Human life on Earth is the result of an extremely for­tu­nate envi­ron­ment. This includes our tem­per­ate posi­tion near a sta­ble star, in a sta­ble area of the galaxy, with neigh­bour­ing bod­ies of the right size and dis­tance to pro­tect us. It is a Rare Earth.

As we evolved, this envi­ron­ment con­tin­ued (and con­tin­ues) to influ­ence our ongo­ing design; not just phys­i­o­log­i­cally, but also technologically.

The Sun emits a wide range of radi­a­tion, from very short wave­lengths to very long. A star of the Sun’s age and size emits a peak radi­a­tion of about 400 to 700 nm, so it’s no sur­prise that we have evolved to use this par­tic­u­lar sec­tion of the elec­tro­mag­netic spec­trum for human ‘eyesight’.

Solar Spectrum

(Image used under Cre­ative Com­mons license from Global Warm­ing Art via Wikipedia)

Once through the Earth’s atmos­phere, the Sun’s radi­a­tion peaks at roughly 555 nm, which we have hand­ily adapted to detect with the high­est sen­si­tiv­ity. This wave­length cor­re­sponds to what we inter­pret as the colour green (the mid­dle of our vis­i­ble spectrum).

Elec­tron­i­cally, we have devel­oped a sys­tem to repro­duce much of these ‘vis­i­ble’ wave­lengths, called the RGB color model, as com­monly used in dis­plays such as tele­vi­sions and com­puter mon­i­tors. This sys­tem does have some lim­i­ta­tions, such as the inabil­ity to repro­duce all of the vis­i­ble wave­lengths, includ­ing vio­let (about 400nm).

The inter­est­ing result is that, because our eyes do not have an equal sen­si­tiv­ity across red ®, green (G) and blue (B) – from one end of the spec­trum to the other – we can com­press these three com­po­nents indi­vid­u­ally so that only the most impor­tant infor­ma­tion is retained with accuracy.

This tech­nique is used by NTSC, the pop­u­lar tele­vi­sion stan­dard used across the Amer­i­cas, Japan and other coun­tries, to use less band­width for non-green colours, as clearly shown in this exam­ple.

Read more about this Chroma Sub­sam­pling tech­nique on Wikipedia (note that in the arti­cle text, the ‘luma’ com­po­nent — or bright­ness — is largely influ­enced by green).