Luminance (relative)

Imagine you have a flashlight and you shine it on a white piece of paper. The brightness that you see is called luminance, which is basically how bright something appears to your eyes.

Now, let's say you shine the same flashlight on a black piece of paper. The brightness that you see will be different, even though you haven't changed the flashlight. This is because the luminance of an object is relative to its surroundings – the same object can appear brighter or dimmer depending on what it's next to.

Relative luminance is a way of measuring this difference in brightness. It's a number that tells you how bright an object appears compared to a reference point. The reference point is usually the brightest object in the scene (like a white piece of paper) or the darkest object (like a black piece of paper).

For example, let's say you have two rooms that both have the same light bulb. In one room, the walls are painted white and in the other room, the walls are painted black. Even though the light bulb is the same, the walls will affect the luminance of the room. The white room will appear brighter because of the higher relative luminance of the walls, while the black room will appear darker because of the lower relative luminance of the walls.

In summary, luminance is how bright something appears, and relative luminance is a measurement of how bright something appears compared to its surroundings.