Embodiments of the disclosure describe an on-head detection technique for an HMD that includes an optical sensor positioned to detect light reflected from at least one of a face of a user or a lens worn by the user. Grayscale image data having a plurality of pixels is received and processed. A method for regenerating a segmented filter comprises maintaining the engine exhaust stream to fuel stream ratio substantially consistent among segments during their regeneration. The first bit line is coupled to a first column of the multi-level cells. The device comprises autoclavable parts andor disposable parts, which can be separated from the breathing system without exposing staff handling the system to contaminated surfaces in the breathing system when changing patients.