I understand that for example light blue is brighter than dark blue. But I don't understand really what brigthness in essence is. Wikipedia states that "brightness is the perception elicited by the luminance of a visual target".
But what are the units of measurement for brightness?
When you say "light blue" is lighter than "dark blue" you may be mixing two concepts together without knowing it. In one sense, the colors light blue and dark blue are two different colors - that is, they are a different proportion of the three primary colors. You could also be talking about different brightness levels within the same proportion of color mixing.
For example, I can take a color that is 33% red, 33% blue, and 34% green - you will always think this is a 'barely greenish' color due to there being slightly more green than the other colors, but that color can be at any brightness level from nearly black to nearly white.
Play around with color pickers on the web to understand this. Here's a good one - I like this one because you pick colors, and then on the side it shows you the brightness levels of that color. I think within the wheel, you'll see colors you could call "light" blue and "dark" blue - and then you'll see they both have brightness levels.
http://www.w3schools.com/tags/ref_colorpicker.asp
The units of measurement for an amount of light IS lumens, but it's not affected by color. It's a measure of the amount of light, and the scale is adjusted to make sense to the human eye. It's basically a measurement of the photon density - not an absolute number of photons. This is perceived as brightness by eyeballs and cameras.
http://en.wikipedia.org/wiki/Lumen_(unit)
Candela is the unit of measurement for total amount of photons period. http://en.wikipedia.org/wiki/Candela
The unit of measure for brightness is the lumen. I know it is used to measure the amount of light emmited from a source.
But in your question, I think you are talking about the reflection of light off a source. I may be wrong but I think it is also measured in lumens.
The presence of black (in your example of dark vs light blue) shows lower lumen units than the "lighter" versions of a color which in essence are just reflecting more white light off their surface.
Each pixel is composed of three colors: red, green and blue. An image with a color depth of 8-bit has 2^8 distinct values per color channel. This value represents brightness. If the brightness is set to zero than it represents black. If the value is set to 256 for red than it means red elicits color at maximum power. In other words, brightness is the value of the color component of a pixel.
In photoshop brigthness is measured in percentages
