How does an LED Half Angle affect my backlight?

The Problem

LEDs emit light in an angular pattern as measured from the centerline of the LED. A lack of understanding of these LED Half Angles can make Backlight Panel design extremely difficult and unnecessarily costly. It is also critical to understand Half Angles in Backlighting Panels will respond differently than the Half Angles reported in LEDs spec sheets. This is due to the fact that spec sheets show half-angle as a stand-alone device but not the half-angle in a custom lighting application.

Here are a couple of issues that could arise if you do not understand an LED Half Angle and why the distribution can change in an application:

  • • Lack of uniformity
  • • Lower intensity
  • • Lower efficiency
  • • Too many LEDs

The Solution

LED Specification Sheet and LED Half Angles

Half Angles listed in spec sheets are assumed to be going through “open air” and not a Backlit Panel. An LED Half Angle is the angle and spatial distribution in which the light emitting from the LED is 50% or greater of the maximum intensity. This is the most effective emission range of the LED. The LED Half Angle is documented on a specification sheet from the manufacturer and will look something like the diagram to the right.
Below, we can see the LED Half Angle going through “open air” or “open space” vs how it changes when going through the medium of an optical panel. With light emitting from the aperture of an LED and entering the Light Panel system, we notice the Half Angle of the light changes, this change and slight bend is caused by a property of the light panel material called the Refractive Index (RI). The Refractive Index (RI) is different from material to material and changes the calculation of an LED Half Angle in a Backlit Panel. (Note: the RI is in the spec sheet of a given material)

Material

Depending on the needs for an application, bear in mind the differences of Refractive Index (RI) between different Backlit Panel materials. Two common materials are Polycarbonate and Acrylic. There are many properties that are different between these two materials, but for now we will focus on the Refractive Index. There is a slight difference between acrylic and Polycarbonate in the LED Half Angle as it passes through the material. Depending on material chosen, this can change LED placement, light coverage, uniformity, and light intensity.

LED Location

LED location is determined by many variables including Refractive Index (RI) of Light Panel material, light coverage requirement, light uniformity requirement, light intensity requirement and mechanical space restraints. Early consideration of backlight panel design can help streamline the design process. By providing flexibility with regards to LED location, the lighting requirements are easier to achieve. By moving the LED location closer or further away from the optical region we can see the difference in light coverage which directly affects dark spots and “hot” spots in the viewable lighting areas and are now able to create 100% light coverage on the intended illuminated zone.

It should be noted that this may cause an undesired loss of efficiency/efficacy, but depending upon the application, tradeoffs can be discussed. If there are mechanical space restraints, and LEDs cannot be moved further away, LEDs may be rotated, or optical features can be added within the light panel itself to redistribute the light and still maintain a short LED transition zone. From this design approach you can achieve 100% light coverage as well. When LED Half Angles are accounted for early in the design process it is possible to both maximize performance and reduce tooling and piece price by adequately accounting for LED type, location and reducing quantity required.

Intensity and Uniformity

Light becomes weaker and experiences energy loss (attenuation) as it passes through any medium (including air). This means that light intensity is strongest at the LED chip, and becomes progressively weaker as the LED is moved further from the Light Panel. LED Half-Angles must be considered early in the design process, before fixing the LED positions on the PCB/FPC. This will optimize the light distribution and optical tooling, while reducing the overall timeline to production.

Conclusion

An understanding of LED Half Angles is critical when designing an optimal backlight panel. In addition to the Half Angle, important points to be taken-into consideration include: location of LEDs, number of LEDs, the materials used for backlighting, and the LED specification. Disregarding any of these key points can decrease efficiency, increase cost, create non-uniform lighting, and low light intensity. To efficiently streamline the design process for your backlighting application, a good solution may be to outsource to an expert.
To this end, the role of the lighting designer/manufacturer is critical to optimizing these variables for maximum performance, while helping to reduce cost of your backlight panel.

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