Dear Karunakar,

you have to have a look at the lamp specifications/data sheet. There you will find a value of the optical power density at a given distance such as it is given for example in the question of the following link:

https://www.researchgate.net/post/How_can_we_calculate_UV_light_intensity.

From that starting value you may scale the power density to your distance. Please note that there is an inverse quadratic dependence on the distance.

So for a power density of pd15 =Po W/cm² at 15 cm for example you have to estimate your power density pd5 at 5cm according to:

pd5 = pd15*15²/5² = Po*15²/5² W/cm²= 9*Po W/cm²

However for a source size of 2cm of your lamp the quadratic law is only a rough estimation with respect to the 5cm distance, because the lamp pellet (2cm) cannot really regarded as a point source when having a distance of only 5cm to the sample.

If you do not have got the data sheet you have to retrieve that starting value Po via an appropriate UV power-meter

An example of 312nm UV lamp (16W) is given here:

http://www.advantage-lab.com/sites/default/files/AL10-16-312%20Product%20Description%20English%20v4.pdf

Here you will have for example 1250µW/cm² at 15cm.

Good luck.

Start with downloading the free light measurement handbook from Alex Ryer. Easily found via internet.

Well Karunakar,

One thing I must agree here with Gerhard M has stated above that the equations talk about a point source where as you have both the detector (2cm) and a UV lamp (source) also of comparable size and then the distance of 5cm does not make any sense of talking like a point source for that matter.

What I think is that with whatever lamp or source that you are using, try a UV detector with known calibration i.e. use some standard measuring instrument to detect the UV light and then compare the response of your "Pellet Detector" under study with this reference measurement.

I hope this work very well for your application.

Best Luck!