Preform, serving as a semi-finished product, for a container intended for containing products therein that are sensitive to radiation in particular light sensitive and food and dairy products, consisting of at least one base layer (1) made of a primary plastic base material, with a certain amount of additives (5) incorporated in it (1), characterised in that said preform (10, 20) is opaque over virtually the whole extent thereof, wherein a relatively low percentage of plastic additives (5) is incorporated to generate sard opaque appearance (22), so as to protect the inner space (9) thereof which is delimitated by it against external radiation (V.sub.1, V.sub.2) particularly electromagnetic radiation, more particularly light, under normal pressure condition.