The optical pulse sensor consists of a photodiode at the focus of a wide angle field-of-view lens, and was used to observe the bright flashes of light produced during a lightning discharge. During the mid 1980s, this instrument was flown on a high altitude aircraft to observe lightning from above cloud top. A bandpass filter was installed at the front of the lens and was used to pass one specific lightning spectral line, such as the neutral atomic oxygen line (777.4 nm) or the neutral atmoic nitrogen line (868.3 nm).