The optical component is a very precise component, and the manufacturing cost is high. If the thickness of the component can be reduced, or even a sheet lens, the size of the optical component can be reduced, thereby reducing the size of the lamp or other equipment, and saving material. cut costs. As the thickness is reduced, the light absorption is also reduced, and the efficiency of the luminaire or instrument is also increased. Therefore, it is one of the goals of optical design to make high-quality sheet-shaped optical parts.
The Fresnel lens is a sheet-like thin lens that has been used in some aspects for its lightness, thinness and low cost. However, the Fresnel lens on the market is mostly concentric circle structure with equal radius. The fabrication lacks precise optical design process, resulting in low image quality, and some even simple corrugated structures, and its optical quality is even worse. . Even a better Fresnel lens is usually formed by dividing a common lens into small segments, which are approximately broken lines, and are formed by simple translation of different distances. The defects in these design methods result in low quality of the Fresnel lens.
LEDs are small in size, but most of the LED lenses on the market are thicker than 10mm, which is a fatal problem for LEDs in some applications, although Fresnel lenses can be used to reduce the thickness of the lens and reduce light absorption. However, how to carry out accurate optical design is rarely reported in the literature.
This article describes the design method for obtaining accurate ultra-thin zigzag lenses with good optical quality and high light utilization. Because the general Fresnel lens is theoretically wasteful, that is, the light passing through the lens theoretically has a part that cannot reach the destination of the design, and the lens obtained by the method has no theoretical waste. In addition, the distance between the small serrations can also be different according to needs, and the zigzag spacing at different positions in the same lens can also be changed, so that the zigzag lens designed by this method has wider adaptability, that is, it can Adapt to different conditions of use and different processing conditions.
This zigzag lens is suitable for secondary optical lenses in which the LED is a light source. For a small-sized light source such as an LED, it is very meaningful to have a small and thin optical lens.
First, the design principle
A single lens is generally a transparent material whose surface shape is curved, and its function is to change the direction of the light to form a desired spatial distribution of light intensity. The disadvantage is that it tends to be relatively thick, so it is bulky and costly, and the absorption is large, especially for lenses with large curvature. For the sake of simplicity, an example of a plano-convex lens is shown in Fig. 1(a). Correspondingly, the conventional Fresnel lens is shown in Fig. 1(b). For the sake of explanation, the pitch of the figure is relatively large.
Figure 1 The formation principle of the traditional Fresnel lens
The design principle of Fresnel lens is to replace the entire continuous large surface with several small faces. Figure 1 (c) shows the design principle of a conventional Fresnel lens. The function of the sawtooth Fresnel lens of Fig. 1(b) is the same as that of the original lens of Fig. 1(a). The traditional design method can be represented by Figure 1 (c). Actually, the Fresnel lens of Fig. 1(b) can be regarded as that a plurality of rectangular portions are deleted from the lens of Fig. 1(a), and the remaining portion is moved downward into a sheet shape to become a Fresnel lens. See Figure 1(c), where the stepped shadow is the part of the multiple rectangles that are deleted.
It is obvious that the Fresnel lens of Fig. 1(b) is thinner than the lens (a), so that the absorption is small and the material is saved. However, this conventionally designed lens is correct only for parallel light, in which case the shaded portion of (c) has no effect on the light. However, in the case of non-parallel light, such as when the LED is a light source, the shaded portion of (c) has an effect on the light. If it is removed into a Fresnel lens, it will cause a lot of stray light. In addition, if the cross section of the lens is replaced by a broken line instead of a small arc, an optical error will also occur.
In order to overcome the above shortcomings, we propose to design Fresnel lenses in two new ways. Here we design for a single LED. For other light sources, the design principle is the same, so in principle it can be extended to other light sources.
The basic idea of ​​the new method is to divide the edge of the deleted invalid part along the light, and the effective remaining part moves along the light while changing its size in a certain proportion, so that the light will not propagate in the lens. Hit the invalid part of the edge and it will be refracted in the original direction. This reduces the scattered light and increases the optical efficiency of the lens.