Now that the basic system is modeled, let’s take a look at the initial performance. The criteria often used in determining design merit are energy efficiency and uniformity (illuminance and luminous intensity). Energy efficiency is defined as the ratio of energy emitted by the display to the energy emitted by the source. In position space, the desired output should be uniform across the display (minimum flux per pixel deviation). In angle space, the output should be uniform within a small (~30 deg) half-cone angle. Remember that we are using this design for a small digital device. If the design application was a television or computer monitor, we would want a much larger half-cone angle (~90 degrees).

Trace rays with the following detector control settings and notice the energy lost due to thresholds.

Looking at the detector viewer we can see that about 40% of the source energy is getting to the detector; this value may vary up to a few percent based upon the random nature of the Monte Carlo ray trace. There is some energy lost due to ray errors, but this is insignificant in this application. Most of the energy is lost due to bulk absorption in the waveguide, however almost 10% is lost due to thresholds. This is common in systems where rays undergo multiple reflections. Eliminating this loss requires reducing the minimum relative ray intensity by a few orders of magnitude. This can be done if the lost energy is significant, but it will noticeably slow down the ray trace. Reducing the threshold to 1E-6 reduces the lost energy to 1% and increases the efficiency to about 46%.

Take a look at the illuminance and luminous intensity distributions. The illuminance is highest on the side of the display opposite the source. This is a consequence of the light guide causing high angles of incidence, and thus TIR, close to the source. The luminous intensity plot shows several peaks rather than the desired uniform distribution at smaller angles. As we will see, this intensity distribution is characteristic of the wedged light guide and BEF.



There are few geometry parameters in the system as it is currently defined that will correct these distributions. The most effective means of doing so is to introduce scattering properties to the wedged light guide. The input, top and bottom faces have the greatest affect on the illuminance and luminous intensity distribution.

Apply a lambertain scattering profile to the input face of the light guide with the following settings.

Trace rays and observe the difference in output characteristics. Make sure “Scatter Rays” is checked in the detector control dialog!



The efficiency of the system increases a few percent and the illuminance uniformity is much improved. The luminous intensity is slightly better, but some hotspots remain that must be resolved.

Now, remove the scatter profile from the front face of the light guide and apply one to the top face. The rectangular volume is defined by default with three face groups, so we cannot set only the top or bottom face to be diffuse. Instead, we will place a scattering rectangular volume coincident with the top face that effectively adds a scatter profile just to this surface. The nesting rule will give precedence to this new volume at the interface if this object follows the rectangular volume in the NSCE. Insert a rectangular volume object at surface 7 with the following parameters:

Y-Pos = 2
Z-Pos = 38.5
X-Tilt = -90
Material: Blank (Air)
X1, X2, Y1, Y2 Half Widths = 37.5
Z Length = 0.01
Lambertian scattering profile: front face only

Leave all other parameters as their defaults. Trace rays and note the change in output.



The illuminance uniformity is degraded, but the luminous intensity hotspots are resolved; the efficiency is also drastically increased. It appears that there is a tradeoff between spatial and angular distribution of the output. If a similar scattering function is applied to just the bottom face we find that the efficiency is reduced.

Based on our results, it would seem the ideal scattering profile would be on the top face of the light guide, and would scatter less light near the source and more towards the opposite end. The array object has the capability to model just such a non-linear pattern.