Flue gas analysis is accomplished at the stack, while the coal is burned at the other end of the plant. As coal bums it emits light. The light emitted by the flame contains information about the condition of the flame. The most important condition in relation to optimization is the air balance. The appearance of a flame will change as the quantity of air mixed with the fuel changes. An instrument can be used to analyze the flame appearance and provide information about its quality. This information can then be used to help optimize the flame quality.