A holographic projection system arranged to detect a change in a wavelength of light from a light source, the projection system comprising: a light source arranged to output light having a wavelength; a spatial light modulator arranged to receive the light from the light source and output spatially modulated light in accordance with a diffractive pattern comprising a computer-generated hologram to form an image, wherein the image comprises a primary image region comprising information for a user and first and second control image regions, wherein the first image control region and the second image control region each comprises an area outside of the primary image region; a detector arrangement comprising a first detector region configured to detect light ir the first control image region and output a first signal representative of a position of the first control image region, and further comprising a second detector region to detect light in the second control image region and output a second signal representative of a position of the second control image region; and a processor arranged to: cause the spatial light modulator to display the diffractive pattern comprising the computer-generated hologram to form the image, wherein the positions of the first and second control image regions are dependent on the computer-generated hologram and the wavelength; receive the first and second signals; determine the relative position of the first and second control image regions based on the first and second signals; and detect a change in the wavelength based on a change in the relative position of the first and second control image regions. a light source arranged to output light having a wavelength; a spatial light modulator arranged to receive the light from the light source and output spatially modulated light in accordance with a diffractive pattern comprising a computer-generated hologram to form an image, wherein the image comprises a primary image region comprising information for a user and first and second control image regions, wherein the first image control region and the second image control region each comprises an area outside of the primary image region; a detector arrangement comprising a first detector region configured to detect light ir the first control image region and output a first signal representative of a position of the first control image region, and further comprising a second detector region to detect light in the second control image region and output a second signal representative of a position of the second control image region; and a processor arranged to: cause the spatial light modulator to display the diffractive pattern comprising the computer-generated hologram to form the image, wherein the positions of the first and second control image regions are dependent on the computer-generated hologram and the wavelength; receive the first and second signals; cause the spatial light modulator to display the diffractive pattern comprising the computer-generated hologram to form the image, wherein the positions of the first and second control image regions are dependent on the computer-generated hologram and the wavelength; receive the first and second signals; determine the relative position of the first and second control image regions based on the first and second signals; and detect a change in the wavelength based on a change in the relative position of the first and second control image regions.