When grow lights are tested, there's usually a mention that the light was running for 30 minutes. I suspect that's because, under test conditions, you want to ensure a "steady state".
Yes, it takes time for things to warm up but with we're not dealing with CRT's and tube. Today it's IC's and diodes that I suspect are designed to reach operational state
very quickly.
What sort of variance are you seeing and are you measuring from the same distance from source?
With the meter that you're using, have you been able to factor that the meter might be changing?
"Re. waiting for it to settle in" - how large is the change and how aggressive are you being with your light levels?
My grow is averaging right around 1kµmol so, if I was seeing a 200µmol range and if I was using, God help me, Photone, I'd wait. OTOH, if the variance is such that it won't taco your leaves, I don't know that it's worth waiting.
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A big thing to keep in mind is that a light meter/PAR meter will tell you how much light is hitting the canopy. Your plants can tell you how much light they can use.
It's handy to have a number. That will tell you that you're in the range of where you should be and it's also helpful because you can use the numbers for your next grow but the bigger issue is to set your light levels and then see how the plants react. If the leaves taco or if they rotate around the petiole (think Venetian blind), then that's too much light. The number on the meter is helpful but how the plant is reacting is the only thing that matters.
If you see a reaction to the new light levels, raise the light 1" or 2" or turn the dimmer down maybe 10% and let the plants go back to normal. If you're in veg, you'll find that you can bump light a little bit every few days.
These are the average PPFD values from my current grow. They are pretty typical but it's a hydro grow and I think plants in hydro mature more quickly than in soil. Your mileage will vary but the idea is the same - a gradual increase in light, based on what the plant can use to create food to fuel its growth.
