The design and development of a non intrusive device to measure local void fraction in a vertical upward flowing two-phase mixture over a region 12.70 mm wide by 1.00 mm high is presented in this article. The device designed, referred to as the optical void detector, operates on the principle of light scattering whereby a beam of laser light reflected back after passing through the two-phase mixture is directed by means of lenses towards a photodiode sensor. The voltage signal produced by the photodiode is dependent upon the intensity of the laser light reaching the sensor which can be used to determine void fraction by virtue of a calibration procedure. Measurements made with the optical void detector focused upon a bubbly flow mixture passing through a tube bundle in the range 0 < alpha < 25% void fraction were validated by means of high speed photography and gamma densitometry. Although very good agreement was observed between the measurements made by the optical void detector and those obtained by high speed photography, the measurements made by the gamma densitometer were consistently lower.