An implantable apparatus for physiological measurement in a host organism, the apparatus comprising: a) a sample chamber that is configured for implanting within the host organism, the sample chamber having a measurement port and live cells that are treated to fluoresce in response to light having an excitation wavelength, and wherein the sample chamber is configured to maintain fluid communication between the live cells and the host organism; b) an optical sensor housing that is configured for implanting within the host organism, and configured to block from the sample chamber, ambient light penetrating into the host organism, the optical sensor housing having: (i) a window disposed to convey excitation light output toward the sample chamber and to receive fluorescent light from the sample chamber; (ii) a coupling that couples the measurement port of the sample chamber in a fixed position relative to the window; (iii) an optical chamber that is partitioned by a beam splitter into an excitation sub-chamber and a detection sub-chamber that is optically separated from the excitation sub-chamber, wherein both sub-chambers are in optical communication with the window; (iv) an excitation source that is energizable to direct light having the excitation wavelength along an excitation path through the excitation sub-chamber and to the window; (v) a detector that is disposed in the detection sub-chamber in a detection path of fluorescent light received from the live cells; vi) a reflective surface disposed between the beam splitter and the sample chamber which directs light into and out of a primary plane such that the excitation path and the detection path define a folded optical axis that extends between the live cells of the sample and the detector; and c) a signal processing apparatus that is in signal communication with the detector and is energizable to acquire and process a detector signal and to transmit a processed signal that is indicative of fluorescent light energy. a) a sample chamber that is configured for implanting within the host organism, the sample chamber having a measurement port and live cells that are treated to fluoresce in response to light having an excitation wavelength, and wherein the sample chamber is configured to maintain fluid communication between the live cells and the host organism; b) an optical sensor housing that is configured for implanting within the host organism, and configured to block from the sample chamber, ambient light penetrating into the host organism, the optical sensor housing having: (i) a window disposed to convey excitation light output toward the sample chamber and to receive fluorescent light from the sample chamber; (ii) a coupling that couples the measurement port of the sample chamber in a fixed position relative to the window; (iii) an optical chamber that is partitioned by a beam splitter into an excitation sub-chamber and a detection sub-chamber that is optically separated from the excitation sub-chamber, wherein both sub-chambers are in optical communication with the window; (iv) an excitation source that is energizable to direct light having the excitation wavelength along an excitation path through the excitation sub-chamber and to the window; (v) a detector that is disposed in the detection sub-chamber in a detection path of fluorescent light received from the live cells; vi) a reflective surface disposed between the beam splitter and the sample chamber which directs light into and out of a primary plane such that the excitation path and the detection path define a folded optical axis that extends between the live cells of the sample and the detector; and (i) a window disposed to convey excitation light output toward the sample chamber and to receive fluorescent light from the sample chamber; (ii) a coupling that couples the measurement port of the sample chamber in a fixed position relative to the window; (iii) an optical chamber that is partitioned by a beam splitter into an excitation sub-chamber and a detection sub-chamber that is optically separated from the excitation sub-chamber, wherein both sub-chambers are in optical communication with the window; (iv) an excitation source that is energizable to direct light having the excitation wavelength along an excitation path through the excitation sub-chamber and to the window; (v) a detector that is disposed in the detection sub-chamber in a detection path of fluorescent light received from the live cells; vi) a reflective surface disposed between the beam splitter and the sample chamber which directs light into and out of a primary plane such that the excitation path and the detection path define a folded optical axis that extends between the live cells of the sample and the detector; and c) a signal processing apparatus that is in signal communication with the detector and is energizable to acquire and process a detector signal and to transmit a processed signal that is indicative of fluorescent light energy.