An optical device, comprising: a colour layer which comprises elongate strips of at least two different colours alternating with one another periodically along a direction of colour periodicity (D CP), the elongate strips extending along the direction which is orthogonal to the direction of colour periodicity; and a light redirecting layer overlapping the colour layer, and defining at least a first image to be exhibited by the optical device, the light redirecting layer comprising at least a first array of refractive and/or reflective anisotropic light redirecting elements, the anisotropic light redirecting elements of the first array each having a primary axis orientated along a first direction lying in the plane of the optical device, characterised in that the anisotropic light redirecting elements of the first array extend across a first region of the light redirecting layer and being absent elsewhere, and are configured such that an incident light bearr lying in a plane perpendicular to the first direction and from a light source off the normal of the optical device will be redirected by the anisotropic light redirecting elements towards the normal of the optical device but within the same plane, whereas an incident light beam lying in a plane which is not perpendicular to the first direction and from a light source off the normal of the optical device will either not be redirected, or will be redirected by the anisotropic light redirecting elements out of the plane of the incident light beam; whereby for a viewer substantially on the normal of the optical device, when the incident light beam lies in a plane perpendicular to the first direction and is from a light source off the normal of the optical device, portions of the colour layer which overlap the first region of the light redirecting layer are illuminated by the anisotropic light redirecting elements relative to the remainder of the colour layer; and wherein the first region is arranged such that, at each location across the first region, the relative proportions of the at least two different colours of the colour layer which are illuminated by the anisotropic light redirecting elements are configured to exhibit in combination a colour of a corresponding pixel of the first image; such that for a viewer substantially on the normal of the optical device, when the incident light beam lies in a plane perpendicular to the first direction and is from a light source off the normal of the optical device, a colour version of the first image will be exhibited by the device. a colour layer which comprises elongate strips of at least two different colours alternating with one another periodically along a direction of colour periodicity (D CP), the elongate strips extending along the direction which is orthogonal to the direction of colour periodicity; and a light redirecting layer overlapping the colour layer, and defining at least a first image to be exhibited by the optical device, the light redirecting layer comprising at least a first array of refractive and/or reflective anisotropic light redirecting elements, the anisotropic light redirecting elements of the first array each having a primary axis orientated along a first direction lying in the plane of the optical device, characterised in that the anisotropic light redirecting elements of the first array extend across a first region of the light redirecting layer and being absent elsewhere, and are configured such that an incident light bearr lying in a plane perpendicular to the first direction and from a light source off the normal of the optical device will be redirected by the anisotropic light redirecting elements towards the normal of the optical device but within the same plane, whereas an incident light beam lying in a plane which is not perpendicular to the first direction and from a light source off the normal of the optical device will either not be redirected, or will be redirected by the anisotropic light redirecting elements out of the plane of the incident light beam; whereby for a viewer substantially on the normal of the optical device, when the incident light beam lies in a plane perpendicular to the first direction and is from a light source off the normal of the optical device, portions of the colour layer which overlap the first region of the light redirecting layer are illuminated by the anisotropic light redirecting elements relative to the remainder of the colour layer; and wherein the first region is arranged such that, at each location across the first region, the relative proportions of the at least two different colours of the colour layer which are illuminated by the anisotropic light redirecting elements are configured to exhibit in combination a colour of a corresponding pixel of the first image; such that for a viewer substantially on the normal of the optical device, when the incident light beam lies in a plane perpendicular to the first direction and is from a light source off the normal of the optical device, a colour version of the first image will be exhibited by the device.