Special topic detailed explanation of the relevant knowledge points of LED lenses

Basic concept of lens

Lenses are made according to the law of light refraction. Lenses are optical elements made of transparent materials (such as glass, crystal, etc.). Lenses are refracting mirrors whose refractive surfaces are two spherical surfaces (part of a spherical surface), or a spherical surface (part of a spherical surface) and a flat transparent body. The images it forms are both real and virtual. Lenses can generally be divided into two categories: convex lenses and concave lenses. A convex lens is thicker in the middle than in the edge, and there are three types: biconvex, plano-convex, and concave-convex; a concave lens is thinner in the middle than in the edge, and there are three types: biconcave, plano-concave, and convex-concave.

LED lenses are generally silicone lenses. Because silicone has high temperature resistance (it can also be reflow soldered), it is often directly packaged on LED chips. Generally, silicone lenses are small in size, with a diameter of 3-10mm. In addition, LED lenses are generally closely connected with LEDs, which helps to improve the light output efficiency of LEDs and the optical system of lenses that change the light field distribution of LEDs.

LED lens is an optical system closely associated with LED that helps improve the light output efficiency of LED and change the light field distribution of LED. High-power LED lens/reflector is mainly used for focusing and guiding light of high-power LED cold light source series products. High-power LED lens designs light distribution curve according to the angle of different LED light output, increases optical reflection, reduces light loss and improves light efficiency (and sets aspherical optical lens). The following focuses on the secondary focusing high-power LED lens made of PMMA material.

1) Classification by material

1. Silicone lens

a. Because silicone has high temperature resistance (it can also be reflow soldered), it is often directly packaged on LED chips.

b. Generally, silicone lenses are small in size, with a diameter of 3-10mm.

2. PMMA lens

a. Optical grade PMMA (polymethyl methacrylate, commonly known as: acrylic).

b. Plastic materials, advantages: high production efficiency (can be completed through injection molding and extrusion); high light transmittance (penetration rate is about 93% at a thickness of 3mm); disadvantages: the temperature cannot exceed 80° (thermal deformation temperature is 92 degrees).

3. PC lens

a. Optical grade material Polycarbonate (PC for short).

b. Plastic materials, advantages: high production efficiency (can be completed through injection molding and extrusion); slightly low light transmittance (penetration is about 89% at a thickness of 3mm); disadvantages: the temperature cannot exceed 110° (thermal deformation temperature 135 degrees).

4. Glass lens

Optical glass materials have advantages: high light transmittance (97% at 3mm thickness) and high temperature resistance; disadvantages: large volume and heavy mass, single shape, fragile, difficult to achieve mass production, low production efficiency, high cost, etc. However, the current price of such production equipment is high and it is difficult to popularize in the short term. In addition, the disadvantage of glass being more fragile than PMMA and PC materials requires more research and exploration. As for the improved processes that can be achieved now, the unbreakable characteristics of glass can only be improved by coating or tempering. Although the light transmittance of glass lenses will be reduced after these treatments, it will still be much greater than the light transmittance of ordinary optical plastic lenses. Therefore, the prospects for glass lenses will be broader.

2) Application classification of LED lenses

1. Primary lens

a. The primary lens is directly packaged (or bonded) on the LED chip bracket and becomes a whole with the LED.

b. Theoretically, the LED chip emits light at 360 degrees, but in reality, the chip is fixed and packaged on the LED bracket, so the maximum light-emitting angle of the chip is 180 degrees (there is also a small amount of residual light in the range greater than 180 degrees). In addition, the chip will have some stray light. In this way, all the light from the chip can be effectively gathered through a lens once, and different light-emitting angles such as 180°, 160°, 140°, 120°, 90°, and 60° can be obtained. However, the light-emitting efficiency of LEDs with different light-emitting angles is somewhat different (the general rule is: the larger the angle, the higher the efficiency).

c. Primary lenses are generally made of PMMA, PC, optical glass, silicone and other materials.

2. Secondary lens

a. The secondary lens and LED are two independent objects, but they are inseparable in application.

b. The function of the secondary lens is to converge the light emitting angle of the LED light source into any desired angle between 5° and 160°. The distribution of the light field can be mainly divided into: circular, elliptical, and rectangular.

c. The secondary lens material is generally optical grade PMMA or PC; glass can be selected in special cases.

3) Classification by specification

1. Penetration type (convex lens)

a. When the LED light passes through a curved surface of the lens (biconvex has a curved surface), the light will be refracted and focused, and when the distance between the lens and the LED is adjusted, the angle will also change (the angle is inversely proportional to the distance). The optically designed lens spot will be very uniform, but due to the limitations of the lens diameter and lens mode, the LED light utilization rate is not high and the edge of the spot has a relatively obvious yellow edge;

b. Generally used for spotlighting at large angles (above 50°), such as table lamps, bar lamps and other indoor lighting fixtures;

2. Catadioptric (cone or cup type)

a. The design of the lens uses penetrating focusing in front, while the conical surface can collect and reflect all the side light. The overlap of these two kinds of light (at the same angle) can achieve the most perfect light utilization and beautiful light spot effect;

b. You can also make some changes to the surface of the conical lens, which can be designed into a mirror surface, frosted surface, beaded surface, striped surface, threaded surface, convex or concave surface, etc. to obtain different light spot effects.

3. Lens module

a. Multiple single lenses are injected into a whole multi-lens. According to different needs, it can be designed into a 3-in-1, 5-in-1 or even dozens of lenses in one lens module; two separate lenses can also be combined together through a bracket.

b. This design effectively saves production costs, achieves consistency in product quality, saves space for lamp mechanisms, and makes it easier to achieve characteristics such as "high power".

4) Materials and production

1. As an optical grade product, LED lens has strict requirements on light transmittance, thermal stability, density, refractive index uniformity, refractive index stability, water absorption, turbidity, maximum long-term working temperature, etc. Therefore, the material of the lens must be selected according to the actual situation. In principle, optical grade PMMA is selected. If there are special requirements, optical grade PC can be selected. At present, the PMMA material of Japan Mitsubishi is the best (VH001 is the frequently selected brand), and the Nantong Liyang, a branch of Mitsubishi in China, is slightly inferior.

2. A dust-free workshop of Class 10,000 or even higher must be equipped. Operators must wear anti-static clothing, finger cots, masks and other anti-static and dust-proof measures, and the workshop must be inspected and cleaned regularly.

3. You must have professional optical injection molding machines such as Toshiba, Demag, Haitian, Chen Hsong and other brands of injection molding machines, and strictly control the injection molding process to obtain qualified products.

4. Product inspection: no bubbles, no dents, no shrinkage marks, no flow lines, no crescents; shape accuracy Rt<0.005 surface roughness Ra<0.0002.

5. The product must be packaged in anti-static and dust-proof PVC and must be completely sealed. The temperature and humidity must be strictly controlled during storage, and it is best not to store it for more than one year.

From the design and production process of LED lenses, it can be seen that the seemingly simple LED lenses have very high requirements for software and hardware from design to finished products, which also causes a large price difference of LED lenses in the market. From the bright prospect of LED lighting popularization, lenses, as an essential component of LED lighting fixtures, will have a better and better market prospect.

5) Design and mold processing of LED lens

1. First of all, it depends on the light source (high-power LED). Different brands of high-power LEDs (such as CREE, Lumileds, Seoul, Osram, Edison, Changsenyuan, etc.) have different chip structures, packaging methods, and light characteristics, which will cause the same lens to be different when paired with different specifications and brands of LEDs; therefore, targeted development (oriented to mainstream brands) is required to meet actual needs;

2. Use optical design software (such as Tracepro, CodeV, Zemax, etc.) to design and simulate optical light dispersion, and obtain the corresponding optical aspheric surface;

3. LED lens itself is a precision optical component, so it has extremely high requirements for mold precision, especially the processing precision of the lens optical surface must reach 0.1μm, and the lens eccentricity must be within 3μm. Generally, the processing of such high-precision molds must have the following equipment: ultra-precision processing machine (for example: PRECITECHNANOFORM350), CNC comprehensive processing machine, surface grinder, milling machine, CNC electric discharge machine, surface profiler, etc.

4. The most precise part of the mold is the optical mold core. First, we select special mold core steel to complete the initial blank. After nickel plating, we use ultra-precision processing machine to process the curved surface with aspheric technology.

VI) Advantages

1. Regardless of the distance, there is not much difference between the lampshade (reflector cup) and the lens. In terms of uniformity, the lens is better than the reflector cup.

2. Use a small-angle LED lens, which is better than a lampshade because it can emit light farther! The light from the lampshade has already been concentrated by the lens (because the LED itself must have a lens) and then by the light cover, which will waste a lot of light. It is better to concentrate the light at the lens, and the light-emitting angle of the lens is easy to handle.

In addition: If space permits, using three 1W units will produce much better results than using one 3W unit.

3. In comparison, the lampshade has a large uniform light distribution range, but the light projection is not good, while the lens is the opposite.

4. LED transparency appears to be more upscale.

7) Consideration of light loss

1. The luminous flux of lamps with bulb shells and lenses must actually meet the light distribution requirements of the standard, and factors such as the transmittance of the shell, lens, and overflow light loss must also be considered. Bulb lamps or high-power ordinary lighting require lenses to diffuse parallel light beams to meet the requirements of the standard. In order to make the optical effect more reasonable, the lamp cover should be divided into small rectangular units in the design. The purpose of this is to break the wave surface of the light wave and make the product have a uniform appearance. In each small unit, an ellipsoidal surface is used because the surface has curvature in both horizontal and vertical directions, so different diffusion effects can be achieved with different curvature radii in two directions. Its fundamental purpose is to overcome the shortcomings of traditional technology, reasonably utilize luminous flux, and achieve uniform and efficient light distribution. In fact, the shell of the bulb type is PC material (injection molding completed), and the spherical, pear-shaped, and cylindrical bulb shells are all non-small units and non-planar whole shells, with large light loss and small light angle.

2. Because one surface of the lens is a curved surface with curvature radii in both horizontal and vertical directions, the incident light can be diffused in both horizontal and vertical directions. Since the curvature radii in the two directions are independent of each other, the two curvatures can be adjusted separately according to requirements so that the light output can be diffused to different degrees in the two directions. Therefore, the lens composed of a bidirectional curvature surface can distribute the light output more freely according to the design requirements, use the luminous flux more efficiently, and reduce unnecessary waste and glare. In addition, due to the use of a smooth transition surface, the lamp has a uniform transition light distribution and a good appearance. Completely transparent PMMA lamps or lampshades will cause dazzling or glaring string light in the center of the light source, but the brightness decreases rapidly around the light source. The lighting of many social occasions and working environments must exclude this unpleasant atmosphere or minimize the light source that causes eye discomfort.

3. The projection of each lens unit on the body is a rectangle, so that each unit can be arranged closely and neatly. The parallel incident light beam is refracted by the lens unit to form a symmetrical uniform diffusion in the horizontal direction and a uniform diffusion with downward deflection in the vertical direction. By adjusting the size of each unit in a group of lenses and the radius of curvature in two directions, the distribution of the outgoing light flux in different solid angles can be adjusted to achieve the light distribution required by the design.

Since the role of the incident curved surface is to deflect and diffuse the light, the number of units, unit size, curvature radius, etc. of each lens group in the product design can be changed according to the actual situation. The actual situation is that the inner pattern (to divide the small units) on the lens of the high-power lens is made by the manufacturer, and only the lens height, angle, and material are considered when selecting.

4. We choose to put the light source inside the focal point of the lens. The farther the light source is from the lens, the less light flux the lens collects, and thus the lower the efficiency of the lens system. According to the calculation formula of a single convex lens: r = (nL-1) f. Among them, r-convex surface curvature radius, nL-lens material refractive index, f-lens focal length. When the lens material is selected, the larger the focal length, the larger the curvature radius. Under the condition of the same lens aperture Φ, the larger the curvature radius, the thinner the lens. The thicker the lens, the more obvious the aberration will be, thus affecting the use effect. Therefore, try to choose a lens with a larger focal length. At the same time, the increase in focal length increases the size of the optical system. Therefore, the focal length of the lens cannot be blindly pursued to the maximum. Since the thickness of the lens is not very large, a Fresnel lens is not used to avoid increasing the tediousness and cost of processing.