How do you calibrate a Colorimeter?

You make several solutions of precisely known concentrations of whatever chemical you're yzing (including one of just water, to use as a control or blank). You put each one in the colorimeter and take a reading. Then you have a calibration - you know how the colorimeter responds to a certain concentration of chemical, so if you put in an unknown sample, you can tell what the concentration is by comparing it to the knowns. You usually do this by making a graph of concentration vs colorimeter response.