Apart from being affordable, the Philips bulb can get especially dim: At the lowest setting our light meter read it at 14 lux, and then 13 lux after five minutes. That put the Philips among the dimmest 60-watt equivalents we tested, only slightly brighter than the Cree, and certainly low enough light to render book text illegible. At a distance of 6 feet, the Philips measured 5 lux at the dimmest setting, holding constant after five minutes. Reflected off the wall at 6 feet, it registered 4 lux at the lowest, again holding steady after five minutes.
Luminous efficacy of a light source may be defined in two ways. The radiant luminous efficacy (LER) is the ratio of the visible light flux emitted (the luminous flux ) to the total power radiated over all wavelengths. The source luminous efficacy (LES) is the ratio of the visible light flux emitted (the luminous flux ) to the total power input to the source, such as a lamp.  Visible light is measured in lumens , a unit which is defined in part by the differing sensitivity of the human eye to different wavelengths of light. Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy (LER) is a measure of how well the distribution of energy matches the perception of the eye. The units of luminous efficacy are "lumens per watt" (lpw). The maximum LER possible is 683 lm/W for monochromatic green light at 555 nanometers wavelength, the peak sensitivity of the human eye.