Green illumination is a feature of narrow band imaging
endoscopy. Narrow band illumination is used in endoscopy to enhance details of
the mucosal surface without using dyes.
achieved by altering the light by using optical filters in front of the light
source to isolate blue and then green light, which penetrate different layers
of the gut wall. Blue light has a shorter wavelength, 415nm, and so penetrates
the superficial epithelium, whereas green light which has a longer wavelength,
540nm, has a greater depth of penetration, and is used to image the submucosa. Narrow
band imaging allows for better visualization of the gut wall vasculature
because red light, which penetrates deeper than the gut wall, is omitted.
Haemoglobin has its highest absorption peaks at 420nm and
580nm. High levels of absorption cause the blood vessels to appear darker, and
enhances micro vessel structure. This is significant in the detection of
cancers in the gut wall as tumours often have an irregular blood supply.