useful life

Definition: Period of time a chemical, photo-sensitive material or disposable item such as a light bulb, battery, ink cartridge etc. can be kept or used before the item deteriorates in quality below a specificied limit. * The limit may vary with how critical an operation is e.g. the useful life for a bulb in a sensitometer is far shorter than the same bulb used in a slide projector. How long is a half? Typically, the useful life of many things is around half the total life e.g. a print developer's useful life when made up is, say, two hours, after which its strength falls significantly below its original strength (and what is industrially acceptable) but it can be used for at least another two hours with increased development time and if a small loss of contrast is acceptable. Some systems do not decay logarithmically, so the useful life can be much longer than half the total life e.g. NiCd batteries maintain their charge up to a point, then the voltage drops very rapidly.

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