When you buy an UVB-NB hand held (usually with a 9w bulb), you must be aware of:
1- Despite it's a 9 watt bulb, it irradiates only 1.15 miliwatts of UVB-NB, per square centimeter, at 5 cm far (2 inches).
2- So, when you expose the patch for 60 seconds, you will transfer 60 x 1.15 mw = 70 milijoules only.
3- 70 milijoules will be the transferred (delivered) energy.
4- If you use a 18w (2 x 9w) hand held, it irradiates only 2 miliwatts of UVB-NB, per square centimeter, at 5cm far (2 inches).
5- So, when you expose the patch for 60 seconds, you will transfer 60 x 2 mw = 120 milijoules.
6- 120 milijoules will be the transferred (delivered) energy.
7- So, before using any UVB-NB handheld device, check with your doctor to determine the correct use. A suitable doctor will tell you how much energy has to be transferred to each patch. You can adjust the exposure time based on the formula: Energy = UVB-NB Power x Time.
Power will/may vary with your specific device.
8- Don't get burned!
The closer the light is to your skin, the more energy is transferred. You have to determine how much energy each patch needs. For example: If you want to transfer 1J (1000mJ) to your hand, if you keep the 9w bulb at 5cm (2") far, it will provide 1.15mw (per cm2). So, 1000mJ / 1.15 mw = 870 seconds (14.5 minutes).
So, the first thing to know is the UVB-NB exposure protocol. Basically, this means how much energy will need to be transferred to each patch. A suitably qualified doctors should be able to tell you, but understand a regular doctor may not know and so a dermatologist should be consulted.
The other way to determine the correct exposure time is via trial and error. In this method you start with 30 second exposure time at 5cm (2") far and wait 12 hours to see if the patch has become pink. If so, keep this time. If not, increase 15 seconds and try again. I don't like this approach, since you will/may eventually get burned.
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