The Optare Sigma is a step-entrance single-deck bus body that was built by Optare between 1994 and 1996 on the Dennis Lance chassis. The body was constructed from Alusuisse aluminium alloy with GRP front and rear panels. It was the single-deck version of the Optare Spectra which had the same lower dash styling.
The Sigma is not a low-floor bus although its floor height of 40cm is considered low for a step-entrance design. It was also similar in design to the Optare Delta. The Vecta has the same body styling as the Sigma but is designed for medium-length chassis, the Sigma is for full length chassis.