How Do I Select an LED for My Backlighting?
Is choosing the lowest cost or brightest LED the best way to select an LED for my backlighting application? The wrong choice can cause higher cost, difficult design challenges, low light, or inconsistent performance. Other appearance problems, such as using the wrong size LED can cause witness lines on the facia/graphic and poor optical coupling leading to an inefficient lighting system which can also cause unwanted light bleed in other areas of the assembly. So, what is the best way to select an LED for your backlighting?
Issues from poor LED selection:
- Wrong size LED or type
- Inaccurate color selection
- Incorrect LED half angles
- Low light performance (How do I tell if my backlighting is bright enough?)
- Power constraints
LED selection for your UI, HMI or display backlighting will be dependent upon the application environment of the product and the resulting performance requirements. An example would be indoor lighting vs outdoor lighting applications and the overall size of the illuminated area (large optical zone vs a small optical zone). This article will get into more detail regarding different applications and understanding those parameters to help you in choosing the right LED.
Touch UI, HMI, and Display Indicators will require different brightness levels and colors depending on your application needs. Keep in mind, environment, functionality and power constraints.
For environment, an indoor application HMI such as a Microwave Oven Touch Interface will have a different LED/lighting specification than an outdoor Display Indicator such as a traffic light. Day light readability for example will dramatically change the LED choice. Correctly identifying your light specification up front is key to proper LED selection.
As for function, the indoor Microwave Oven UI example would need a seamless ultra-thin backlight requiring a low profile LED while an outdoor Display Indicator will have more room for a thicker light panel allowing for a taller high-bright LED choice.
Another area to accommodate for is power constraints. The power constraints will be different if your application is portable (Battery powered) or if your application is an indoor display indicator with access to an AC wall plug. This also must be factored into your LED decision.
Choosing the LED package size is important to selecting an LED. The height profile being the wrong size can lead to numerous adverse issues. For example, using an LED that is too tall can cause witness lines in unwanted regions and lead to delamination of graphic/facia. To large or too small of a package can cause alignment and tolerancing problems. The LED Package size needs to be chosen within the Mechanical Constraints available while still providing the primary goal of optimized lighting performance. Choosing too large an LED can cause allow too much light into the system bleeding into areas not intended to be lit and, conversely, too small may cause too little light into the system resulting in Low Light Performance. Both affect the efficiency of the system and should be validated early on in the design process by consulting with lighting experts to balance the mechanical, electrical and optical requirements and avoid issues down the line.
LED Type - Top Fire vs Right Angle
Top Fire and Right Angle LEDs are very different and understanding these differences will help you determine your LED selection. Right Angle LEDs can be used for Edge Lit Panel technology, they emit a uniform block of light with excellent coupling capability. This LED can be more costly and they are less bright than their Top Fire LED counterpart on a unit basis, but many times not as many are required when paired with an optical light panel reducing overall system cost while increasing light performance especially with regard to uniformity. The Top Fire LEDs are generally less costly, and are brighter so typically used for point source of lighting. This style of illumination is not uniform in and of itself (hot spot vs dim spot “scalloping” can occur) and many times require alternative diffusing techniques that can add to the overall cost and design time of the end application. Therefore, it is important to evaluate the light objectives up front and then choose an LED type accordingly.
If your Touch UI, HMI or Display indicator has colored illuminated regions, consider the option of using White LEDs vs Colored LEDs. White LEDs are brighter and can be used with a colored graphic/fascia to achieve a desired look or function. Exact color matching can be achieved more closely with white LEDs and a colored tuned facia. The option of using White LEDs also limits the color changing effects within the same illuminated region. A colored LED will provide a more vibrant color than it’s White LED counter part. Colored LEDs also provide the option to change colors within the same icon or space.
The temperature (measured in “Kelvin”) of an LED is also something to consider. “White” usually comes as “Warm” or “Cool” This is common knowledge in the lighting industry, but a brief description is as follows: “Cool” white is sometimes described as a slight blue and appears brighter to the eye. A “Warm” white is described as soft, more relaxing for the eyes and has a slight orange/red tint. Consider the color temperature when selecting an LED to be sure it is in line with the desired product look and reach out to the experts for technical support.
LED Half angle is the angle measured with respect to the LED’s light emission center line at which the radiant intensity falls to 50% of its max value. The value of the Half Angle helps determine the optical tuning on the light panel to accomplish a uniform illumination across the application. The Transition Area is the area in which the light within the light panels enters the intended illuminated region. The refractive index bends the half angles even further. Keep the refractive index in mind when selecting a material as your illumination medium.
Half angles are measured in the X-Y plane, but also in the Z-axis. So the mechanical aspects of the design and the tolerance stack up of the design will influence the “air gap” between the LED and the back lit panel. The greater the air gap, the more light will escape above or below the panel prior to entering the panel. This will lead to lower overall efficiency and should be avoided.
As bright as possible is not a quantitative value and is subjective. As mentioned before the illumination requirements are important to keep in mind when selecting an LED. The requirements will be different and is dependent upon the application. See How Bright Article.
Light intensity is a quantifiable and standard unit of measure and should be defined at the outset of the project. Also, keep in mind the full system needs to be taken into account when establishing your lighting specifications, for example, identify not just the LED brightness on a spec sheet, but also the attenuation of the materials (facia, etc.) that light will pass through before reaching the eye of the user. Understanding these details will better inform your LED decision making and lead to successful execution. Lighting experts can offer support with light measurement equipment and testing to benchmark your application and remove the subjectivity.
The LED selection must also be made based on the available power budget. In many cases, especially for hand-held type products, the power limitation prohibits using the ideal LED type and quantity to provide proper illumination. In these instances, the use of custom tuned optic used in conjunction with the correct powered LED can help with light delivery while limiting the power consumption requirements (i.e. can use lower power LED and/or fewer LEDs needed).
Conclusion on How to Execute
Selecting an LED can be a seamless process if all variables of application are taken into consideration within your UI or HMI or Display Indicator backlighting illumination. Please keep in mind these three key concepts when selecting an LED; environment, functionality and power constraint.