There is a some of discussion on the interneet forums about IPL pulsing formats and websites with some manufacturers claiming that their IPL device delivers a square pulse that provides constant energy to tissue during the exposure, and is clinically better. This claim is not quite accurate, somewhat misleading. and an example of marketing leading the physics.
The square pulse as advertised for these devices provides a constant power level, and constant irradiance at the tissue, not constant energy. The energy or fluence delivered is accumulated over time period of the pulse, and is the integral of this pulse shape. It is triangular, or ramp shaped, with respect to time. The longer the IPL is radiating tissue the more energy the tissue absorbs. Whether you use a true square pulse or trapezoidal pulse will make very little difference. The trapezoid pulse makes a graph of energy accumulation look like a triangle with a rounded top. A multi-pulse system just makes the energy accumulation look like a staircase function, or a triangular sawtooth ramp instead of a smooth triangle.
The differences in these systems are very small compared to the measurement errors and other fluctuations. It would be hard to demonstrate a clinical difference, everything else being equal. I am not aware of any science showing there is a clinical difference between these systems. There are just too many variables involved to do a controlled study comparing devices that would show the subtle difference between them, and any if these system can be made to work well with the right settings.
The change in spectral output with the pulse shape due to non-constant, or non-square pulses, has become another red herring thrown out in marketing literature. The spectral output is dependent on the temperature of the source, and is indirectly related to the power of the lamp, however the spectrum of a flash lamp is quite wide, and filters, internal reflections, spot size, hot spots, and other variable have at least as great an influence as the small differences in the final accumulated energy spectrum due to pulse shape.
Different IPLs may have somewhat different spectral outputs, but they also have different filters, different light guides, and reflectors. You should be skeptical of any study comparing the different devices since there are so many variables that need to be controlled. The settings and filters for each device are determined by testing, and all these devices work well, but you will find each requires difference settings and filters for the same result. Almost all these devices use the same off-the-shelf lamps in any case.
More important factors for choosing an IPL are cost, speed, spot size, uniformity across the treatment site, and versatility (filters). I believe uniformity across the spot size, the method of propagating the light to tissue, and surface cooling are far more critical to consistent results than any small deviation due to pulse shape.