A measuring instrument includes a housing, a number of infrared light emitting devices, a number of infrared light receiving devices, at least two light sources, and a controller. The housing includes a first sidewall and a second sidewall substantially parallel to the first sidewall. The infrared light emitting devices are positioned on the first sidewall. The infrared light receiving devices are positioned on the second sidewall and receive infrared lights emitted by the infrared light emitting devices. The controller is electrically coupled to the infrared light receiving devices and the light sources. When one of the infrared light receiving devices does not receive infrared light emitted by a corresponding infrared light emitting device, the controller turns the light sources on.