Mobile Displays

Introduction

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To download the associated FRED file: mobile-displays.frd

Mobile devices such as smartphones, e-readers, and watches are becoming ubiquitous in today’s world. Precise optical engineering is required to optimize the performance of mobile features such as the camera system, sensors, and the display.

A key design objective for a mobile display is uniform illumination over its area and range of viewing angles. In addition, it should have high optical efficiency to reduce power consumption and increase battery life. Edge-lit LED screens accomplish this by using compact and efficient LED lights coupled into a transparent waveguide. Elements such as reflective back surfaces, microstructure patterns, brightness-enhancing films, and diffusers can be incorporated into the display to increase efficiency and uniformity. In this FRED model, an edge-lit LED smartphone display is virtually prototyped. Uniform illumination is achieved by incorporating a gradient diffuser along the waveguide.

Edge-Lit LED Screen with Diffuser

Waveguide

The first component in the system is a rectangular waveguide. A block with dimensions [25 x 40 x 1 mm] semi-width, semi-height, and semi-depth is created with the following properties:

LED Array

The LED will be modeled as a small rectangular Lambertian emitter of dimensions of [1.8 x 0.7 mm] x-semi-aperture and y-semi-aperture embedded within the edge of the waveguide to maximize optical efficiency. This can described by FRED’s Detailed Optical source type.

An array of 5 identical side-by-side LEDs is created along the top face of the waveguide by right-clicking the completed LED source and selecting “Edit/View Array Parameters…” The spacing between LEDs is set to be 10 mm.

Reflectors

A back-reflector will increase optical efficiency of the display by recycling light that would have exited its backside. The back-reflector is modeled by creating a [25 x 39 mm] semi-width and semi-height reflective surface placed just behind the back face of the waveguide and translated 1 mm vertically (away from LED array). A small front-reflector is also added just in front of the embedded LEDs by creating a [25 x 1 mm] semi-width and semi-height reflective surface placed just in front of the front face of the waveguide on the LED end. The front-reflector and truncated back-reflector also reduce excess light that refracts directly out of the bottom of the display, increasing uniformity.

Figure 3. Display geometry, including LED array (central LED shown in yellow), front-reflector (red), and back-reflector (green).

Diffuser with Scripted Scatter

Without a diffuser, light will either refract out of the waveguide or get guided to the end via total internal reflection. The purpose of the diffuser is to gradually scatter trapped light out of the waveguide for uniform illumination. To counteract an exponentially decreasing irradiance gradient from the LEDs, the diffuser needs to have an equal and opposite effect. An exponential diffuser with maximum scatter at the end of the waveguide achieves this.

A new Scatter model is created using the “Scripted” option. Assuming that local y-position along the diffuser (g_Ypos) ranges from -40-40 mm, we create a variable “p” (probability of scatter) based on the following exponential function:

p = aExp(b(-g_Ypos+40))-1

The parameters “a” and “b” are constants that can be adjusted. In this model, a=4 and b=0.04. Additionally, scatter occurs only for rays with y-positions sufficiently above the LED array (y>25 mm from bottom). This abrupt scatter “cut-on” counteracts the high-irradiance region near the light source.

To make the simulation more efficient, a “Monte Carlo” Raytrace Control can be applied to the scattering surface. This feature ensures that rays do not split at each scattering event. The following Raytrace Control is created and assigned to the surface:

Figure 4. Assign the scripted scatter function (“Tailored Scatter” in this case) to the front or back of the waveguide under the Scatter tab of the surface.

Evaluating the display

The irradiance exiting the mobile display before and after the scripted gradient diffuser is shown:

Figure 5. Log(10) of irradiance distribution from mobile display without diffuser (left) and with exponential diffuser (right). Logarithmic scaling gives a better idea of what the human eye perceives. 250,000 rays were traced in this simulation.