Chemiluminescence

Chemiluminescence (or Chemoluminescence) is a kind of luminescence. It is a process of making light from a chemical reaction. Chemiluminescence in biological systems is called bioluminescence. 
The light made in chemiluminiscence is not directly related to heat. A simple two step example of a reaction of A, and B, that produces; C, D, and light.
C* is an excited state of C. 
This reaction is simpler than most chemiluminesence reactions. The excited state happens when electrons are pushed into a higher orbit by the energy of a chemical reaction. The excited state is less stable than the ground state. The electrons in the excited state fall to the ground state (lower energy), emitting light. 
The amount of measurable light made is called "radiant intensity": ICL (photons emitted per second). 
Analytical applications.
The device needed to measure the light made is simple. It needs something to hold the sample, and a photomultiplier tube. There are three ways for this to be used in chemical measurements. 
Analysis of gases.
The method measures small amounts of atmospheric pollutants. A common method measures the amount of nitrogen monoxide by reacting it with ozone. The light made has a wavelength range of 600 to 2800 nm.
Analysis of liquids.
Luminol is the most well known type of compound used for chemiluminescence in liquids. 
Analysis of cell organelles.
Ca2+(calcium) inside of different parts of cells like the mitochondria can make light when it reacts with a protein from jellyfish named aequorin. Nitric oxide(NO) is in cells and is a way for cells to talk to each other, this can be measured with the compound luminol.