A light mask type optical device, comprising: an optical element, made of transparent plastic material, and it includes: a light inlet face; a light outlet face opposite to the light inlet face; and a plurality of grating structures of micro meter (μm) scale, disposed on the light outlet face, each of the grating structures includes a first slant face, a second slant face, and a plane surface located between the first slant face and the second slant face, to form into an inverted trapezoid, an indent portion formed on the light inlet face opposite the grating structures, wherein when the light is emitted from a light source right above the light inlet face, a part of the light is output from the light outlet face, while the other part of the light is reflected by the first slant face and the second slant face to form into a circulating light, and that is combined with a direct downward light, to impinge onto the light outlet face, as such an incident light is enhanced by a sideway light, and to be concentrated to a centre of the light outlet face, in achieving reduced glare and increased lumen, characterized in that an angle formed by the first slant face and the second slant face is between 70 to 100 degrees and that the grating structure of μ m scale is formed having the following parameters; a first width W1 for a width of the plane surface being 0.15 mm - 0.85 mm, a second width W2 for the width of the grating structure being 1 mm-3 mm; the first height H1 for a height of the grating structures being 0.35 mm - 1.05 mm; while the second height H2 for the height of the indent portion being 0.02 - 0.08 mm. an optical element, made of transparent plastic material, and it includes: a light inlet face; a light outlet face opposite to the light inlet face; and a plurality of grating structures of micro meter (μm) scale, disposed on the light outlet face, each of the grating structures includes a first slant face, a second slant face, and a plane surface located between the first slant face and the second slant face, to form into an inverted trapezoid, a light inlet face; a light outlet face opposite to the light inlet face; and a plurality of grating structures of micro meter (μm) scale, disposed on the light outlet face, each of the grating structures includes a first slant face, a second slant face, and a plane surface located between the first slant face and the second slant face, to form into an inverted trapezoid, an indent portion formed on the light inlet face opposite the grating structures, wherein when the light is emitted from a light source right above the light inlet face, a part of the light is output from the light outlet face, while the other part of the light is reflected by the first slant face and the second slant face to form into a circulating light, and that is combined with a direct downward light, to impinge onto the light outlet face, as such an incident light is enhanced by a sideway light, and to be concentrated to a centre of the light outlet face, in achieving reduced glare and increased lumen, characterized in that an angle formed by the first slant face and the second slant face is between 70 to 100 degrees and that the grating structure of μ m scale is formed having the following parameters; a first width W1 for a width of the plane surface being 0.15 mm - 0.85 mm, a second width W2 for the width of the grating structure being 1 mm-3 mm; the first height H1 for a height of the grating structures being 0.35 mm - 1.05 mm; while the second height H2 for the height of the indent portion being 0.02 - 0.08 mm.