Through the Lens (TTL) flash metering
Technology - Technology
TTL metering means that the amount of (flash) light is measured behind the lens. Normal daylight can be measured "off-line" (=before the exposure). Obviously this is not possible in flash photography because you have just one single flash burst and no constant light. Therefore the light from a flash is measured by one or more sensors below the light channel (behind the mirror and curtain). A certain amount of (flash-) light is reflected from the film/sensor towards these sensors (see picture below). The camera measures the received light and stops the flash output if necessary.
 

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