 5.848958e+02*b^2 +
-1.461635e+03*b^3+1.251033e+03*b^4
But then something interesting happened. When I
analyze LED, it gives a value of 79*106 cd/m2. So, it
jumps to this upper limit calibrated with the sun previously.
(I had similar results for EOS 7D with the lens 16-35mm,
at 16mm)
3. 3. Does photosphere
compress the response curve, so at the upper end all values above certain
threshold will have the same number?
4. 4. Any additional
suggestions on properly obtaining and calibrating HDRI for this purpose?
Thank you,
Yulia Tyukhova
Fulbright Scholar, "Intern LC"
Architectural Engineering Graduate Student, UNL-Omaha, NE, USA
B.E. and M.E. in Lighting Engineering (MPEI), Moscow, Russia
[email protected]
[email protected]
+1 (402) 996 0910
PKI 247
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