I’ve been wanting to understand PAR and light levels better anyway, so I did a bit of math to follow up on Cornell’s DLI recommendations.

### Unit conversions

- I’m assuming a quantum meter that measures PAR flux as Photosynthetic Photon Flux Density (PPFD) using units of micro-moles per square meter per second (µmol/m2/s)
- Cornell specifies daily light integrals of PAR in moles per square meter per day (mol/m2/d)
- 1 mole is 1,000,000 micro-moles
- There are 60*60 = 3600 seconds in an hour
- A 12 hour light cycle would provide 3600 * 12 = 43200 seconds of light per day
- A 16 hour light cycle would provide 3600 * 16 = 57600 seconds of light per day
- An 18 hour light cycle would provide 3600 * 18 = 64800 seconds of light per day

### Lettuce: 12 hour light cycle – target DLI of 15 mol/m2/d

For a 12 hour light cycle, here’s how I calculate the target PAR measurement to satisfy Cornell’s minimum recommendation of 15 mol/m2/d for lettuce:

- We need 15 mol/m2 of PAR, so that’s equal to 15 * 1,000,000 = 15,000,000 µmol/m2
- The lights will be on for 12 hours during that day, so that’s 43200 seconds
- We need a PAR flux of 15,000,000 µmol/m2 / 43200 seconds which reduces to about
**347 µmol/m2/s** for the reading we want to see on the quantum meter

### Lettuce: 16 hour light cycle – target DLI of 15 mol/m2/d

A 16 hour light cycle gives 4 more hours to hit the target DLI, so the PAR flux can be lower. The target measurement would go down to:

15,000,000 µmol/m2 / 57600 seconds = **260 µmol/m2/s** on the quantum meter

**But**, lettuce is long day facultative. We don’t want it to bolt, so running the lights for more than 14 hours a day might not be a good idea.

### Tomatoes: 18 hour light cycle – target DLI of 20 mol/m2/d

Since tomatoes are day-neutral, we can run the lights for longer. For a light cycle of 18 hours on and 6 hours off, the target PAR flux on the quantum meter would be:

20,000,000 µmol/m2 / 64800 seconds = **309 µmol/m2/s**

### Conclusions

In the the other thread I linked to earlier, @serein calculated a PAR of 48 µmol/m2/s. According to the Cornell Lettuce Handbook, that’s almost enough for germinating seeds, but it wouldn’t be adequate for vegetative growth. [**edit**: I’m not sure if that was for the PFC 1.0 LEDs though, he might have been using other lamps.]

[**edit # 2**: I fixed a couple math errors for the lettuce.]