James Prescott Joule
Confused by terms?
It’s really easy!
The joule (symbol J) is the derived unit of energy in the International System of Units. It is defined as:
How should I pronounce Joule?
In common speech, we screw up a lot and use the term energy when we mean power and vice versa, energy is the ability to do work, while power is the rate at which work is done.
OK so now you’re more confused.
You’re not the only one!
POWER vs ENERGY
We should not use these words interchangeably, they are very different terms.
Power is meaured in Horsepower, Watts and a few other terms. For the purpose of this discussion, we’ll stay with Watts or milliwatts (thousandths of watts) which is typically the power level of phototherapy panels and booths used in the home and clinic.
Energy for the purpses of our discussion is measured in Joules or millijoules. A joule is a watt-second or one watt of power delivered over a one second period.
So let’s talk about UV Phototherapy. The doctor wants to record in the chart the dosage that any given patient has received during a treatment, this is typically stated in terms of “mJ” (millijoules) in the case of UVB and UVB Narrowband and in “J” (Joules) in the case of UVA/PUVA treatment.
To detemine how many J or mJ a patient receives the clinic one needs to know the power coming from the lamps and then set a time to deliver the correct dose.
In a typical (not all) home UVB Narrowband units the typical output power is 2.5 mw/sq. cm. at roughly 6” from the unit when the unit leaves the factory.
The formula used to calculate is
Time(Minutes) = (Joules/cm2 x 16.67) / (Power in milliwatts/cm2)
Therefore to calculate the time for 350 mJ (350 mJ = 0.35J) with a unit generating 2.5 mw/sq. cm.
Desired dose = 350 mJ (0.35 J)
Phototherapy Power = 2.5 mw/ sq cm.
(0 .35J) then (.35 * 16.67)/2.5 would be 2.3 minutes.
We know that a system in a doctors’ office / clinic must be calibrated using this formula of perhaps a DOSIMITERY system. The typical home user dod not need to worry about this, this is why we recommend the simpler home treatment protocol at <CLICK HERE>, it’s so much easier as the typical home user does not have a UV power meter to help with clibration and system setup.