What is the difference between brightness and luminance? (HSB vs. LAB)

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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?

Answer

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.

Attribution
Source : Link , Question Author : curious , Answer Author : Ryan

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