The factors that affect whether an endoscopist sees a polyp are not well studied. Polyp detection rates vary widely, even amongst experts. Techniques that highlight lesions advanced in recent years. Chromoendoscopy, spraying dye on the bowel lining, has been shown to help pick up more precancerous polyps in one of three studies in normal patients. Autofluorescence endoscopy (AFI) and narrow band imaging (NBI) use light filters to produce a false colour image of the bowel lining where polyps stand out. These techniques have been used with some success in the oesophagus and stomach but little work is available for the colon.