Because liquid crystal itself does not emit light, a backlight unit (BLU) is needed to provide illumination rays. The brightness and volume of a BLU largely determine the performance of a display. Ultra-thinness has become a popular trend, which requires a light-emitting diode (LED) BLU to minimize its thickness as soon as possible. The direct-lit BLU has the advantages of high brightness, high energy utilization rate, and good uniformity. In order to reduce the production cost, the number of LEDs in the direct-lit module is decreasing, and the distance between LEDs is increasing, which makes the optical distance (OD) larger. Free-form lenses have been widely used in direct-lit BLUs to reduce the thickness and increase the distance-height ratio (DHR). However, when the OD is quite small, the size of the designed lens is relatively small, which results in a large processing error. Furthermore, the illuminance uniformity will be reduced when the lens designed based on the point source method is used for the extended light source. A lot of complicated optimizations are needed to improve uniformity. Therefore, an ultra-thin lens based on the surface microstructure is designed in this paper. The lens first collimates the rays emitting from an LED and then reflects the rays by the microstructures, which can increase the size of the illumination spot during the limited OD. The curvature of the designed surface is small, which can help avoid the influence of processing errors.

In this paper, a lens design method based on the surface microstructures is proposed for an ultra-thin BLU that consists of an array of LEDs with a pitch of Δpitch,x×Δpitch,y (Δpitch,x is pitch of x direction, and Δpitch,y is pitch of y direction). This design is different from the traditional double free-form surface lens which uses the free-form surface with a large curvature to refract the light at a larger angle. In this paper, the free-form surface is used to collimate the rays from the light source, and then the collimated rays are reflected to the bottom by the surface microstructure. After that, the rays are reflected again by the reflection film at the bottom so that the spot size can be effectively increased under the small OD. In addition, the collimation effect of a traditional total internal reflection (TIR) lens designed based on a point light source is not ideal under an extended light source. Therefore, the proposed method uses the edge ray principle to improve the collimation of the rays passing through the free-form surface under the extended light source. It requires no complicated optimization when the point light source is replaced by the extended light source.

The design is carried out in a BLU with a mini-LED number of 3×3, an array pitch of 15 mm×15 mm, and an OD of 3 mm. Based on Snell's law, the paper firstly designs three free-form surfaces to collimate the ray emitted by the light source and then designs surface microstructures to reflect the collimated rays to the bottom (Fig. 2). In addition, based on the edge ray principle, the paper optimizes the free-form surfaces so that it can improve the collimation of the rays when the light sources are changed to the extended ones (Fig. 10). The simulation model is built in LightTools. The simulation results show that the uniformity can reach 82% for a 3×3 LED array with an OD of 3 mm and a DHR of 15 mm. Compared with that of the traditional double free-form surface lens, the uniformity is improved by 40.7%.

Free-form surface lenses have been widely used in direct-lit BLUs to reduce the thickness and increase the DHR. However, when the OD is small, the size of the designed lens is relatively small, which results in a large processing error. In addition, the illuminance uniformity will be reduced when the lens based on the point source method is used to form the extended light source. In this paper, an ultra-thin lens based on the surface microstructure is designed according to the TIR principle. The free-form surfaces are used to collimate the rays from the light sources. Then the collimated rays are reflected to the bottom by the microstructures. The rays are reflected once more by the reflection film. Therefore, the spot size can be effectively increased under the small OD. In addition, the edge ray principle is used to improve the collimation of the rays passing through the free-form surface under extended light sources. With no larger curvature, the designed lens can avoid the influence of processing error and achieve satisfying illumination. The proposed method does not need a lot of complicated optimization work, which presents high practical application value.