x and y directions if the dominant periodicity coincides with the nominal periodicity of the filter the filter response is in accordance with the nominal periodicity if as shown in fig7 the dominant periodicity p 1 does not coincide with the nominal filter periodicity pf then the filter response is in accordance with the dominant periodicity p 1 so long as p 1 lies within the filter band an acceptable signal 13 is produced the scale 1 may be a flat plane or a curved surface it may be continuous in one or more directions for example as a band or cylinder a second diffraction grating may be used between the grating 2 and the detector array 4 to match the pitch of the interference pattern to the pitch of the detector array for example by making use of moir u00e9 effects the grating 2 may be a phase grating or a ronchi grating the grating 2 and or the detector array 4 may be tilted by rotation about an axis in a plane containing the x and y directions in order to increase the tolerance of the read head to misalignment with respect to the scale particularly to increase the tolerance on distance u the patterns of marks illustrated for the scale and grating are chequered but other regular patterns would give satisfactory results e g rows of circles.