The difference between instant start ballasts and rapid start ballasts is a key issue in fluorescent lighting controls. Instant start ballasts give an initial burst of electricity that brings a fluorescent lamp to full brightness – well instantly. The negative of instant start ballasts is that if a lamp is turned on and off frequently, then the lamp life is shortened substantially. A rapid start ballast warms the lamp before it is turned on which maintains lamp life. This is a key consideration if you are installing occupancy sensors which will turn lights on and off frequently.
In T-8 fluorescent fixtures you have the option of instant start or rapid start ballasts. In T-5 lamps, all ballasts are rapid start.