An apparatus for testing a subterranean formation penetrated by a wellbore is disclosed. According to the method, a transmitter element emits light, which is received by at least one receiver element and the light emitted from the transmitter element is scattered by an object, wherein an optical guide is provided between the transmitter element and the receiver element. A transfer film including a spring-back layer is also disclosed. These protein hydrolysates can be used in the preparation of food compositions, wherein these protein hydrolysates provide for novel and unexpected flavours. Such photoconductors may exhibit a photoconductive gain, i. The light guide part includes a main light guide part and a sub-light guide part formed as a single piece with the main light guide part or partially overlapped with the main light guide part.