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Looking for RGB or RGBW values of a sunrise

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Twoflower:
For a small project I need a table of RGB or RGBW values over time of the sky-color from the morning just before the sun rises until the sun is nicely over the horizon**. In the end I would like to play back this using a LED strip. Actually two strips: One RGB and one white. Thus the RGBW. But I think I can convert an RGB table to RGBW.

As I don't have a single pixel RGB sensor I need to find another way. My idea so far was to setup a cam looking into a white, opaque globe to get the sky colour as whole. Do a picture every minute or so. And reduce the rescale the picture to 1x1 pixel to further obscure the light and get the RGB value from the sky from that remaining pixel.

Any other ideas, or is there somewhere an existing table I can use? An existing table would be nice as here the winter fall is coming and the chances to capture this is not that high any more.

**I'm speaking of a rising sun as seen from Earth. And a nice one is preferred ;-)

Twoflower:
Thanks evb149.

As I said, I'm looking for a nice one. So a location with low particle count (including water). OK, the longitude and altitude wasn't specified. So the altitude should be about sea level (I like the reds of the rising sun). About the latitude: Here I'm probably not that picky, lets say somewhere in the range of +/-50° would be acceptable. To my understanding that mainly changes the speed of the phases and I can adjust that. The sun position should be between 8° below horizon (blue hour) and 15° above horizon (no visible reddish sky any more). Looking at all the data, couldn't be a simulation a source of this table?

Using the white balance and lux value is some kind of different scale which should be possible to transform into the requested RGBW table. But one or two values are not enough to calculate a nice transition between the points. And so far I haven't found any table that provides more than 2 or 3 values unknown how they timely relate. That's the reason why I'm looking for a higher resolution on the time axis. Probably 20 entries are enough to extrapolate the steps in between. I wouldn't complain if I can get more steps.

About the dynamic range. That's also right, I'm limited to 12bit resolution per channel. And that's plenty for my application. Probably except that the lower end might be still to bright for my taste.

Twoflower:
Thanks again evb149.

I scanned through the papers. And that provided the right keyword in one of them: "CIE daylight model". Even if the model needs to be purchased it can be used as Google search term. Lots of interesting information pops up. Including a Daylight Visualizer from a company producing windows (free download). Probably I need to play with it to see if I can abuse it and model a integrating sphere instead a room with windows.

ajb:

--- Quote from: Twoflower on September 17, 2017, 07:53:12 pm ---As I don't have a single pixel RGB sensor I need to find another way. My idea so far was to setup a cam looking into a white, opaque globe to get the sky colour as whole. Do a picture every minute or so. And reduce the rescale the picture to 1x1 pixel to further obscure the light and get the RGB value from the sky from that remaining pixel.

--- End quote ---

I'm not sure integrating the whole sky will get you the effect you want, since the quality of light at sunrise is highly variant depending on viewing direction.  You may get more useful information from an integrating screen aimed towards the sunrise, or perhaps multiple captures with the screen aimed at different parts of the sky.  You'll also need to be careful that the white balance of the camera is fixed through the whole capture, otherwise the changing white balance will confound your normalization process. 

Another thing to consider is that if you are looking to illuminate a room or something with your RGBW LED strips you're going to have a very difficult time replicating the way a sunrise illuminates a scene.  This is down to how the illumination spectrum interacts with the reflectance spectrum of the objects under illumination.  An object illuminated by an RGBW "orange" is going to look quite different than when illuminated by a sunrise "orange" depending on its pigmentation.  The only real way to do better here is to use more wavelengths--IE, instead of red, green, and blue, you might want deep red, red, orange, yellow, green, and blue.  Or even more--some LED stage lighting fixtures use seven different wavelengths to achieve smooth color mixing and less erratic interactions with the pigments in the scenes they illuminate. 

So if you wanted to really closely replicate the illumination of a sunrise, you'd want to take a spectrograph of the sky over the course of a sunrise and then try to match that spectrum as best you could with a handful of different wavelengths (and possibly multiple color temperatures of white as well).  It still wont' be perfect, but it will be far better than RGBW.  Of course if you're just illuminating a white screen or something, then RGBW may be just fine.

Twoflower:
ajb: Yes you're absolutely right. The automatic setting would spoil the process. On the other side one could probably use the while balance result the Cam would use from the EXIF data. But for sure that will be the first thing I will do: Fixing the exposure settings.

And I also plan to calibrate the final setup with the same approach. Because the environment is not neutral white, but it is static. The idea is that set some specific RGBW values, taking a picture and compare what I wanted to see and what I actually get. As the equipment is very compute limited that factor will then included into the lookup table in a offline process. Unfortunately I can't fit more than 2 LED stripes into the aluminium profile. So one RGB and one White has to be sufficient. then again I don't want to have a 100% replica of a 2*10^30 kg fusion reactor shining from 150mio km through 500km gaseous molecules in my home. It would be nice to come close without getting a sunburn and the heat. But so far I need some starting point with the possibility to improve the result in a iterative process.

Unfortunately I don't have a spectrograph, just a Canon EOS 5DMk3. That's the reason for using this one to collecting the data. Plus the converting from the continuous result from the spectrograph to discrete RGBW values. But that is probably doable.

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