In the recent months of study I've learned some new ways to classify different things happening with light that aren't usually referred to properly. In fact, many people are largely unaware of these
concepts consciously and so have trouble expressing them.
These aren't wavelength concepts in regards to color, but mostly physics and human physiology.
First, lets start with light sources and their hierarchy. The Sun is the ultimate light source to us. It is the brightest, and is unarguably number 1 in the hierarchy. It is also one of the most diverse lights we can see. It contains pretty much the entire range of the visible spectrum. This is another reason it is our standard for comparison. When you start to create a scene in your mind, you should compare it's light sources to the sun. The hierarchy after this is somewhat debatable. I would classify street lights and other powerful man-made lights next, then indoor weak lights after that, and finally Moonlight.
You may see things differently, and that is totally ok. Many factors can affect how bright the light appears. Surely you've seen how spotlights and flashlights can be focused into a stronger beam. This is the law of conservation of energy at work. When light is blocked, it has to go somewhere. It will continue to bounce until it's kinetic energy has played out. This is also why closer proximity makes a light brighter; the longer the distance from the object reflecting light, the more photons escape perpendicularly to the wavelength propagation and the less gets reflected. You and I could be talking about the same light source, but have we disclosed the variables that can change the perception of it?
Different types of lights are created in two main ways: incandescent and plasmic. Incandescent lighting is when an electric charge heats a piece of metal until it glows hot enough to produce light, or heats elemental powder into gas, which gives off light. This is basically light bulbs and flourescent bulbs. The idea is the same as a blackbody radiator, which is a device used in a spectral analyzer to determine the colors of things. Plasmic light is what happens when electricity gives off light. This is LED monitors and lightning. LED is a light emitting semi-conductor diode, which is a basic logic gate that can only send electricity one way.
Now to the important part, which is the difference between luminance and illuminance. We all know the difference, but there seems to be a common lack of these terms in art instruction/discussion. It is a really important thing to understand the difference. Luminance is a light source, usually one of those mentioned above. If it is not giving off light, it is reflecting light. It is illuminated. What makes the difference in our images, in terms of making something look like it is giving off light? It is incredibly variable, and the variable is the aperture which is receiving the light. The Luminous Flux vs the Luminous intensity.
Luminous flux is the total amount of light being emitted from all sources in the scene which could potentially reach the aperture (eye or camera). The luminous intensity is the total amount of light hitting the aperture. Think of luminous intensity as a percentage of the luminous flux. Our eyes have a baseline electric charge called Univariance, which measures the electromagnetic value in the photons and dilates or contracts our pupils accordingly. When the luminous intensity of a color exceeds the amount of light that is ideal for the aperture, overexposure happens. The eye is adjusting to the total amount of light, not the brightest value. This is why looking at a light source in a dimly light room (tungsten light bulb) will burn in to your eyes rather than adjusting to looking directly at the light.
So my theory in a nutshell is this: any color in a scene which is overexposed appears to be luminous. It will take work to figure out what will appear that way in a scene, but you start to get a feel for different situations after a while. Make comparisons. Let's say the sun is the brightest and has a value of 100 on my fake luminous flux meter. What percentage of that is hitting my eye? It doesn't have to be too exact, as long as you are making a judgement. Now let's compare the other light sources to the sun. What value on the luminous flux chart do you think a street light would be? Again, what percentage of that is hitting the eye? Hopefully this will help to make it easier to understand what you're seeing on histograms.
There are a ton of other things that can be done with exposure in a scene, but this has gone long enough, and I've covered the concepts that I intended. I leave you with the list of terms.
Getting this stuff right is pretty much everything when it comes to making surface values look right, and it will be much easier to research when you know what terms to use. Look deeper. I still have a long way to go with this concept.
Aperture-the eye or camera
Luminance- Creating light through heat/electric means
Illuminance- Being lit by a source and reflecting light. Don't forget the moon is reflecting, not creating.
Luminous flux- The total light that is radiating in your scene.
Luminous intensity- The total light hitting the eye or camera. Expressed as a percentage of the luminous flux.
Univariance- The electrical charge in the eye that determines the size of the aperture.