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I've been trying to understand the difference between the brightness or value component (as part of HSB/HSV) and the luminance component (as part of LAB).

I've read this short article on xrite which claims that luminance is objectively measurable, while brightness is subjective.

From what I understand from the article, since luminance is measurable in candela/square meter, it's possible to get consistency across monitors that are properly calibrated. While brightness is measured in % and differs from monitor to monitor, so 70% brightness would be different across monitors, even if they were calibrated.

That said, is this the only difference? Specifically, do they have different effects on colors?

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user287001 answered on a technical level much better than I would've known. My answer is going to be brief and focused on what the main difference is that I see in a practical application.

HSB is not a color mode

That bares repeating now, HSB is not a color mode.

HSB is simply an alternative way of looking at the RGB color mode. But its not its own color mode. You can't go to Image Mode and change it to HSB. You will never see the Channels defined as HSB.

On the opposite site Lab is a color mode and you will get separate channels for each.

Let's say I put in a Hue of 150, Saturation of 100, Brightness of 100. Then another box with a Hue of 150, Saturation of 100, and Brightness of 50. I go into the Channels and its pretty hard to follow:

Red: enter image description here

Green: enter image description here

Blue: enter image description here

First Box #1 & #2 in RGB and HSB. Second Box #3 and #4 in RGB and HSB: enter image description here

Now let's see similar but in Lab. This first box is Lab 100, -72, 46. The second box we'll lower the L to 50.

Lightness: enter image description here

a channel: enter image description here

b channel: enter image description here

Lab info panel: enter image description here

Notice in looking at the a and b channels the two squares are identical. This is confirmed in the info panel. Adjusting the Lightness didn't alter the color at all.

My point being the two aren't really comparable. Brightness and the HSB is a way of explaining RGB differently. Lightness in LAB is an actual color mode that gives you different channels. The Lightness channel as you see doesn't shift the Hue at all because Hue only reside in other channels. The Brightness however as you can see in the 4th screenshot did shift our colors adjusting the amount of Green and Blue.

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  • I should have written "perceived" colors as part of "Specifically, do they have different effects on colors?" but it does seem to me that the example in LAB is a bit more on the warm side, is it not?
    – curious ♦
    Aug 30, 2019 at 18:17
  • @Emilie its not. I used a different color than in the RGB example. Had I entered the same HSB numbers into LAB color mode it didn't give me 100 L which is what I wanted for the start of the example so I just picked a green.
    – Ryan
    Aug 30, 2019 at 18:45
  • Ok so the perceived color is the same, but behind the scenes, the way it's spread between channels is separate, is that right?
    – curious ♦
    Aug 30, 2019 at 18:46
  • Yes. Well partially yes. The perceived colors can be the same. HSB doesn't really have anything to do with perception, its just a way to pick colors.
    – Ryan
    Aug 30, 2019 at 18:47
  • Yes, HSB uses RGB channels and LAB uses LAB channels :) -- and we look at them through a RGB monitor and... argh back to square one x) Great answer though!
    – curious ♦
    Aug 30, 2019 at 18:52
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HSV is an alternative numbering system for RGB colors in computer software. Photoshop uses it, too, but V (=value) is named B (=brightness) in Photoshop.

V (=Photoshop's B) is calculated from RGB numbers with the following formula:

V = the biggest of R, G and B divided by 255

V doesn't present actual amount of produced light, because RGB=128,127,128 has the same V (=50% if rounded to integer) as RGB=128,0,0

Computer screens have an adjustment named Brightness. It affects the amount of light produced by the backlight of the screen or it modifies the apparent brightness with some other method. The exact method and how it is presented in numbers is proprietary. Thus it's useless to compare brightness settings between screens.

More: As soon as one modifies the brightness adjustment of their display, they ruin its color calibration (if it was calibrated).

CIELAB color system (=Photoshop's LAB) uses L, which is often said to present perceived lightness. But it's not absolute, it's compared to a standard illuminant. That illuminant isn't absolutely fixed in the CIELAB system, but one used is named D50, which is an attempt to simulate diffuse daylight at certain conditions.

I have read from several sources that Adobe uses D50 as its reference white in Photoshop's LAB color mode. This is one of the sources: http://www.color-image.com/2011/10/the-reference-white-in-adobe-photoshop-lab-mode/

L=100% means the color is seen by the assumed standard watcher as bright as D50. It's not direct sunlight, it's more like what's seen in the sky near the horizon in the middle of the day.

If one gives certain CIELAB color numbers, he assumes certain color and relative lightness. A computer+program+display can produce that color if it's possible to produce in that system and the system is properly calibrated, but the meaning of the numbers LAB is known as soon as one knows the used standard illuminant.

RGB numbers or their equivalent HSV numbers mean a certain color only after one has also specified the used RGB implementation (sRGB, NTSC, Adobe RGB etc...). So, if you have two monitors which are both calibrated to show for ex.sRGB range absolutely right, you can believe both of them show sRGB range in the same way.

Absolute also means that the maximum luminous intensity given by RGB = 255, 255, 255 should be as said in sRGB specs. It says that max bright white should create luminous intensity 80 candelas/square meter.

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  • So just to check I'm understanding properly, on calibrated monitors with a color profile, both V and L will look consistent, but L refers to a standard also has uses other than just in monitors (proof reviewing, diffuse daylight, etc, according to the article)
    – curious ♦
    Aug 30, 2019 at 3:01
  • @Emilie You have understood it. I suggest you also read this: color-image.com/2012/02/… because it explains shortly why 6500K white is normal in monitors despite the fact that D50 means color temperature about 5000K. BTW Thanks for radical sentence straightening in your edits. Many of us who have learned English as adults have kept a great part of the twists of our native tongues also when writing or speaking in English.
    – user82991
    Aug 30, 2019 at 8:43
  • You're welcome! Non-native speaker here so it may not be perfect but I helped where I can :)
    – curious ♦
    Aug 30, 2019 at 14:30
  • My question was in part driven by this answer, if you don't mind taking a look: graphicdesign.stackexchange.com/a/127988/18168
    – curious ♦
    Aug 30, 2019 at 14:31
  • @Emilie the linked story creates flashy colorings but it doesn't make the wanted explicitly predefinable hue quantization + remapping. I guess this isn't lack of knowledge, but the question was ambiquous when the linked answer was written.The question was edited much later to a better shape. It was fixed to be a perfectly fitting question for User Wolff's answer which uses HSB extracting filter plugin. The question was fixed by the questioner, not Wolff. But the linked answer became obsolete..
    – user82991
    Aug 30, 2019 at 21:27
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Lab: L = Luminosity a = Green/Magenta b = Blue/Yellow it is an exact way to define a specific colour, much like CMYK and RGB.

you may have noticed that there is no way to specifically adjust Red/Cyan. Red/Cyan typically affects density (brightness, luminosity, whatever you want to call it). so you must reduce Magenta and Yellow if you want to increase Cyan, do the opposite to increase Red.

a quick search on the interwebs showed that Lab designed to approximate human vision.

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