A colour model that defines colours by the three parameters hue (H), lightness (L), and saturation (S). It was introduced by Tektronix Inc. Hue lies on a circle, saturation increases from centre to edge of this circle, lightness goes from black to white. This model uses the same hue plane as the HSV model, but it replaces value (V) by an extended lightness axis so that the maximum colour gamut is at L=0.5 and decreases in each direction towards white (L=1) and black (L=0). The HLS colour model is represented by a double hexagonal cone, with white at the top apex and black at the bottom.