Some photoelectric observations of lightning

During the summer of 1965, a calibrated silicon photodiode and an oscilloscope were used as a fast-response lightning photometer covering the visible and near-infrared regions of the spectrum from 0.4 to 1.1 μ. Simultaneous still photographs of the discharge channels were taken to determine how the photoelectric pulse profiles depend on the type of lightning (i.e., cloud-to-ground versus cloud-to-cloud) and the geometry of the channels. These observations represent the first attempt at a photometric study of lightning with correlated photographs of the discharge channels. The photodiode detector consisted of an Edgerton, Germeshausen & Grier model 560-561 ‘lite-mike’ and ‘detector head’ This system provided a voltage output, proportional to the light flux input, which was recorded as a function of time on a fast oscilloscope (Tektronix type 555). The photodiode and associated circuitry were linear over a wide range of incident light levels (within 5% over 7 decades) and had a response time of less than 1 μsec. The relative spectral response of the detector was calibrated by the manufacturer and is shown in Figure 1. The manufacturer also provided a pulse calibration factor of 7.7 mv/mw of incident light power at 800 mμ. The area of the photosensitive detector was 0.0728 cm2.