According to an aspect of the present invention, a near infrared spectroscopy (NIRS) sensor assembly, system, and method for measuring a characteristic of a biological tissue is provided. The NIRS sensor assembly includes a light source, at least one light detector, and a subject contact layer. The light source is operable to emit light at one or more predetermined wavelengths. The light detector has an active area for detecting light emitted by the light source and passed through the biological tissue. The detector is operable to produce signals representative of the detected light. The subject contact layer is disposed to cover one or both of the light detector and the light source. The subject contact layer has at least one optically transmissive portion.