Both the VASI an PAPI units have two bright lights per unit, and then a filter, before the light exits the unit. The fliter/lense is one piece and has red on the bottom and white on the top. There is a metel bar the comes out, from the filter that keeps the red and white light seperate, so you dont see both lights at any alt. Then whole unit is tilted to the angle that will give the light condition needed for that runway. On a PAPI each unit has a slight difference in how much it is tilted, so that they dont change colors at the same time. Its a very basic system, but does the job very well.