This document applies to basic safety and essential performance of cerebral tissue oximeter equipment, that employs light at multiple wavelengths to derive a quantitative measure of oxygen saturation of haemoglobin within the volume of tissue sampled under the probe attached to the head. The cerebral tissue oximeter equipment can be based on continuous light, frequency domain or time domain technologies. This document applies to ME equipment used in a hospital environment as well as when used outside the hospital environment, such as in ambulances and air transport. Additional standards may apply to ME equipment for those environments of use. NOTE 1 Cerebral tissue oximeters are sometimes referred to as near infrared spectroscopy equipment in medical literature. Not included within the scope of this document are: invasive tissue or vascular oximeters; oximeters that require a blood sample from the patient; equipment measuring dissolved oxygen; ME equipment, or part thereof, that measures path-length-dependent haemoglobin change. The requirements for functional near-infrared spectroscopy equipment are found in ISO 80601-2-71[4]; ME equipment, or part thereof, that measures arterial saturation based on pulsatile changes in tissue optical properties (SpO2). The requirements for pulse oximeter equipment are found in ISO 80601‑2‑61[3]; ME equipment, or any part thereof, that claims to monitor tissue in parts of the body other than the head. This document also applies to cerebral tissue oximeter equipment, including cerebral tissue oximeter monitors, cerebral tissue oximeter probes and probe cable extenders, that have been remanufactured. If a clause or subclause is specifically intended to be applicable to ME equipment only, or to ME systems only, the title and content of that clause or subclause will say so. If that is not the case, the clause or subclause applies both to ME equipment and to ME systems, as relevant. Hazards inherent in the intended physiological function of ME equipment or ME systems within the scope of this document are not covered by specific requirements in this document except in 201.11 and in 201.7.2.13 and 201.8.4.1 of the general standard. NOTE 2 See also 4.2 of the general standard. This document can also be applied to ME equipment and their accessories used for compensation or alleviation of disease, injury or disability. This document is not applicable to remote or slave (secondary) equipment that displays StO2 values that are located outside of the patient environment. NOTE 3 ME equipment that provides selection between diagnostic and monitoring functions is expected to meet the requirements of the appropriate document when configured for that function.