The wavelength region of the IR sensor
As with most SI units it is convenient to quote our dimensions in values of 1 up to 999 followed by the multiplier (m&Omega;, k&Omega;, M&Omega; etc.). As we can see from your graph the visible light spectrum is in the range 400 nm to 750 nm. To keep the units consistent we often refer to the IR wavelengths in nanometers as well.


  The infrared waves typically have wavelengths between 0.75 and 1000µm.


This is 750 nm up.


  a - The wavelength region from 0.75 to 3µm is known as the near infrared region.


So 750 nm to 3000 nm.


  b - The region between 3 and 6µm is known as the mid-infrared region, and


3000 nm to 6000 nm.


  c - infrared radiation which has a wavelength greater higher than 6µm is known as far infrared.


At this stage it's probably worth switching to &mu;m.



Figure 1. Section of OP's image with wavelengths marked in nanometers.