Temperature Readings

I know I have read quite a bit in regard to temperatures of the indoor grow room and what is ideal for flowering. My question is when they say anything above 90f is bad are they measuring ambient temps of room or at the tops of the plants? You always hear keep the light as close as possible without burning the plant which in my case is aprox. 8-10 inches with 400 watt HPS but if this is case the temps measured at the tops is going to be much higher than the ambient temp. of room. So can somebody tell me a little more specifically as I would greatly appreciate any help. I can maintain ambient temps of 81f but I dont want to flower yet until I have this answer as I can exhaust more heat from room if I have to but I am on pretty tight budget. Thanks guys

I personally hang a thermometer probe 1-2" above the tops of the plants, directly under the light source. If you're trying to stay under a threshold, directly under the lights is going to be the ambient temp, plus whatever direct heat the light source is adding, and that's the point of extreme for your plants (it's also where the most growth is usually happening).

Thanks for taking the time to reply. Makes sense. The tops are exploding with growth right now.

No, you measure ambient air temps with the thermometer at the plant tops but shaded from direct light, and you should really keep those temps in the seventies.

i keep my temp at 23 celcius or 73 in Fahrenheit for flowering... and that is at the bottom of my plants but at the top of the pots that they are growing in.. and i have never ever had a problem with anything

i keep mine 400w a foot away