In principle, the frequency f (and the wavelength λ=c/f) of an electromagnetic wave can take any value and the whole range of possible frequencies is called the electromagnetic spectrum. Different regions of the spectrum are conventionally given different names (with associated spectral ranges smoothly depending on specific science sector):
gamma-rays λ< 1 pm
x-rays 1 nm >λ>1 pm
Ultraviolet (UV) 400 nm >λ>1 nm
Visible (VIS) 700 nm >λ> 400 nm (blue: 455 – 492, green 492 – 577, yellow 577 – 597, red 622 – 700)
infrared (IR) 1000μm >λ> 0,7 μm (Near-IR - NIR: 0,7-1,3; Short-Wave IR SWIR: 1,3-3; Medium IR - MIR: 3-6, Thermal IR - TIR: 6-20; Far IR - FIR: 20-1000)
Radio waves λ> 1 mm (Microwaves MW 1 m >λ> 1mm). Optical range (usually referring to the spectral range from VIS to TIR) and microwaves are the most important spectral region for remote EO systems.