Dear Zhenya Zhelyazkova,

Whether the product you refer to emitsat the wavelength of 254nm or not? Ideally you should measure it using a UV power meter. The intensity should be equal to W/m^2. So the formula you mentioned can be derived from j/m^2=W/m^2*time=J/(sec*m^2)*sec

Hope it is clear.

Ultraviolet radiation dose calculation for algal ... - Core

core.ac.uk › download › pdf

PDFby E Navarro - ‎2014 - ‎Cited by 6 - ‎Related articlesJan 22, 2014 -

Please look at the following below links which may help you in your analysis:

https://www.epa.gov/sunsafety/calculating-uv-index-0

https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1017/S1350482703002032

http://www.csgnetwork.com/uvindexcalc.html

Thanks

Safety readings require the lower range, and efficacy requires a range up to at least10 mW·cm-2

Low pressure mercury lamp looks like normal fluorescent tube but without white coatingin side of the tube. The colour looks pale-bluish - and I would not recommend to look at it for long and at any close range, especially at lest without glasses, even 'normal' glasses block the 254nm line.

It usually assumed for low pressure mercury lamps that 40% energy (of 9 Watt in your case) go into 254nm line.

Then, you may get power per area at distance R as Power_per_area = Power/S

where S= 4*pi * R^2 is area of the sphere.

Still, to be more certain I would try to get some UV intensity meter e.g. https://www.thorlabs.com/thorproduct.cfm?partnumber=S130VC

You can measure the UV intensity, however you need UV spectra-photometer suitable range of nm below 400 nm..