A scatterometer for measuring a property of a target on a substrate includes a radiation source, a detector, and a processor. The radiation source produces a radiated spot on the target. The scatterometer adjusts a position of the radiated spot along a first direction across the target and along a second direction that is at an angle with respect to the first direction. The detector receives radiation scattered by the target. The received radiation is associated with positions of the radiated spot on the target along at least the first direction. The detector generates measurement signals based on the positions of the radiated spot on the target. The processor outputs, based on the measurement signals, a single value that is representative of the property of the target. The processor also combines the measurement signals to output a combined signal and derives, based on the combined signal, the single value.