Lighting. Light is also associated with emotions and pretty much everyone learns this intuitively. In a broader concept we have waves and both light and sound are waves, which means that sound and light shares very similar wave's properties. In here I'd like to comment that some people, for different reasons, may not feel the same under the same lighting or auditory conditions. There are certainly some cultural differences when we compare the color choices among different countries, but I'm not going to discuss that.

Source. Every light must have a source. In here I'd like to comment that sometimes this rule can be broken. In older Tom Raider games there is light underwater without any sources. This is physically wrong, but it doesn't look wrong in the game because it was an artistic choice and/or a technological limitation. It's pretty common for movies to alter the environment's brightness for artistic reasons. In Horror movies for example or during night scenes, there is a blue ambient light that is artificial but it's done that way because without light we can't see anything. An example of this is Alan Wake. All night scenes have an ambient light that doesn't match the real moonlight, but we have an artistic choice here. In this site I have given examples of bad lighting.

Lighting composition. Pretty much the same discussion about contrast in regards to geometry and textures. We have guiding lines with textures and geometry. Light also has the power to guide the player's attention and to create shapes by means of contrast.

Light's colors. Hourences makes a pretty good explanation here in regards to color systems. With light we have two opposite extremes: full bright and full dark, which translates to white and black. When we see something what we see is light being reflected and when light is reflected off a surface it contains information of it. Either there is light being reflected or no light at all. Black light doesn't exist because black is the absence of light itself. That's why we always add light to the world, unless there is some weird engine out there where we subtract light from the space. On the other hand, textures use a color system where the basic colors are not the same used for lights. The resulting confusion is that some colors, counter-intuitively, are non existent in lights. For example: gray light doesn't exist. What exists is white light that is more intense or less intense.

Unfortunately I think that most teachers never explain the differences at school and we are often left with an incomplete comprehension of colors and light. We always learn that there are three basic colors and mixing them in equal parts should result in white. However, how do we combine colors to result in black? That's the error! Black is not a combination of colors, but the absence of all colors! Any surface under the absence of light is going to be black no matter what color it is.

When we want to make surfaces darker we have to make the texture itself darker. Because lowering the light's brightness is going to make the whole space darker. If I were to explain this using physics I'd say that to make a surface darker means to change how much light it reflects and how much light it absorbs. In here I'd add that if we have a conflict between a lighting artist, a texture artist and a level designer, we have a problem.

Lighting composition. Hourences uses the same graph to discuss that lack of contrast and excessive contrast are to be avoided. This discussion can be very long because when it comes to light and contrast, there are two main types of contrast: intensity and colors. For example: black and white create the most extreme contrast of all. Yellow and red creates less contrast than red and blue. But we can always have contrast in the form of increasing or decreasing saturation too. Add in size, radius, number of lights, placement, alignment and there are literally thousands of different ways to compose lights.

About emotions there are certainly scientific research in this area relating to multiple fields. Hourences advocates against running away from clichés. For example: all pictures of the Sahara desert show the predominance of yellow, brown, orange, colors; all pictures of the Antartic land show the predominance of blue, white and grey. We can be creative, but at the same time, if we forget about clichés, we may end up with a lighting mood that doesn't match the emotions that we want to convey.

The last topic about lights that Hourences discusses regards realism. First, photorealism as in renders that take hours to complete is almost never a good idea. Think about movies, they always change lighting in a way or another due to the artistic choices made. Second, there isn't enough processing power to simulate light with complete physical accuracy. I'd say that we don't need to. The main issue here is about design and/or arts. Shadows, brightness, saturation, colors, contrast, all that depends more on designing and artistic choices than on technology itself.