Method for designing lens of lighting device

A method for producing a streetlight, the method comprising:

• providing an LED device as a light source;
• and
• providing a lens to distribute light of the light source to an irradiation surface, the lens having a first curved surface for refracting the light of the light source and a second curved surface for refracting the light having passed through the first curved surface, the providing a lens including:
• determining the irradiation surface that is an area onto which light having passes through the second curved surface is to be irradiated;
• forming a reference circle disposed within the irradiation surface;
• designing a free curved surface of the first curved surface by using light emitted from the light source as an input direction vector and a coordinate within the reference circle as an output direction vector to calculate normal vectors of the first curved surface;
• and
• designing a free curved surface of the second curved surface by using light refracted by the first curved surface as an input direction vector and a coordinate within the irradiation surface as an output direction vector to calculate normal vectors of the second curved surface.