i think the sun is too strong on my lettuce, it's fading a bit and got some weird red spots on it (kinda looks like its turning purple) not really sure what's up with it.
http://www.cornellcea.com/attachments/Cornell CEA Lettuce Handbook .pdf
See page 10 and 13.
Uniform light distribution is required in the Pond Growing Area. A supplemental light intensity within the range of 100-200 µmol/m 2 /s (for a total of 17 mol/m 2 /d 1 of both natural and supplemental lighting) at the plant level is recommended. It should be noted that 17 mol/m 2 /d is the light integral that worked best for the particular cultivar of boston bibb lettuce that we used. For some cultivars, 15 or mol/m 2 /d is the maximum amount of light that can be used before the physiological condition called tipburn occurs.
Also:
An overhead fan (paddle fan - Figure 14) is used to blow air vertically down onto the lettuce plants at the rate of 140 cubic feet per minute. The airflow increases plant transpiration. This increase in transpiration increases the transport of nutrients, especially calcium, from the roots to the young, fast-growing lettuce leaves. The greater rate of nutrient transport provides sufficient amounts of calcium to the leaves and, therefore, prevents tipburn. Without this airflow, lettuce must be grown under reduced light levels (for example at 12 mol/m 2 /d instead of 17 mol/m 2 /d but realize that this data is only for cultivar Ostinata which is no longer available), which slows the rate of growth. The actual daily light integral target that can be achieved with and without vertical airflow before tip burn occurs is a function of cultivar selection, spacing and airflow. The numbers given above are examples of what has been successful in our situation and are not the only solution and no attempt was made to establish airflow maxima and minima.
This is another paper, and uses another light schedule.
http://www.agriculturejournals.cz/publicFiles/71067.pdf
The results reveal the following: (1) judged by the dynamics of anti-oxidative enzyme activity, there was no light stress to occurr in the 100, 200 and 400 μmol/m2 s treatments, a mild light stress occurred in the 600 μmol/m2 s treatment, and a serious light stress occurred in the 800 μmol/m2 s treatment; (2) increased light intensity gradually reduced the contents of soluble protein and nitrate in lettuce, whereas the content of soluble sugar remarkably increased. The biomass of a single plant of lettuce in the 600 µmol/m2 streatment was the highest and second highest in the 400 µmol/m2 streatment but was the lowest in the 100 µmol/m2 s treatment. No significant difference in the biomass of single plant was observed between the 400 and 600 µmol/m2 s treatments. Based on these results, the range of 400 µmol/m2 s to 600 µmol/m2 s is a recommendable light intensity for lettuce production.
So basically if you start going towards 600 PPFD you will start getting light stress on the lettuce.
So try to stay around 200-600.
900 PPFD is nice for marijunana because it can take that much light with ease, but for lettuce you need a mild light intensity. Like 200-400 PPFD.
It's not the colour of the light that's the problem, it's the intensity.
If you are having problems, lower light intensity.
Or maybe add a fan for airflow.
Lettuce is no marijuana, marijunana loves to be in the spot lights.
Lettuce is like a shy kid.
Also, I'm not completely sure but I bet hydroponic lettuce is also able to take a higher light intensity without tip burn than soil grown lettuce.
