Evolution of Vacuum Tubes
X-ray image depicting a snapshot of the evolution of vacuum electron tubes. Left to right: a 1922 Western Electric 216A; a 1936 RCA 6F7; and a 1942 Sylvania 6SL7GT. Vacuum tubes brought the dawn of the age of electronics in the early twentieth century before they were replaced by transistors after the 1950s. Today, most types of vacuum tubes have been substituted by transistors or semiconductor devices. Image taken with an X-ray system (ZEISS Xradia Context).
—Herminso Villarraga-Gómez, Carl Zeiss Industrial Metrology, Wixom, MI, USA