The visible spectrum is the portion of the electromagnetic spectrum that the human eye is capable of perceiving. The electromagnetic radiation in this range of wavelengths is called visible light or simply light.
There are no exact limits in the visible spectrum: a typical human eye will respond to wavelengths of 400-700 nm, although some people may be able to perceive wavelengths from 380-780 nm.
The color temperature of a light source is defined by comparing its color within the light spectrum of the light that emits a black body heated to a certain temperature. Therefore this color temperature is expressed in Kelvin, although does not expressly reflect a measure of temperature, since it is only a relative measure.
It is usually not perceptible to the naked eye, but by direct comparison between two lights as might be the observation of a sheet of plain paper under tungsten (incandescent lamp) and another under a fluorescent lamp (daylight) simultaneously.
Lux and Lumen
The difference between the lux and the lumen is that the lux takes into account the surface on which the light flux is distributed. 1000 lumens concentrated on one square meter illuminates the surface with 1000 lux. The same thousand lumens, spread over 10 square meters, produce an illuminance of only 100 lux. An illuminance of 500 lux is possible in a kitchen with a single fluorescent tube. But to illuminate a factory at the same level, will require tens of tubes. In other words, illuminate a larger area at the same level of lux requires a greater number of lumens.
The lux (lx)
It is the derivative unit of the International System of Units for the illuminance or lighting level. Equal to one lumen / m². It is used in photometry as a measure of luminance, taking into account the different wavelengths by the luminosity function, a standard model of the light sensitivity of the human eye.
The lumen (lm)
It is the unity of the International Measurement System for measuring luminous flux, a measure of the light power emitted by the source. Luminous flux differs from radiant flux in that the former includes the variable sensitivity of the human eye to different wavelengths of light and the latter involves all electromagnetic radiation emitted by the source according to the laws of Wien and Stefan-Boltzmann regardless of whether such radiation is visible or not.
The luminous efficacy of a light source is the relationship between the luminous flux (lumens) emitted by a light source and power (Watts) W.