An optical unit, comprising: an image pick-up device configured to convert a reflected light reflected from an original document to an electric signal, after the light is irradiated on the original document from a light source; a lens unit configured to read full-colour including three wavelengths of Red, Green and Blue and image the reflected light on the image pick-up device; and a holding member configured to hold the image pick-up device and lens unit; wherein a focal length of the lens unit is adapted to fluctuate in a first direction in response to temperature fluctuations and a distance between the image pick-up device and the lens unit is adapted to fluctuate in a second direction in response to temperature fluctuations and the said first and second direction are aligned with each other; characterised in that the holding member is adapted to constrain the relative separation of the image pick-up device and the lens unit such that the difference between the fluctuation of the focal length of the lens unit caused by temperature fluctuations, and the fluctuation of the distance between the image pick-up device and the lens unit caused by temperature fluctuations is less than the effective focal depth of the lens unit for the one of the three said wavelengths of Red, Green and Blue for which the peak width of the modulation transfer function, MTF, of the reflected light to the image pick-up device is the narrowest; and in that the focal length of the lens unit is adapted to increase in the said first direction in response to an increase in temperature and the distance between the image pick-up device and the lens unit is adapted to increase in the said second direction in response to an increase in temperature. an image pick-up device configured to convert a reflected light reflected from an original document to an electric signal, after the light is irradiated on the original document from a light source; a lens unit configured to read full-colour including three wavelengths of Red, Green and Blue and image the reflected light on the image pick-up device; and a holding member configured to hold the image pick-up device and lens unit; wherein a focal length of the lens unit is adapted to fluctuate in a first direction in response to temperature fluctuations and a distance between the image pick-up device and the lens unit is adapted to fluctuate in a second direction in response to temperature fluctuations and the said first and second direction are aligned with each other; characterised in that the holding member is adapted to constrain the relative separation of the image pick-up device and the lens unit such that the difference between the fluctuation of the focal length of the lens unit caused by temperature fluctuations, and the fluctuation of the distance between the image pick-up device and the lens unit caused by temperature fluctuations is less than the effective focal depth of the lens unit for the one of the three said wavelengths of Red, Green and Blue for which the peak width of the modulation transfer function, MTF, of the reflected light to the image pick-up device is the narrowest; and in that the focal length of the lens unit is adapted to increase in the said first direction in response to an increase in temperature and the distance between the image pick-up device and the lens unit is adapted to increase in the said second direction in response to an increase in temperature.