Optical transient detection

Optical transient detection identifies transient luminous events, such as lightning flashes, by their temporal signature rather than by spatial imaging. Rather than resolving a scene structurally, the method looks for rapid photon-count pulses at a specific wavelength associated with the event of interest.

Implementations built for lightning detection center on the 777.4 nanometer neutral-atomic-oxygen emission line produced by lightning discharges. The Geostationary Lightning Mapper (GLM) ^[1] is a single-channel near-infrared detector with a bandpass of about 1 nanometer around this line, operating at a 2 millisecond frame rate with a nadir footprint of about 8 kilometers (widening to about 14 kilometers at the edge of its field of view). It detects total lightning, including in-cloud, cloud-to-cloud, and cloud-to-ground discharges, day and night, at a flash detection efficiency of 70 to 90 percent. GLM flies on GOES-16, GOES-17, GOES-18, and GOES-19.

The MTG Lightning Imager (LI) ^[2] detects optical pulses in a wider band of about 1.9 nanometers centered on the same 777.4 nanometer oxygen line, using four cameras, each operating at 1 kilohertz, to cover four quadrants of the observed disc. It provides a sub-satellite spatial resolution of about 4.5 kilometers and continuous day and night detection of intra-cloud, cloud-to-cloud, and cloud-to-ground lightning. LI flies on MTG-I (Meteosat-12).

Because the method detects a temporal pulse rather than forming a conventional image, it requires only a single spectral band and does not depend on daylight, remaining tolerant of cloud cover for lightning-discharge detection.