[HDRI] RGB value for different light sources

[Mahlab, Seyedeh Farzaneh]

Hi every one
I am taking HDR images of different white LED light sources with different color temperature emitting on white diffusive background. I am trying to calculate CCT of each light source based on RGB extracted. But I am not sure how wide an HDR image can cover the differences between white color, like cool white and warm, based on RGB. I also illustrate them on chart to see how RGB values are acting for different light sources. As shown in the following:
[cid:***@01D28091.23CDF880] [cid:***@01D28091.23CDF880]




So my question is : 1)is HDR able to give me the difference between white colors (different CCT) based on RGB value ?( or would I be able to extract CCT from an HDR image ?)
2) when the light source is getting cooler, R values getting slightly lower. Is this point of view acceptable here ? B and G value are almost the same. Is there a reason for that ?
Thank you very much !

[Gregory J. Ward]

Hi Seyedeh,

If your LED light sources are of the RGB type, then you should not rely on a digital camera to determine the correlated color temperature. The reason is that the RGB spectra are fairly saturated, and most cameras do not measure saturated colors accurately.

If your LED light source uses white LEDs, then you *might* get a reasonable CCT result if you convert the images using the dcraw freeware tool to a linear output format such as 16-bit TIFF. For example, I would use:

dcraw -w -4 -t 0 -T input.raw

The dcraw program has a much better notion of the correct color space for most cameras than Photoshop or the camera itself. Since Photosphere and hdrgen take the JPEG from the camera, their color results are not as reliable. Also, you don't need HDR to measure a light source, as you don't care about light/dark detail. You just have to make sure your exposure is not clipped.

Cheers,
-Greg

Date: February 6, 2017 1:53:22 PM PST
Hi every one
So my question is : 1)is HDR able to give me the difference between white colors (different CCT) based on RGB value ?( or would I be able to extract CCT from an HDR image ?)
2) when the light source is getting cooler, R values getting slightly lower. Is this point of view acceptable here ? B and G value are almost the same. Is there a reason for that ?
Thank you very much !

[Mahlab, Seyedeh Farzaneh]

Thank you. So it means that I need to convert HDR images to TIFF format using dcraw? I was trying to run this code but it is giving me "command not found" error. Do I need to install dcraw software?
I also took raw images from the light source during photography. I know it is possible to convert them to HDR format. So which result would be more accurate for my porpuse ? using dcraw or raw2hdr ? Thanks

[Gregory J. Ward]

You have to find, download, and compile dcraw.c. Google will help with that. If you want to use raw2hdr, which calls dcraw, you will further need to download and install exiftool. Both are highly recommended and easy to install on most systems. No dependencies. (A miracle these days.)

Cheers,
-Greg

Subject: [HDRI] RGB value for different light sources
Date: February 8, 2017 2:36:31 PM PST
Thank you. So it means that I need to convert HDR images to TIFF format using dcraw? I was trying to run this code but it is giving me “command not found” error. Do I need to install dcraw software?
I also took raw images from the light source during photography. I know it is possible to convert them to HDR format. So which result would be more accurate for my porpuse ? using dcraw or raw2hdr ? Thanks