The Lux Meter is usually used to measure illumination. The illumination is how level of luminous flux is falling on a surface area. The luminous flux is visible component that is defined in radiant flux (light power) divided by relative sensitivity of human eyes over the visible spectrum. This means the Lux is well fit to light level from sense of human eyes.
However there is a difference between spectra response of ordinary silicon photo diode and human eyes, it cannot be used for the lux meter. Some photo diodes for illumination sensor have a color compensation filter on the window to correct its spectra response.
DIY Lux Meter: [Link]