In cinematography, an extremely important factor in determining a lighting look of an entire film (or a part, whatever you're going for). There is a simple formula, key plus fill over fill, that is used to calculate the ratio. The only parts of this formula are the key light and the fill light. The light reading for the key side, added to the light reading for the fill side, divided by the fill side makes a ratio, and the ratio can be used much faster to maintain a look throughout multiple shots than guessing. Also, ratios make it easier to identify a lighting style (such as chiaroscuro or it's opposite, no-tan) by the ratio. For example, a no-tan lighting ratio would be 1:1. For every bit of key light, there is an equal amount of fill light. This is confusing. I'm sorry. Hopefully the hardlinks to the lighting words will help.