I use the Photone phone app. It can measure PPFD which is µmols/m²/s (micro-mols per square meter per second). The app is free but you need a diffuser. One can be made from paper or they sell a plastic clip on diffuser for $26. I just bought the clip-on diffuser but haven't used it much. The paper diffuser seems to work but I want more accuracy.Question how does one measure ymols? I am not familiar with them. It may be a stupid question.
EDIT: I forgot to mention the app also measures lumens (amount of visible light perceived by the human eye) and DLI. Daily Light Integral (DLI) is probably the most important light measurement, according to my research. It measures how many photons are striking your plant over a 24 hour period. An Internet search will find many references for the proper DLI for each phase of your plant.