The business I am in we use Greenlee 6060 to measure light in all new buildings and when setting up ems systems. Te are not super expensive (relative) but do fine for us. My company has about 50 of these meters. Ont thing I noticed is if outside and you turn measurement instrument away from sun even 10 degrees It really affects your light reception.
I've only heard of light meters for photography to get general light output in lumens/lux... wouldnt something that measured individual wavelength intensities be much more useful for plant growth?