A colonoscopy a procedure used to detect changes or abnormalities in the large intestine (colon) and rectum. Sigmoidoscopy is used to check the sigmoid colon, which is the lower part of your colon or large intestine.
A long, flexible tube (colonoscope, sigmoidoscope) is inserted into the rectum, which has a tiny video camera at the tip of the tube. This allows for real-time visualisation of the lower gastrointestinal tract. The tube is also used to blow air into your colon, which makes the colon swell up a bit to improve visibility.