Pages 9-10 (and others) from Notebook 6 by Forrest M. Mims III (September 8 and 12, 1973) discuss the use of near-IR LEDs as dual function emitters/detectors. The first witness statement on page 10 is by H. Edward Roberts, then president of MITS, Inc., and the designer of the Altair 8800 microcomputer in 1974. Visible light LEDs and diode lasers can also function as either emitters or detectors. Their advantages include two-way communication over a single optical path or optical fiber and the fact LEDs can function as spectrally selective photodiodes having a bandwidth as little as several nanometers. The latter characteristic means they do not require an optical filter, and this makes them ideally suited for use in inexpensive sun photometers and radiometers. Their major drawback in this role is that LEDs are more temperature sensitive than silicon photodiodes.