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I ignitron A three terminal device capable of switching extremely large currents (hundreds of amperes) at high voltages. It has electrical characteristics similar to an SCR. It works by controlling a gas discharge between an anode and cathode. The anode and cathode are well insulated, so when the device is off, it can separate several thousand volts. The device is turned on by initiating a gas discharge. This is accomplished with a minute arc to the cathode with the ignitor terminal. Once the discharge is initiated, the current flow between cathode and anode maintains the discharge. Ignitrons are used in high current regulated voltage supplies or for controlling automated spot welding equipment.

illuminance Luminous flux density on a surface, i.e., luminous flux incident per unit area of a surface, when the latter is uniformly illuminated. Synonymous with illumination, a more general term. Also synonymous with the intensity of illumination. Practical units of measurement are lumen per square meter or meter-candle or lux, lumens per square foot or foot-candle, lumens per square centimeter or phot. 1 phot = 10,000 lux = 929.03 foot-candles. Note that luminance of a source is the number of lumens emitted per solid angle (steradian) by a unit source area. Visual acuity and other properties of vision depend on the illumination, and minimum values are tabulated for various occupations in lighting codes. The eye has maximum efficiency between 10 and 100 foot-candles.