How do I get Clean Isolated backlighting for my HMI?
HMI backlighting zone isolation has limitations when it comes to the desired look, functionality, manufacturability, quality, and cost. Poor light isolation can cause unwanted light bleed between zones and an undesirable experience for the user such as low light intensity and non-uniformity. Illuminating an entire HMI surface simultaneously is not a desired look or functionality for many programs. A common desired feature is to illuminate icons or regions independently for the user to easily identify individual functions. So how do I get clean isolated backlighting for my HMI?
Issues and limitations for isolated backlighting:
-Light in unwanted areas
-Low light intensity
With isolated backlighting, the key is to create a balanced design of low cost, easily manufacturable, and desired look with functionality. This process can be executed easily and streamlined if there is early consideration for backlighting in your design.
One solution to illuminate regions independently on an HMI is with Molded-in Gasketing with Side
Fire LEDs. A molded-in gasket is a polymer (typically white or black opaque) used within an edge lit panel to isolate an illuminated optical zone. An embedded gasketing design creates an enclosure around the illuminated region and prevents light from entering or escaping. This solution offers regions to be illuminated independently from one another, color variation, and on/off variability. This option comes available with Black or White gasketing depending on application requirements with White having good reflective properties to allow for brighter illumination and Black having good absorption preventing light bleed into unwanted regions. This is a good option because the thickness can be as thick or thin as needed for the application from 0.015 inches to 0.125 inches matching the light panel thickness required.
Egg Crate Design
An additional solution for HMI light isolation is known loosely as the "Egg Crate Design" . This configuration works by utilizing Top Fire LEDs (User facing LEDs) in conjunction with a separate, molded light-box assembly. This allows the walls around the LEDs to block the light between zones in a conical fashion, underneath the individual graphic areas to be lit. This method can be effective but requires a very thick clearance (sometimes 1 inch or more) within the assembly stack up. Also, since top fire LEDs are not inherently uniform, additional reflectors, masks, diffusers and/or printed graphic compensation are typically required to achieve adequate uniformity. This can add to design and component costs depending on performance requirements.
Individual Light Guides with Divider Walls/Tray
A similar option to the egg crate design is to utilize a molded tray with divider walls for optical light zone segregation, however, instead of top fire LEDs, side emitting LEDs are used paired with individual light guides per illuminated zone. The advantage here is the uniformity can be achieved through edge-lit diffusion in a much thinner overall package. This allows for great dead front and light blockage (prevents light bleed in unwanted regions) but creates increase in labor demand by a "one panel at a time assembly" as individual light guide "chicklets" must be handled and placed in the tray.
Another approach that has been used for isolated backlighting are "Empty Slots" This technique is employed with edge-lit panels and the idea is to create open cutouts between the optical lighting regions disrupting light from traversing between the segregated light zones. This is likely the most inexpensive method for isolated backlighting but the downside is it does not fully eliminate light bleed between segregated zones since the channels cannot run fully around a zone or end-to-end on a panel without compromising the mechanical structure, so some light will escape around the edges with molded material remaining in place. Some light can also "jump" the clear slots depending on brightness LEDs and design space available for channel width.
Display Functionality for HMI Isolate Backlighting
In terms of functionality, the independently illuminated zones allow for a broader spectrum of applications such as white goods, automotive, display indicators, and more with next level performance. As seen is the attached figure, zone isolation using Isolated Backlighting can be used for Speedometer (automotive) applications to independently light display indicators such a "Drive", "Reverse", "Neutral", etcetera. These applications in conjunction with Lighting Isolation Solutions allow for features such as on/off variability, multiple color options and independent illumination.
There are many zone isolation solutions available to effectively backlight your HMI product. Take early consideration into your backlighting design and weigh the pros and cons of each solution to determine the best approach for your application.