Three types of watches are covered by this specification. Types I and II are general purpose watches intended for use in low or no light situations. Type III watch is intended for use by bomber pilots and navigators in low or no light situations on prolonged flights. The intended use, available maintenance and storage of the watch will determine which type of watch is required.

The watch covered by this specification is military unique. It has the unique military requirement of being able to be read after 8 hours in total darkness without any intervention on the part of the user, for example, by the push of a button. No commercial equivalent exists.

following criteria is a guide for watch type selection to match user requirements:

Type I: Analog, short life (2 years), non-maintainable, antimagnetic, water-resistant Type II: Analog, long life (5-10 years), maintainable, antimagnetic, water-resistant, high altitude, corrosion-resistant Type III: Analog, long life (5-10 years), maintainable, antimagnetic, water-resistant, high altitude, corrosion-resistant, with elapsed time ring