A method of determining the presence of aliasing or avoiding aliasing within certain limits in correlation measurements in a time-of-flight system having an imaging sensor and being for use with an illumination unit, the illumination unit being adapted to illuminate a scene with modulated light at a first and at a second different frequency, the system being adapted to obtain correlation measurements between a correlation signal and modulated light reflected from the scene, the method comprising: a) detecting reflected light from at least one object in the scene; b) determining by one or more first correlation measurements a first correlation between a first correlation signal and the reflected light received at the imaging sensor, the first correlation signal being equal to the modulated light having a periodic waveform of the first frequency and the first correlation measurement providing a value of a parameter related to a distance from the imaging sensor to the at least one object; the method characterised by c) determining by one or more extra-added second correlation measurements a second correlation between the reflected light received at the imaging sensor and a second correlation signal, the second correlation signal having a periodic waveform of the second frequency lower than the first frequency; determining the second correlation measurement using different phase offsets for the second frequency; and determining in-phase and/or quadrature components of the detected reflected light, d) using the in-phase and/or quadrature components from the second correlation measurement for determining for that value of the parameter the presence of aliasing, and wherein the first and second correlation measurements are combined into aliasing indicators and the presence of aliasing is determined from indicators being the sign of the in-phase and/or quadrature components of the detected reflected light. a) detecting reflected light from at least one object in the scene; b) determining by one or more first correlation measurements a first correlation between a first correlation signal and the reflected light received at the imaging sensor, the first correlation signal being equal to the modulated light having a periodic waveform of the first frequency and the first correlation measurement providing a value of a parameter related to a distance from the imaging sensor to the at least one object; c) determining by one or more extra-added second correlation measurements a second correlation between the reflected light received at the imaging sensor and a second correlation signal, the second correlation signal having a periodic waveform of the second frequency lower than the first frequency; determining the second correlation measurement using different phase offsets for the second frequency; and determining in-phase and/or quadrature components of the detected reflected light, d) using the in-phase and/or quadrature components from the second correlation measurement for determining for that value of the parameter the presence of aliasing, and wherein the first and second correlation measurements are combined into aliasing indicators and the presence of aliasing is determined from indicators being the sign of the in-phase and/or quadrature components of the detected reflected light.