Photometry based on the work of the American astronomer Harold Lester Johnson (1921–80). It falls into three natural parts: UBV, RI, and infrared photometry. UBV photometry was based on a photomultiplier sensitive to the wavelength range 300–600 nm. The U and B magnitudes were formed by splitting the old photographic magnitude into blue (B) and ultraviolet (U) parts. The V magnitude is similar to but more accurate than the old visual system. Johnson later extended this work into the red and infrared with two further filters designated R and I, using a photomultiplier sensitive to wavelengths 0.3–1.1 µm. The third innovation was the extension into infrared photometry with J, H, K, L, M, and N filters covering the wavelength range 1.25–10.4 µm.
Subjects: Astronomy and Astrophysics.