# Light intensity at a distance -- UV light to kill bacteria

#### ahk

Summary
If I've got a bulb producing about 5J/s of useful energy at 253.7nm then how much useful energy do I have at .254m (10")?
I have obtained seeds of a rare rhododendron species.

These seeds are contaminated (experimentally determined). While I have tried several common methods, none decontaminated the seeds.

Lately, I have read that so many joules of UV light at approximately 254nm will prevent replication and inactivate or kill bacteria, fungus, mold... even in the spore stage.

I don't need a perfect number as it seems that too much UV (within reason) won't damage seeds.

My light bulb is 25W. I have read that an average Ag bulb of this wattage produces about 5W at 254nm. Using 5W at 254nm and .254m as my distance, when I try to use (what I think is) the appropriate equation (Intensity=W/m^2), I get a ridiculous answer of 77.5W/m^2.

Can anybody help?

Thanks.

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#### gleem

Summary: If I've got a bulb producing about 5J/s of useful energy at 253.7nm then how much useful energy do I have at .254m (10")?

My light bulb is 25W. I have read that an average Ag bulb of this wattage produces about 5W at 254nm. Using 5W at 254nm and .254m as my distance, when I try to use (what I think is) the appropriate equation (Intensity=W/m^2), I get a ridiculous answer of 77.5W/m^2.
Your source is generating 5w of power which represents the flow of energy. At .254 m this flow is spread out over the surface area of 4π(.254)2 m2 = 0.811 m2 so the flow of energy can be expressed in terms of watts/cm2. So 5W/0.811 m2 = 6.17 W/m2 = 6.17x10-4W/cm2

Lately, I have read that so many joules of UV light at approximately 254nm will prevent replication and inactivate or kill bacteria, fungus, mold... even in the spore stage.
I believe UV dose is usually expressed in watts⋅sec/cm2.

#### BillTre

Gold Member
2018 Award
UV dose to kill (or inactivate) different species various among the different species, sometimes by a large amount.
Here is a pdf (from a different thread) that has a information on a lot of different organisms UV sensitivity. There are many other similar lists around.

"Light intensity at a distance -- UV light to kill bacteria"

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