We have produced a novel water substitute phantom suitable for film dosimetry, while retaining the radiological property of a conventional water substitute phantom. The novel phantom excludes the effect of light, which is known to affect the accuracy of results on conventional phantoms. The effect of light was eliminated by appropriately adjusting the quantum of the carbon black to that of a conventional phantom material. Through comparison of the novel phantom with a conventional phantom it was shown that the absorbed dose determined by conventional phantom was 15% higher for 10 MV X-rays and 18% higher for an 18 MeV electron beam, attributable to the contamination of Cerenkov light. Although the net optical density of the film increased with time owing to the optical permeability of the phantom, that of the novel phantom did not vary with time. The novel phantom was therefore shown to be unaffected by such local light and by the optical transmission of the phantom.