A system (10) for measuring the luminance and temperature of a light source element of a spectrophotometric device. The system includes a photodiode (20), a luminance measuring device (40), a temperature measuring device (60), a first switch (100), and a controller (80). The photodiode (20) is operable to receive light signals (22) emitted by the light source element and passing through a subjects body tissue. The luminance measuring device (40) is operable to measure luminance of the light signals (22) received by the photodiode (20). The temperature measuring device (60) is operable to measure the temperature of the photodiode (20). The first switch (100) is operable to connect the photodiode (20) to the luminance measuring device (40) or the temperature measuring device (60). The controller (80) is operable to control the connection of the first switch (100).
