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I checked the HSB values (using Photoshop Elements) of the same pixel before and after a saturation adjustment in Lightroom - here is the information:

Before Saturation: RGB: 159, 189, 233, Gray: 28% HSL: 216 degree, 32%, 91%

After Saturation: RGB: 99, 183, 255, Gray: 32% HSL: 208 degree, 61%, 100%

I don't know what algorithm Lightroom uses. It seems it doesn't simply increase S in HSL color space.

Does it use other color spaces, or use some functions to change all the 3 variables in HSL color space?

For saturation enhancement, one post talked about the way to do it in Gimp: Is there an equivalent to Adobe's "Vibrance" in Gimp? - the way to change saturation, while H and V remains the same (in Gimp the color space is HSV)

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I attempted to duplicate what you are describing. I created an image consisting of the color you specified. I had to guess at how much saturation you were adding in the adjustment.

In Lightroom (Develop>Basic>Color>Presence>Saturation) I added 100 saturation. (I'm not sure what steps you were going through, but if you specify, this may help get a clearer answer.)

I then exported this to a new file. Measuring the results, the hue barely changed. The resulting color was 134,187,253 HSB: 213 deg, 47%, 99%

I did this all in sRGB.

I think the shift you are seeing is caused by something else occurring coincidentally during your process. One possibility is a shift based on color profile. I tried the process again, beginning with ProPhoto RGB and exporting to sRGB, and this caused a large shift in hue, similar to what you are experiencing.

Try examining your color space and the conversion options. There are several options in the Adobe Conversion Engine (ACE), based on intent: Perceptual, Saturation, Relative Colorimetric, and Absolute Colorimetric.

You will always have a shift, though, if the color you've created in one color space is not available in the destination space. Here is some light reading on the topic: http://help.adobe.com/en_US/creativesuite/cs/using/WS6A727430-9717-42df-B578-C0AC705C54F0.html

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I am not sure. I don't currently have lightroom at my disposal, but I think it has to do with the colour profile conversions, and color spaces.

If an image is reduced to another color space it has less information, so the colors "wraps" to the lower color space model.

Initial color space:

Target color space:

Here is for example a conversion (in PS) from pure rgb model to a cmyk simulation, and the cian and magenta shifted a little in a hue wheel.

Initial image:

Converted image:

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HSL and RGB are 'color systems' maybe 'color models' not "color spaces".

Lightroom does tricky things behind the scene. If I remember, computations are done in a "ProPhoto RGB" color space. But if I sort of understand why in HSL the lightness parameter needs to be modified with the Saturation parameter, I do not understand why the Hue changes

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There is this script "Eg-Color vibrance" on http://registry.gimp.org/

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