Because the use of the optical window can make the measurement independent of factors such as the strength of the optical signal, the measurement distance, noise and the photoelectric output value.
Because the use of optical windows can make the measurement not affected by the intensity of the light signal, measuring distance, noise and changes in the value of the photoelectric output.
Because the use of the light window can make the measurement independent of the intensity of the light signal, the measurement distance, the noise and the change of the photoelectric output value.