here is a turnkey solution for measuring flashlight output in lumens…
Texas_Ace BLF Calibrated Lumen tube / Sphere No math skills needed
the calculation of Lux has additional factors, that include distance, and illuminated area (beam shape):
for example:
both lights produce the same lumens, but the LUX are different because the light is spread over different size areas.
more info:
How to measure light in foot-candles, lumens and lux
and here is a Spectrometer
if you get interested in measuring Quality of light, in addition to Quantity of light.