I can't believe I havn't seen this answer. For $125 or less on a dream piece of equipment, I would recommend people buy a PAR sensor. No need to spend extra for the meter. Plug it into a multi-meter.
Best single purchase I ever made for keeping planted tanks.
or u can read later for my lux meter that i siliconed so its water proof. its 25 bux so far in product and silicone. and its about to get calibrated. i estimate 63 lux per PAR..