Light is electromagnetic wave with specific wavelength (frequency). Humans are tailored to sense this wave in close range so-called visible spectrum.
Spectrum visible [source]:
For plants it is important the light of specific wavelength which can be absorbed by molecule of chlorofil.
Plant absorption band[source]:
Example of light sensitive components:
Absorption of light have similar effect in all cases. The current flow grow with more incoming light. But this is little tricky. Not all the wavelengths excites the component with the same rate. That is why ordinary lux metrs are probably not the best solution for growers. The usual lux meter sense just the light from visible spectra but plants usable light lie even out of this range and vice versa the green light which our eyes (and lux meters) are most sensitive is badly utilizable by plant. Concrete wavelength for particle can be found in it's datasheet.
Digital light sensor. It communicates with arduino by I²C protocol. It's advantage is that output is directly in lux. So after connecting to arduino it is function luxmeter! The handicap is inappropriate sensing spectra (as mentioned before). Sensing particle is phototransistor. It's sensing spectral band is on picture [source: datasheet].
Spectral bend [source:datasheet]:
Very interesting modul. It includes matrix 8×8 with photodiodes. Every two lines are sensitive to different light color (wavelength). Primary is this modul used for color differentiation. But his ability of differentiate intesity in concrete color spectra can be very valued by growers because it is very valuable detail of light intesity in green, blue and red part of spectra. Disadvantage is that the ouput is just relative so it is not tabulated (For this purpose one must calibrate this sensor).