For HID lighting. Just hang the thermometer from the light. Adjust the length, so it reads about 78F/25C after about 10 minutes. Now you also have a gauge to adjust your light height.
I have done that in the past to gauge the distance, but I found I was ending up with my lights further and further away. The radiant heat was getting right to the thermometer as it was right out in the light and it was getting very hot and I was ending up with semi-stretched nodes.
Thanks to RIU several weeks back I learned more about ambient and radiant heat. The plants can handle much higher radiant temps. Its the ambiant temps that have to be kept in the mid 70's, whereas the radiant temps can climb much higher. So I am not sure if the thermometer out in the open under the lights will give you the reading you want.
I am by no means the end all be all of knowledge. I am always learning and looking for the best methods and the correct info to achieve the desired results. Feel free to correct me if I am wrong or explain. You may have a different thermometer type in mind.
Theres a few threads on here about it. heres a more recent one:
https://www.rollitup.org/general-marijuana-growing/124410-radiant-heat.html I also saw another posted a few days back as well, and there are alot of older threads about it.