The invention relates to a method of producing a screen (6) having a striped structure of electroluminescent material on a display window (3) of a colour display tube (1). In present day colour display tubes (1), this screen (6) is produced using a photochemical process for exposing a photosensitive material which is applied to the display window (3). Normally, the exposure device used for this process comprises two lenses, a first lens (28) for correcting the rotation of the image of the elongated light source (22) and a second lens (27) for taking care that the landing position of the light on the display window (3) will be representative of the landing position of the electron beams (7), (8), (9) in the colour display tube (1) when it is operated. Unfortunately, the prior art system has the disadvantage that the line-growth factor is not constant over the entire screen (6). As a result, a change in the amount of light in the exposure process leads to a change in the distribution of the line width over the screen (6) and hence to a change of the luminance distribution. This invention provides a solution to this problem by introducing a third lens element (35) into the exposure device. This lens element (35), that breaks the four-quadrant symmetry of the prior art system, enables a deliberate and controlled rotation of the image of the elongated light source (22) on the screen (6) in such a way that the line-growth factor is made constant over the entire screen (6). In a preferred embodiment, the first lens (28) and the third lens (35) are integrated to form one lens (36).