Who has HPS and a PAR meter?

Why do I ask? because I'm curious to know what the spectrum of the glowing arc tube is right after the HPS is turned off. It should change as it cools. I'm wondering if it's mostly far red, red or what. Because if it's far red then it would be an end of day far red treatment. If it's normal red then it would counter the far red in the spectrum when the light is on. Maybe it's far red and that's why HPS plants stretch, which is the effect of end of day far red. A video would be good, showing the changing spectrum as it cools. I don't think anyone ever did this before so it may be a first.

Doesn't it take like only 5-10 seconds to stop glowing??

goofy81 said:

Doesn't it take like only 5-10 seconds to stop glowing??

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lol about

Precisely. So if it puts out far red in that glow it would act as "end of day far red". If you're not familiar with that you can search it easy enough. But we need to find out just what the spectrum of that glow is.

I guess you'd actually need a spectroradiometer to see the spectrum, not a PAR meter. Not many people have one of them, since they cost plenty. You can see what I mean on the Spectrum King YouTube channel. It shows the whole spectral graph. Anyway, it's known that HPS has a red/far red ratio of 1.05, so it's pretty much half and half. That's why you need to shine a red light on the plants for 5 minutes after the HPS shuts off to neutralize the far red stretching effect. Apparently the red that comes from the afterglow of the HPS doesn't last long enough to completely neutralize it, or else the last light from it is far red instead of red.

goofy81 said:

Doesn't it take like only 5-10 seconds to stop glowing??

Click to expand...

Yeah you're right, it's probably not long enough to have an effect. There's almost 50% far red in the normal HPS spectrum though, which is why it makes plants stretchy. I just thought the glow might be even more far red dominant but as you said, it's probably too short a time to have much effect anyway.