The invention is a light source apparatus applied to the detection of oral cavity. The light source apparatus comprises an LED unit and a control unit. The LED unit contains the three primary colors LEDs. The control unit provides operating power for the LEDs and controls the LEDs to produce a mixed light. The chromaticity of the mixed light from the LEDs is at a specific range in a CIE chromaticity diagram. The light source apparatus is applied to illuminate the diagnostic areas to produce high color difference and contrast between abnormal and normal area.
