LLLT Q&A : “how it is possible to have higher power density than the applicator’s total power”

QUESTION FROM KOREA
Dear James

The THOR 810nm 1W laser cluster (5 × 200mW) has a 1/e2 power density of 5.96W/cm2. Just wondering how it is possible to have higher power density than the applicator’s total power

The formula is power/beam area

ILLUSTRATION
When you drive your car at 60 kph that is 60 kilometres per hour you don’t have to drive for one hour and you don’t have to drive for 60 kilometres.
e.g. You can drive at 60kph for one minute* and you will have travelled only 1 kilometre
*(1/60th of an hour)(or to put it another 1 minute = 0.01666 of an hour)

Math: 1 kilometre / 0.01666 hours = 60kph

LASER
Same for laser irradiance (power density)

If the laser was 5 Watts and if the beam area was 1cm2 then you would have 5W /cm2
but 5W/cm2 does not need a 5W laser (same as you don’t need to drive 60 kilometres or drive for one hour)

e.g. a 1W laser with a beam that is 0.2cm2 is also 5W/cm2
Maths: 1W / 0.2cm2 = 5W/cm2

ALSO

If a 200mW laser (0.2W) has a beam area of 0.4cm2 then that is also 5W/cm2

so on the THOR 1W laser cluster we have five beams of 200mW
each beam is approx 0.04cm2
so each beam is approx 5W/cm2