A light emitting device (300, 400, 500), comprising: at least one light source unit, which includes a laser unit (210, 310, 410) and a collimating lens (220, 320, 420) corresponding to the laser unit (210, 310, 410), wherein a light emitting surface of the laser unit (210, 310, 410) is a rectangle, wherein a divergence angle (α) of a laser light emitted by the laser unit (210, 310, 410) in a cross-section that goes through a long side of the rectangle is less than a divergence angle (β) of the laser light in a cross-section that goes through a short side of the rectangle; and a focusing lens (330, 430, 530), wherein the collimating lens (220, 320, 420) collimates the laser light emitted by the laser unit (210, 310, 410) onto the focusing lens (330, 430, 530), and the focusing lens (330, 430, 530) focuses the laser light from the collimating lens (220, 320, 420) onto a predetermined rectangular area of a target plane to form a predetermined light spot (370a, 470a, 470b), wherein the laser unit (210, 310, 410) is located on an optical axis of the collimating lens (220, 320, 420) at a predetermined position away from a focal point of the collimating lens (220, 320, 420), and characterized in that the distance between the laser unit (210, 310, 410) and the corresponding collimating lens (220, 320, 420) is less than the focal distance of the collimating lens (220, 320, 420) and the difference between the off-focus collimating lens position and the ideal collimating lens position is less than 0.5 mm, wherein the predetermined light spot (370a, 470a, 470b) has a predetermined aspect ratio of 4:3 or 16:9, and either the light emitting device comprises a group of light source units, the group including a plurality of light source units, wherein the focusing lens focuses the laser light emitted by the plurality of light source units of the group to a common position on a predetermined rectangular area of the target plane to form the predetermined light spot, or the light emitting device (400, 500) comprises two groups of light source units, each group including a plurality of light source units, wherein the focusing lens (430, 530) focuses the laser light emitted by the plurality of light source units of each of the two groups to a common position on the target plane to respectively form two individual light spots (470a, 470b), and wherein the two individual light spots (470a, 470b) collectively form the predetermined light spot. at least one light source unit, which includes a laser unit (210, 310, 410) and a collimating lens (220, 320, 420) corresponding to the laser unit (210, 310, 410), wherein a light emitting surface of the laser unit (210, 310, 410) is a rectangle, wherein a divergence angle (α) of a laser light emitted by the laser unit (210, 310, 410) in a cross-section that goes through a long side of the rectangle is less than a divergence angle (β) of the laser light in a cross-section that goes through a short side of the rectangle; and a focusing lens (330, 430, 530), wherein the collimating lens (220, 320, 420) collimates the laser light emitted by the laser unit (210, 310, 410) onto the focusing lens (330, 430, 530), and the focusing lens (330, 430, 530) focuses the laser light from the collimating lens (220, 320, 420) onto a predetermined rectangular area of a target plane to form a predetermined light spot (370a, 470a, 470b), wherein the laser unit (210, 310, 410) is located on an optical axis of the collimating lens (220, 320, 420) at a predetermined position away from a focal point of the collimating lens (220, 320, 420), and characterized in that the distance between the laser unit (210, 310, 410) and the corresponding collimating lens (220, 320, 420) is less than the focal distance of the collimating lens (220, 320, 420) and the difference between the off-focus collimating lens position and the ideal collimating lens position is less than 0.5 mm, wherein the predetermined light spot (370a, 470a, 470b) has a predetermined aspect ratio of 4:3 or 16:9, and either the light emitting device comprises a group of light source units, the group including a plurality of light source units, wherein the focusing lens focuses the laser light emitted by the plurality of light source units of the group to a common position on a predetermined rectangular area of the target plane to form the predetermined light spot, or the light emitting device (400, 500) comprises two groups of light source units, each group including a plurality of light source units, wherein the focusing lens (430, 530) focuses the laser light emitted by the plurality of light source units of each of the two groups to a common position on the target plane to respectively form two individual light spots (470a, 470b), and wherein the two individual light spots (470a, 470b) collectively form the predetermined light spot.