They converge light only in the curvature direction. Common types include plano-convex and bi-convex lenses, widely used in laser collimation, beam focusing, and other applications. The core of a positive cylindrical lens is to realize unidirectional convergence by "bending light toward the thicker part due to refractive index differences at different positions".

• Structure: At least one surface is a convex cylinder (convex along one direction). In the convex (curvature) direction, the lens is thicker at the center and thinner at the edges. In the perpendicular (non-curvature) direction, it is flat with uniform thickness.
• Optical Principle: When parallel light is incident: In the curvature direction, edge rays pass through a thinner region, while central rays pass through a thicker region. Due to the refraction of the optical material, the thicker region deflects light more strongly. Finally, all rays converge toward the central axis, forming a clear real focal line (detectable on the focal plane). In the non-curvature direction, light always passes through a flat surface with uniform thickness, so the propagation direction remains unchanged without convergence or divergence.