The rated lamp life is defined as the time required for what proportion of the lamps to fail?

The definition of rated lamp life refers specifically to the time period in which 50% of a particular type of lamp is expected to fail. This means that when a lamp is said to have a certain rated life, that duration reflects the point in time where half of all tested lamps under standard conditions would still be operational. This measurement is significant because it provides a standardized benchmark for consumers and manufacturers to gauge the longevity and reliability of lighting products.

The other options indicate proportions (like a quarter, a third, or two-thirds) that do not align with this established definition of rated life, as they suggest different measures of performance or longevity that are not commonly used in the lighting industry. Therefore, the rated lamp life is universally recognized as the lifespan at which 50% of lamps will have burnt out, making the choice representing half the most accurate and relevant.