Technologie et applications des guides de lumière
Les guides de lumière peuvent sembler simples, mais ils jouent un rôle essentiel dans la conception des équipements électroniques ; leur utilisation permet de simplifier le tracé des circuits imprimés, de faciliter le travail des ingénieurs en ergonomie et d'obtenir une conception plus compacte.
Cet article abordera le fonctionnement des conduits de lumière, la technologie sur laquelle ils reposent, les principes physiques sous-jacents, ainsi que les choix et les options qui s'offrent au concepteur d'équipements.
Table des matières
Un guide de lumière est une tige en plastique transparent solide ou une fibre optique servant à acheminer la lumière d'une LED montée sur une carte vers un autre emplacement où elle peut être facilement observée, comme le panneau de voyants d'un module électronique.
Ils offrent plusieurs avantages, notamment une indication claire de l'état de fonctionnement et une conception ergonomique simplifiée. Le trajet optique non conducteur ne nécessite ni soudure ni raccordement par sertissage, ce qui élimine tout risque de court-circuit dû à des fils pincés ; aucune soudure ni aucun raccordement par sertissage n'est nécessaire.
Les guides de lumière constituent des composants essentiels dans de nombreuses applications électroniques modernes. L'utilisation d'un guide de lumière représente une solution économique permettant d'éliminer bon nombre des étapes qui seraient autrement nécessaires pour acheminer la lumière d'un endroit à un autre.
Les conduits de lumière trouvent des applications dans divers secteurs industriels.
En fonction de l'équipement et de la conception de l'interface utilisateur d'un appareil, un produit peut nécessiter plusieurs indicateurs, voire un grand nombre d'entre eux. Pour les conceptions nécessitant plusieurs indicateurs, les conducteurs de lumière constituent une solution idéale en raison de leur faible coût, de leur excellente communication visuelle et de leur flexibilité de conception.
Santé
Dans le secteur de la santé, les solutions d'indication et d'éclairage par LED permettent la conception d'interfaces homme-machine (IHM) essentielles dans toute une série d'applications de technologie médicale.
Les applications médicales des guides de lumière comprennent notamment :
- Technologies pour la vie connectée
- Dispositifs médicaux à usage unique
- Appareils médicaux connectés
- Interface homme-machine dans le domaine médical
Automatisation et contrôle industriels
Les conduits de lumière contribuent à garantir la sécurité, la fiabilité et la conformité dans des environnements difficiles, tels que ceux rencontrés dans les opérations industrielles. Les technologies intégrées d'indication et d'éclairage améliorent la surveillance des ateliers de production afin de protéger les équipements contre les conditions dangereuses.
Parmi les applications industrielles, on peut citer
- Commandes de machines industrielles
- Usines numériques
- Industrie 4.0
- Bâtiments intelligents
Véhicules électriques, conduite autonome et secteur automobile, transports
Les guides de lumière répondent aux exigences en matière d'indication et d'éclairage par LED de ce marché en pleine expansion, qui englobe les véhicules électriques et leurs infrastructures de recharge, ainsi que les moyens de transport non automobiles.
Les applications dans le domaine des transports comprennent :
- Véhicules électriques
- Véhicules autonomes
- Transports connectés
- Mobilité hybride et électrique
Énergies renouvelables
Les conduits optiques garantissent un fonctionnement fiable dans les conditions les plus difficiles. L'exposition à l'eau, aux températures extrêmes et aux vibrations exige que la conception des produits soit conforme aux normes ISO strictes.
Les conduites de lumière sont utilisées dans de nombreux domaines :
- Énergie éolienne
- Solar power
- Greenhouse operations
- Agriculture
Communications
As part of the expanding infrastructure driving worldwide connectivity, light pipes direct light throughout complex systems: from servers and edge computing, to data centers and 5G system infrastructure.
Sample applications in communications include:
- Data centers
- Cloud computing
- Storage
- 5G/IoT
A light pipe is placed very closely to an LED light source on one end of a PCB board. Its optical-grade materials help carry the rays of light to the desired destination, typically at the user interface of a device. Light pipes transmit roughly 80 to 90 percent of the light from the LED source, depending on design and spacing. The closer the light pipes are to the LED(s), the more efficiently they can transfer light. Of course, the light pipe(s) used will need to be rated for the same amount of heat that is produced by the LED or light source, so that must be a design consideration as well that can dictate the minimum allowable distance between the light pipe and the light source.
Refractive Index: Medium Matters
Light doesn’t always behave the same way, which makes light pipe design that much more complex. Why?
Let’s go back to physics class and learn about refraction.
Refraction is defined as the change in direction of a wave from one medium to another or from a gradual change in the medium itself.
So, in addition to all the other variables that go into light pipe design (size, shape, brightness, distance to LED, viewing angle, and material used, to name a few), designers must also consider the Refractive Index.
The Refractive Index is used to determine how fast light travels through a medium.
Calculated as n = c/v, c is the speed of the light and v is the velocity of the light in that specific medium.
This number represents how much of the light is bent or refracted when it hits the medium. It also calculates how much light is reflected when reaching the medium.
In a vacuum, light breaks at a rate of 1.0. But no one designs light pipes to be used in this type of vacuum, so additional consideration must be given to optimize performance.
Plus, any number above 1.0 shows the decreased speed at which it travels compared to in a vacuum.
Refractive Indices:
Air, the rate is 1.003
Water, the rate is 1.33
Acrylique, which is what most light pipes are made of — it’s ~1.49.
Refractive Index isn’t the only number that needs to come into play when it comes to how a surface affects light.
There’s also total internal reflection (TIR), which is the angle at which light is reflected when none of it shines through to a surface exit, instead it is reflected from one surface to another where it is visible.
Source: https://www.physicsclassroom.com/class/refrn/Lesson-3/Total-Internal-Reflection
TIR is a phenomenon that occurs when the angle of incidence is greater than a certain limiting angle, referred to as the critical angle.
Why does TIR matter in light pipe design?
Often, light pipes are used to guide light around corners. To help ensure that the light bounces at the critical angle and travels to the endpoint with maximum effectiveness, understanding the critical angle of various materials is essential.
For clear polycarbonate, the critical angle is 39, and for acrylic, it’s 42.
Understanding how the light reacts in different scenarios will lead to better, more optimized light pipe performance.
For example, when using a right-angle light pipe, here’s how TIR would affect the specifications:
When designing light pipes, most people assume light behaves like electric current and ignore TIR.
Because light has unique properties, designing a light pipe like the right-angle illustration shown above will result in significant light loss at the TIR. To avoid light loss, incorporate the critical angle to create a geometry to guide the light around the critical angle without having the light escape. This concept also applies to light pipes with round corners (or any geometry your design includes).
Two types of light pipes are commonly used: rigid and flexible pipes.
A rigid light pipe is designed to carry light shorter distances in moderate to high vibration conditions and in damp or dusty environments. Rigid light pipes are available in many configurations including vertical, right angle and multi-levels and are designed for distances up to 3 inches. A rigid light pipe is constructed from polycarbonate.
Rigid light pipes are available in numerous configurations, including straight, right angle, single-unit, or multiple-unit, stacked or vertical arrangements Rigid light pipes with an IP67-rated heavy-duty threaded retention provide maximum protection against liquid and dust incursion. Complete solutions are available that include the light pipe, the LED, and a surface-mount adapter.
Overall, rigid light pipes are an economical way to move light from an LED to the indication area on the front panel when direct access is available.
A flexible light pipe is preferred when light must be moved over longer distances or around obstacles between the LED and a front panel or other display location. A flexible light pipe assembly consists of an adapter, which provides less to no light bleed, and a flexible optical fiber with a lens cap. Flexible light pipes are immune to electromagnetic interference (EMI) and do not conduct or transfer electrostatic discharge (ESD) pulses.
Flexible light pipes are more complex and cost more than rigid light pipes; they solve accessibility challenges for engineers by giving them the flexibility to bypass components on the printed circuit board (PCB) that prevent a straight-line access to the indication area on the front panel.
Figure 5 shows the various parts of a flexible light pipe design. The components consist of the LED at point A; a SMD-22 adapter that ensures no light bleeds into an adjacent pipe; a plastic optical fiber (POF); and a lens cap at point B that is bonded to the POF.
Flexible light pipes make it easy to add additional indicators or replace a damaged assembly because the lens cap at point B can be up to 330 feet from the LED light source at point A. The light pipe and lens need far less room near the panel face than the LED and its circuitry. If a human-factors engineer finds another indicator is necessary and space is an issue, a large redesign can be avoided by adding another indicator with a flexible adapter routed from another LED that may be in a completely different location from the first LED. The coupling of design time and design space radically improve cost concerns in a variety of industries and applications. Five different POF jacket colors improve installation and visual identification in the field by technicians, as well as clarify references in user manuals.
Rigid and flexible light pipes offer you two distinct options. Each one has advantages and disadvantages. Here are some guidelines to help you make the right choice.
Consider a Rigid light pipe if these parameters apply to your project:
- Light only needs to travel a short distance
- Low cost is a top priority
- The printed circuit board is located close to the front panel with simple access available
Look towards using a Conducteur de lumière flexible if:
- The light must travel a long distance
- The design of the equipment does not allow for a direct path from the LED to the display lens
- There is a possibility that additional indicators may need to be added later
- The human factors design is not yet finalized
- The LED output must travel through a high-EMI environment before reaching the lens cap
The design engineer must pay close attention to the pairing of the light pipe and the LED to minimize light loss and ensure best performance. The LED must be effectively matched to the entrance of the light pipe to allow for proper light capture with minimal light loss.
Of course, light pipe position can impact the light properties. To minimize light loss, a general rule is to use an LED with a narrow viewing angle of 160 degrees or less and position the light pipe and LED no more than 0.05” apart. There’s a little more to it than that to get the best product for your design. That’s where light simulations come in. Contact our team to run a simulation and determine the ideal light pipe for your design. And, don’t forget the constraints you may have based on the type of light pipe you use: Rigid light pipe design constraints Can’t bend Length limits: Press fit: 1.20” Custom: 1.5” Standard: up to 2” with shorter versions available Flexible light pipe design constraints Bends up to 30 degrees
Light pipe position
Light bleed occurs when light from the LED can be visible around the LED and light pipe area. Light bleed causes false-readings, muted and blended colors, crosstalk between adjacent light pipes, and unwanted glow in the enclosure. When a rigid light pipe is located over the LED, it allows for a certain amount of light bleed at the base of the light pipe. A flexible light pipe is designed to provide less light bleed with a coated optical fiber and a light resistant black adapter.
The maximum flexibility occurs when the designer can choose between designing with individual components (LED and light pipe) or using a complete solution that includes both elements.
Light pipes are designed to carry light from Point A to Point B as effectively as possible.
Point A: Where the input or light source is located. In most cases, it’s the surface-mount (SMD) LED.
Point B: The exit surface, or where the end-user sees the light at the interface or indicator panel of the device.
Light loss, or light bleed, occurs when some of the light traveling from Point A to Point B escapes or shines elsewhere, so less light is available to shine at the exit point.
Light Loss
More Light
To minimize the amount of light loss, designers can use an LED with a narrow viewing angle instead of one with a wide viewing angle to help close the distance between the light pipe and LED where light could escape. The wider the gap between the light pipe and the LED, the more room for light to escape before it enters the light pipe and travels to the exit point.
Note: there will always be some level of light that gets lost traveling from where the LED is located and the exit point where the end-user sees the light.
No matter which type of light pipe is chosen, there are multiple options available to optimize the design.
A surface mount (SMD) LED is mounted on the surface of the PCB and does not require holes. Through-hole LEDs are mounted by inserting leads through the PCB and soldering to provide a stronger connection between layers, which also can withstand some environmental stress conditions. An SMD LED tends to be smaller and brighter than the through-hole equivalent. Both through-hole and SMD LEDs are available in a wide range of sizes, colors, luminosities and lower power wattage.
LED color. SMD LEDs and through-hole are available in a wide range of single colors. SMD LEDs have additional options, including white or single, bi-color and tri-color, in an array of color configurations. Light pipe adapters are available for use with both types of LEDs. In addition, the designer can choose an adapter with a built-in LED or one intended for use with a separate LED.
LED size. Surface mount LED are based on industry standard sizes and through-hole LEDs are sized in millimeters (1.8mm, 3mm, 5mm). For example, an 0402 package (SM0402) is based on approximate dimensions (length × width) at roughly 0.4mm long by 0.2mm wide.
LED intensity. The light output (brightness intensity) of SMD LEDs and through-hole is commonly measured in millicandelas (mcd) or lumens (lm). The higher the mcd or lm rating, the brighter the light intensity.
The lens is mounted or molded on the end of the light pipe; it provides a convenient visual indicator and also protects the light pipe from mechanical disturbance. Some lens parameters are:
Lens profile. A Fresnel lens is designed to focus light and provides reduced shadowing and glare, and a wide viewing angle. A simple flat lens profile or dome lens is also used for increased viewing angle.
Lens color. Diffused, smoked, and color (blue, black, green, gray, red, yellow) lens options are available, depending on model.
Lens size and shape. A variety of lens sizes is available, including diameters of 2mm, 3mm, 4mm and 5mm. Round, oval, and rectangular shapes are available.
Applications in many industries must operate in harsh environments where it is critical that the lens cap appropriately sealed against the elements with the appropriate Indice de protection IP. The IP rating of a lens cap indicates the level of protection it provides against the incursion of solids, including dust, and liquids, including moisture or water. An IP rating is expressed in the form “IPXY”, where X and Y are numeric values.
The first digit in an IP rating indicates the level of ingress protection against solid objects: from IP0x (no protection) to IP6x (complete protection against dust). The second digit refers to protection from liquids: from IPx0 (no protection) to IPx9 (protection against water jets).
A light pipe design includes more than the LED, pipe, and lens. Additional components are needed to make up a complete system, and there are several options for each one.
There are various mounting options for light pipes depending on your application and manufacturing requirements.
If flexible and rigid were the only options, your options might be limited. Light pipes are designed to accommodate a wide range of mounting types.
In addition to the rigid vs. flexible decision, there are several other considerations when choosing a light pipe for your design. Your power source and overall design footprint can dictate which mounting type you should use with your light pipe.
THR-22
THR-5-22
SMD-22
