An apparatus is described for scanning a circuit structure. The apparatus has a linear sensor (16) for detecting light intensity as a function of position along the sensor. A transport mechanism (12) moves a circuit structure (10), such as a PCB or a wafer relative to the sensor. The circuit structure is illuminated with an illumination system (14) that comprises a hollow cylinder (144) with a mainly diffusively and/or specularly reflecting inner wall surface. The cylinder is arranged in parallel with the sensor and has a first slit (40) and a second slit (42) located so that a virtual plane runs through the sensor, the first and second slit to a location for the circuit structure under inspection. The illumination system furthermore comprises a linear light source (146) in the cylinder or the inner wall of the cylinder. In an embodiment the illumination system comprises a splitting mirror (22) in the cylinder to reflect light to the circuit structure.