a transparent sheet; and

a plurality of metal reflectors entirely embedded in a plurality of grooves formed in a front surface of the transparent sheet in a one-dimensional grid array direction and with a constant period (d),

wherein each of the metal reflectors has a constant width portion and an end portion having a shape that gradually becomes thinner in a linear or smoothly curved cross-sectional shape toward a tip taken perpendicular to the front surface of the transparent sheet,

the constant width portion of each of the metal reflectors has an average width (a) of 200 nm or less,

the metal reflectors have an average thickness (b) along a direction from the front surface of the transparent sheet to the tip of the end portion in a backward direction thereof so that a ratio (b/a) of the average thickness (b) to the average width (a) is from 4 to 25, and

the end portions of the metal reflectors have an average length (c) toward the tip so that a ratio (c/a) of the average length (c) to the average width (a) is from 1.2 to 4.7, and a ratio (c/b) of the average length (c) to the average thickness (b) is from 0.127 to 0.315.