An apparatus for curing a liner of a pipeline, said liner including a resin which is curable by exposure to electromagnetic radiation of a specific wavelength or a specific wavelength range, said apparatus comprising: a housing defining opposite first and second ends, an outer wall in any appropriate geometrical shape composed of a set of curved or planar surface elements, each of said curved or planar surface elements extending longitudinally between said first and said second ends of said housing, said surface elements being of identical configuration, and an inner wall defining a substantially unobstructed through-going passage extending longitudinally through said housing between said first end and said second end, a pair of power supply wires for the supply of electrical power to said apparatus and extending from said first end of said housing, a plurality of LED's irradiating electromagnetic radiation of said specific wavelength or said specific wavelength range, said plurality of LED's being positioned and arranged at said curved or planar surface elements, said plurality of LED's being connected through an electronic circuit to said pair of power supply wires, and said plurality of LED's being connected in thermal conductive relationship to heat dissipating elements freely exposed at said inner wall of said housing in said through-going passage of said housing for allowing a stream of cooling fluid to pass through said passage for dissipating heat from said heat dissipating elements and cooling said LED's, characterized in that said housing defines an innermost wall dividing said substantially unobstructed through-going passage into an inner passage centrally located within said substantially unobstructed through-going passage and extending between said first and second ends, and, an outer passage defined between said inner wall and said innermost wall and coaxially enclosing said inner passage. a housing defining opposite first and second ends, an outer wall in any appropriate geometrical shape composed of a set of curved or planar surface elements, each of said curved or planar surface elements extending longitudinally between said first and said second ends of said housing, said surface elements being of identical configuration, and an inner wall defining a substantially unobstructed through-going passage extending longitudinally through said housing between said first end and said second end, a pair of power supply wires for the supply of electrical power to said apparatus and extending from said first end of said housing, a plurality of LED's irradiating electromagnetic radiation of said specific wavelength or said specific wavelength range, said plurality of LED's being positioned and arranged at said curved or planar surface elements, said plurality of LED's being connected through an electronic circuit to said pair of power supply wires, and said plurality of LED's being connected in thermal conductive relationship to heat dissipating elements freely exposed at said inner wall of said housing in said through-going passage of said housing for allowing a stream of cooling fluid to pass through said passage for dissipating heat from said heat dissipating elements and cooling said LED's, characterized in that said housing defines an innermost wall dividing said substantially unobstructed through-going passage into an inner passage centrally located within said substantially unobstructed through-going passage and extending between said first and second ends, and, an outer passage defined between said inner wall and said innermost wall and coaxially enclosing said inner passage.