An adjustable reflector device for light fixtures is disclosed. The device comprises a reflector sheet made of a resiliently flexible material with at least one reflective surface folded and flexed along two parallel pivot axes into a concave arrangement of one central section and two flexible wings behind a high-intensity discharge lamp, such that the degree of concavity of the reflector can be adjusted to a plurality of predetermined degrees relative to the lamp by flexing or bending the wings radially around the pivot axes without the use of tools, thereby allowing horticulturists in the field to adjust the geometry of the light beam emitted in order to provide plants positioned below the fixture with a uniform pattern of light radiation of different intensities as needed at different stages of plant growth.
