How much light energy actually emerges from IPL filters and reaches the skin surface?

When we set a fluence on an IPL unit, it is based on ALL of the light energy coming from the handpiece. However, each filter will absorb some of that energy – it is supposed to! The filters are designed to ‘block’ certain wavelengths, so that only the desired wavelengths reach the skin surface.

So, to effectively treat blood vessels, we must apply green and yellow wavelengths. Hence, a filter in the region of 500 to 550nm is necessary for this treatment.

But, how much light energy is absorbed by these filters?

I built a computer model based on the spectral output of a Xenon lamp to answer this question. This model allows me to calculate the amount of energy will be absorbed by filters, based its distribution across the wavelength range 350 to 1200nm.

The table below shows how much light gets though each filter:

Filter cut-off wavelength (nm)% light energy emerging

This is very interesting! Only 51% of all the light energy above 700nm gets through the filter. So, if the fluence on the screen is set to 20 J/cm2, then the actual energy emerging from the filter is only about 10 J/cm2.

The above image shows how much energy, in which wavebands, gets through. The 400nm filter allows almost all of the energy (88%) through, whereas the 700nm filter is just over half. The ‘boxes’ show how much energy is contained in those wavebands. You can see that there is a significant difference between the filters.

I measured the output from an IPL recently – the measurements agreed pretty closely to my calculations! (There was less than 10% difference – which is pretty good.)

The problem is, without a meter, there is no way you can be sure what is actually hitting the skin. So, you’ll have to go by the numbers available to you on the screen – even though its wrong!! In other words, a setting of 20 J/cm2 with a 500nm filter is not the same as a 20 J/cm2 with a 700nm filter. The actual delivered energies will be different in each case!

(Incidentally, through the years of testing IPLs with my meter, I have found that most don’t produce the fluences they claim on the screens! Even with no filters. But that’s a different story…)

This probably doesn’t help, but it’s better to know these things (I hope!!)


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