ASA exposure index ASA scale British candle DIN number Hefner candle Scheiner scale bougie decimale candle candle lumen candle power candle-foot candle-hour candle-meter decimal candle exposure exposure meter f-stop film rating film speed flux foot-candle intensity international candle lamp-hour lens opening light light quantum lumen lumen meter lumen-hour lumeter luminous flux luminous intensity luminous power lux photon quantum shutter speed time exposure unit of flux unit of light