How to measure digital image's brightness in RGB 8-bit scale

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I have a digital image and I am looking for a image processing tool that can help
me measure the complete image's brightness in RGB scale or CMYK scale.
I see that Photoshop Cloud Creative tool provides for measure the brightness for each
pixel in the image. But I need a way to measure the brightness for the complete image.

Please help.

Thanks a lot in advance.
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sj net

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Posted 5 years ago

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Chris Cox

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There is no one, single brightness. You need to better define whatever it is you are trying to accomplish.
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sj net

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Hi Chris,

I am helping my daughter with the following Science experiment that was already done by another student.

Summarized requirement for me: To measure the amount of fading in the digital pictures that are taken. More details are below.

The Effects of Glass and Plastic Protective Covers on Paper Fading


The goal of this project is to measure the effect of glass and plastic covers on the amount of fading of paper exposed to sunlight.

The project was done by exposing 4 identical pieces of black construction paper to sunlight for 12 days.
Each was either uncovered or covered with a different type of glass or plastic: window glass, museum
glass, acryllic plastic, uncovered. The four samples, plus an unexposed control sample, were
photographed with a digital camera. The amount of fading was found by measuring the brightness of the
digital picture in both RGB and CYMB color scales using Photoshop.


I found that the paper protected by museum glass faded the worst, window glass was second worst,
acryllic plastic did second best, and the unprotected paper faded the least. On the CYMB scale, the black
faded the most, followed by cyan and magenta (nearly the same) and then yellow, which faded very little.

I conclude that using glass or plastic covers to protect construction paper exposed to sunlight from fading
is not useful. This is unexpected since I thought that the covers would absorb some ultraviolet light, which
I thought caused fading.
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Chris Cox

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Ok, they could select an area and use the average filter to average the values inside the selection, then read out the values with the eyedropper. Or they could set the eyedropper to a larger area to average the values and sample from the center of the region (51x51 should be good).

But you should not use CMYK values - because you'll be converting from RGB to CMYK with unknown black generation curves. Use the RGB or Grayscale values. (it is interesting to note that some papers/pigments have a hue change after fading)

Also, you can only make that measurement if you make sure the camera or scanner exposures are EXACTLY the same in each case. One way around that is to photograph all the samples at one time (with labels) under very diffuse light (like a cloudy day) - otherwise you'd probably have to shoot under controlled lighting with manual camera exposure/time settings. Unless you do that, the camera auto exposure will adjust the brightness of the samples to be about the same, invalidating the experimental results. It would also be best to shoot RAW and minimize the image processing - since the JPEG algorithms in most cameras will do funny things to your color values.

Note that museums control their lighting to minimize UV in the first place. And different kinds of glass and plastic will have different UV absorption. But you will get some fading just from visible (non-UV) light, and some materials will fade with age, exposure to air, or heat.