The light guide, especially for motor vehicle signal lamps, comprises a collimator with a collimating wall for binding and routing light rays, and a light guiding body that continues the collimating wall, is of a material with refractive index (n), and is integral, spatially shaped, planar, and fitted at its end with an output emitting surface providing a signal light function. The first height (d) of the body at its beginning where it adjoins the collimating wall, is bigger than its second height where it passes into the emitting surface. The body is adapted to emit light rays generally within angle (ω) of diffusion from the optical axis (x), and comprises a transitional surface that is, in its profile towards the emitting surface, inclined towards the longitudinal axis of the profile. The ratio of inclination height (a) and inclination length (b) of the transitional surface is defined as: a b = tg ⁢ ⁢ ( 1 2 ⁢ arc ⁢ ⁢ sin ⁡ ( sin ⁢ ⁢ ω n ) )