LUX and HPS lamp question

I understand that during flowering the optimal lux should be between 45k and 70k. I’m using a 400w HPS in a 2x4 tent. At 12 inches from the lamp I’m only reading 27,500 lux with a brand new lamp. This would seem to indicate that to reach 45k I would need 2 400w fixtures.

2 fixtures in this tent is going to produce a ton of heat. Am I missing something? Thanks for your help!

Update. I changed out the 400w HPS to a 1000w HPS and now have 45000 LUX at 12 inches directly under the lamp.

I expected the 1000w to really pump up the LUX to the higher end of 60 to 70k.

I must be missing something. I’ve read so many posts where people are using 250w HPS in a 2x4 tent with great results.

Can anyone help?

I’m not a high pressure sodium person, but I can tag @Hellraiser @Oldguy @dbrn32


I don’t have a lux meter. Never really sience’d it that far to be honest. :thinking:

I’ve just based it on 50 w / sq ft and covered my space accordingly.
Currently got 2200 w covering 50 sq ft and I should have another 300 w to bring me up to where I should be for proper coverage.
I’ll find out after this grow if I’m gonna add more light or cut back on plant count.

Fraid I’m not much help on this one @Covertgrower


Are you sure there is not a x10 multiplier that needs to be used with your meter?


The meter in auto mode displays a base number followed by a multiplier of either 10x or 100x. I believe my readings are accurate.

Thanks for your input. With a 2x4 tent (8 sq ft) x 50w/sf = 400watts

But this LUX measurement of the 400w doesn’t get close to the recommended level for flowering. Even with 1000w I’m just getting the lower end of the LUX requirements.

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Is there any chance that the LUX chart is incorrect?

What is your ballast rated for? 1000w? 600w?

I use a 400 w ballast for the 400 and 1000w ballast for the 1000w lamp.

I don’t do lux but grew under HPS for 20+ years. I used a 600 watt HPS in 3x3 tents the last 7 years and that was more than enough light to grow great dense buds in that space, a 1000 watt HPS in a 2x4 is a bit much and I’d be surprised if you can keep the temps in check with that.


This is the first time I’ve checked with a LUX meter, always did the 50w per sq ft and been fine. I’m just curious as to why the lux reading is so low compared to the recommended chart levels.