Optical dating is a method of determining how long ago mineral grains were last exposed to daylight. It is useful to geologists and archaeologists who want to know when a particular event occured. Ages can be determined from a few hundred years to 100,000 years, and can be reliable when suitable methods are used and proper checks are done. The accuracy obtainable under optimum circumstances is about 5%. It is essential to contact an optical dating laboratory before collecting samples for optical dating.