The formal relationship of EV to luminance or illuminance has limitations. The standard international unit for the illuminance of ambient light is the lux. If illuminance is interpreted loosely, measurements with a hemispherical sensor indicate "scene illuminance". Useful daylight illuminance focuses on the direct sunlight that falls into a space. Two national standards now allow for variation in illuminance when using lamps of different spectra. Often task lighting refers to increasing illuminance to better accomplish a specific activity. A poorly placed lamp may render text illegible, regardless of illuminance level. The relationship between illuminance and CCT was not supported by subsequent work. The eyes of the larvae are underdeveloped, but are capable of discriminating changes in illuminance. The foot-candle is a non-metric unit of illuminance that is used in photography. Illuminance is measured with a flat receptor. An average 60-year-old person requires three times more illuminance than an average 20-year-old boy.