A method is disclosed for detecting the formation of or presence of light-interacting domains, e.g. particles or clumps of particles, in a supporting medium which comprises: disposing the supporting medium between a source of multipoint illumination and a scintillation detector, illuminating the supporting medium with a multiplicity of discrete light beams directed through the medium toward the scintillation detector; and causing relative motion between the light beams and the supporting medium. If light-interacting domains of critical size are present in the medium scintillation is detected. A viewer and test card for carrying out the method are also disclosed.
