According to the manufacturer, 98% of the energy output is emitted in the 2 - 20 micron range. I am not able to test how accurate this is.
There does seem to be a difference in what is supposed to be the ideal FIR (Far-Infrared-Sauna) range for humans. I've read one article that suggests that around the 9 micron range is ideal (which is around the FIR (Far-Infrared-Sauna) range that humans output). One brochure I've got states that chinese researchers have found that around the 16 micron range is most beneficial for humans.
When selecting the particular heating elements that are used in these heaters (custom ordered with lower wattage heating elements) I used a purely subjective evaluation, "How did using two of these heaters in the bathroom for sauna feel compared to the "Healthmate" FIR (Far-Infrared-Sauna) sauna that I had been using.