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

[ahk]

TL;DR

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.

 

[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]

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.