Final answer:
The light intensity of the output phosphor is measured in 1. lumens, which represents the luminous flux or total visible light emitted by a source.
Step-by-step explanation:
The light intensity of the output phosphor is measured in units called lumens. Lumens (lm) are the SI-derived unit for measuring the luminous flux, which is a measure of the total quantity of visible light emitted by a source. The luminous flux is particularly relevant when comparing light bulbs for their brightness or energy efficiency, such as comparing an old incandescent bulb with a new LED (light-emitting diode) or compact fluorescent lamp (CFL) bulb for equivalent brightness. To assess the brightness of different bulbs, one must compare the lumens each bulb puts out, as the wattage alone can be misleading.
Luminous intensity, on the other hand, measures the wavelength-weighted power emitted in a specific direction and is measured in candelas (cd). The candela takes into account the luminous efficacy of monochromatic radiation of a certain frequency. In contrast to lumens, candelas can be thought of as the luminous intensity in a particular direction, approximately equivalent to the light from a common candle.
When considering energy-efficient lighting, LED bulbs have become the standard for their ability to emit a specified number of lumens while using less power compared to incandescent and CFL bulbs. This illustrates the importance of considering luminous efficacy in units of lumens per Watt when determining the progress in lighting efficiency.