A system for measuring the luminance and temperature of a light source element of a spectrophotometric device is provided. The system includes a photodiode, a luminance measuring device, a temperature measuring device, a first switch, and a controller. The photodiode is operable to receive light signals emitted by the light source element and passing through a subject's body tissue. The luminance measuring device is operable to measure luminance of the light signals received by the photodiode. The temperature measuring device is operable to measure the temperature of the photodiode. The first switch is operable to connect the photodiode to the luminance measuring device or the temperature measuring device. The controller is operable to control the connection of the first switch.