The invention relates to an illumination system (10), a luminaire (102), a collimator (30), and a display device (200). The illumination system according to the invention comprises a light source (20) and the collimator. The light source is configured to emit a substantially Lambertian light distribution around an axis of symmetry (22). The refractive collimator is configured to redirect light from the light source so as to at least partially illuminate an illuminating surface (50), in which at least a part of the illuminating surface is substantially parallel to the axis of symmetry. The refractive collimator comprises a concave entrance window (34) and an at least partially convex exit window (40) for refracting light towards the illuminating surface.The illumination system according to the invention has the effect that a height of the illumination system may be reduced by virtue of the use of the refractive collimator.