LED optical design that thing

Due to the rush to bring products to market, early attention paid little attention to the optical properties of solid-state lighting SSL and the opportunities it brings. LED luminaires designed by OEMs and luminaire manufacturers still have the same look and function as other luminaires in the history of 130 years of electric light source. Bailey said: "Because the technology is far worse than today, we can't realize the interaction between people and products. We invented the light, but we have not been able to go one step further." When the comfort, glare, uniform and other lighting needs are put on the agenda On the agenda, manufacturers and lighting designers are re-examining the way they want to go.

Difference interpretation

To understand the difference between LEDs and traditional (filament) sources, you must know how they work. The diode is a photon-filled hole generated by the two poles of the semiconductor to generate current luminescence. Incandescent lamps generate light by high temperature thermal radiation.

Figure: LED lighting principle

Source: OMS lighting

Light distribution is another difference between different light sources. The light distribution of a conventional light source is a spherical shape of 360 degrees, and the distribution of the LED light source is 180 degrees. Peter Ngai, vice president of research and development at the Acuity lighting brand, said: "So we must customize optics based on this feature."

The directionality of the LEDs and the forward illuminating characteristics mean that retrofitting traditional luminaires is by no means just as simple as changing a light source. For example, a reflector for an incandescent or compact fluorescent lamp may have no effect at all for the LED source. Because the back of the LED does not emit light, the reflector can be reflected without light. In most of the retrofit cases, Nagi said, “The light distribution is very different. If you want to replace existing fixtures with LEDs correctly, you need to replace the previous reflectors, lenses and other internal optics, and replace them with LED light sources.

The quantity is also critical. Bailey said that LED luminaires use multiple diodes (equivalent to multiple small light sources), each of which "generates a large amount of light", which requires control of their brightness and proper redistribution, otherwise it is prone to discomfort Glare and pixelated distribution, rather than continuous uniformity throughout.

Figure: Example of LED pixelation application. In the whole lamp, it is necessary to avoid pixelation of the light source point as much as possible.

Source: OMS lighting

optical instrument

For traditional luminaires, manufacturers use a combination of reflector cups and lenses to spread and project light onto the target. But these optics, such as the frosted glass bowl (DOME), can cause up to 50% of light loss. Maria Topete, a senior application specialist at Prius, said: A lot of light has been absorbed.

Therefore, in the LED application of this performance-name game, those suppliers of lamps using traditional optics, "have suffered a fatal blow." Topete said, “So when we design, we become smarter. Those optical solutions that look similar have actually many ideas and techniques that are better.”

LEDs, like traditional light sources, want to eliminate glare and must enlarge the light-emitting area to make it diffuse. Topete said: "Although OEMs want to get as much light as possible from the smallest package, this can cause visual discomfort.

LED luminaires typically have two optical layers. Primary optics, using a lens similar to a glass hemisphere, aligns and distributes light according to the expected luminosity specification; secondary optics further parallels, diffuses, or directs light according to the needs of light distribution and orientation. In this case of secondary optics, the size of the light source is increased. In the opposite case, the primary optics use a diffuser to control light, and the secondary optics use a lens or reflector to control light.

Figure: LED optical design structure

Source: Network

Adjust traditional technology

Topete says that the use of optics depends on several factors, including the application scenario, but in any case, the key to efficient use of light is to "allow light to exit" and reach the illuminated surface.

Since the LED's illumination angle is within 180 degrees, "In most cases, the lens is a suitable method to control the light output," Nagi said. The small size of the LED means that the complete lens is comparable to the size of the light source, and a compact, or Fresnel lens can be used to calibrate the light. In this way, light loss and dispersion can be reduced from the source.

Manufacturers are also improving the quality of lenses. Bailey said that we are changing the inner surface of the lens to form a plurality of matte surfaces, thereby largely eliminating the reflected light caused by the reflection of the internal surface of the lens.

The increase in diffusion technology allows for better control of light while maintaining luminous efficiency. Topete says that unlike glass or polycarbonate lenses that are frosted or internally filled with scattering particles, manufacturers can customize the shape and surface of the appropriate plastic lens for LED applications, which can reduce light loss by about 10%.

Figure: An embedded LED light panel that uses a prism diffuser to evenly distribute light.

Source: Courtesy Hubbell Lighting

In contrast, the LED emits less heat forward, so it can be attached to a lens or a diffusion medium in front of it, together with materials such as films and sheets. And this is obviously not possible with traditional light sources that generate high heat.

Luminaire manufacturers and OEMs also use parabolic or hyperbolic reflectors to control light and prevent light from entering the field of view directly, thereby reducing glare. However, although a parabolic reflector causes sufficient light to be spherically distributed, it may not be able to control the escape of light near the central axis of the source, which is a significant fraction of a 180-degree LED. As a result, some luminaire manufacturers have attempted to use LED arrays in different directions inside the parabolic reflector to improve control of the light output direction.

Future new technology

Existing optical technologies do not fully meet the needs of luminaire applications.

Manufacturers are tapping the potential of refractive technology. Bailey said that we are developing prisms to replace the reflector cup. Total internal reflection (TIR) ​​optics combines a reflective cup and a refractive lens to control the direct and reflected light of the LED. The beam of the central axis of the LED can enter and pass through the refractive lens, while the total internal reflection surface can control the beam range.

Figure: The small size of the LED allows the luminaire to be smaller and more efficient, with higher precision optics. The picture above shows a TIR total internal reflection lens.

Source: Courtesy Hubbell Lighting

In order to make effective use of internal reflection, luminaire manufacturers also use "photoconductive materials" to distribute the intense light from the LEDs mounted on the side of the luminaire. Nagi said, "The optical design of the light-guide material is to make better use of the light. After the light is injected, they can be redistributed due to the total internal reflection characteristics of the light-guide material."

Figure: A practice of using an ultra-thin light box with a light guide

Source: Network

Lighting manufacturers also use holographic films such as polycarbonate and polyester to experiment. The microstructure of these diffusers eliminates the imaging and pixelation effects of the LEDs, changing the distribution of light to form different beam angles, and efficiently transmitting light.

Injection molded acrylic and plastic optics have also attracted the interest of LED and luminaire manufacturers. Optical engineers are constantly experimenting with micro-surfaces and micro-features on lenses. Bailey said: "These seemingly minimal features on the lens have a huge impact on the presentation of products and light."

Figure: A downlight using a high-precision injection molded acrylic wall mirror.

Source: Courtesy Hubbell Lighting

Manufacturers now have another new technology for customizing LED optics: 3D printing. Norwegian Luxexcel's patented technology, "Optical Printing Method", enables diode-level printing. Utilizes an improved, multi-format, industrial-grade inkjet printer that eliminates the need for any tools, any molds, or die casting, and requires only CAD files to print out flat and 3D optics, including lenses, prisms, microstructures, and Multi-color, multi-surface laminate. Luxexcel's marketing manager said that several luminaire manufacturers around the world have begun experimenting with this technology.

Figure: Green Linear Prism Printed by Luxexcel

Source: Courtesy Luxexcel

Figure: A spherical microlens printed by Luxexcel that allows light to be parallel, divergent or diffuse, and to mix colors.

Source: Courtesy Luxexcel

The size, performance and reliability of LEDs have opened the door to optics and luminaire design, which manufacturers are just beginning to realize. At the same time, advances in electroluminescence and remote phosphor technology are equally impressive. Bailey said that the lighting industry has been "rebirth of the bath... this is an exciting time, we can use this new technology to freely create beautiful light."