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In evolving technologies such as augmented/virtual reality (AR/VR) optics, it can be important for designers to assess the properties of materials in their systems by including light scattering in design simulations.

In this article, we provide examples of how bidirectional scattering distribution function (BSDF) and total integrated scattering (TIS) measurements can improve your AR/VR designs with the help of ²ÝÁñÉçÇø optical measurement systems.

What is the Difference Between AR and VR Systems?

AR systems combine a virtual image or text information with the real environment, presenting them together as a visual display.

Examples of AR systems include:

• Helmet-mounted displays (HMDs) for helicopter pilots
  
• Head-up displays (HUDs) in vehicles
• Gaming and entertainment systems

VR systems simulate the entire visual environment, such as an immersive display for gaming.

AR/VR System Goals and Challenges

AR/VR technologies must meet specific performance requirements for:

• Autonomy
  
• Comfort
• Ergonomics
• Immersion
• Price
• Design process

In addition, optical performance of AR/VR applications can be subject to many challenges during the development of systems and prototypes in the areas of:

• Resolution
  
• Image quality
• Eye box size
• Brightness
• Efficiency
• Transparency (AR)

If you need to determine the optical properties of materials used in your AR/VR application, ²ÝÁñÉçÇø has solutions to characterize any flat surface for your design project.

Benefits of Optical Measurements for AR/VR Systems

Qualifying and quantifying the image appearance through your AR/VR system can be challenging given the number and range of performance requirements. Designers benefit from realistic product simulations before prototyping to reduce costs and production time.

You can use ²ÝÁñÉçÇø optical measurement tools to obtain data that augments the physical realism of product simulations. ²ÝÁñÉçÇø tools can measure optical properties of materials to help you assess:

• Material transmission
  
• Emitter directivity
• Optical surface quality
• Stray light
• Beam splitter performance

Once you have measured optical properties of materials with ²ÝÁñÉçÇø tools, you can then import the data into design software to include in product simulations and virtual prototypes.

How to Characterize an Optical Surface

During an optical design process, accurate simulation results rely on accurate optical properties. Geometry alone does not determine light distribution; it¡¯s the optical properties that determine how the energy and direction of the rays change. For this reason, it¡¯s important to know as precisely as possible the optical characteristics of the materials that will be used. BSDF is a mathematical function that characterizes how light is scattered from a surface. You can use this data to model the roughness of a surface in your optical system.

²ÝÁñÉçÇø provides several instruments to measure BSDF:

• ²ÝÁñÉçÇø REFLET
  
• ²ÝÁñÉçÇø Mini-Diff VPro
• ²ÝÁñÉçÇø Mini-Diff V2

Another useful measurement for accurate design simulations is total integrated scattering (TIS). TIS describes how much light is deviated (or scattered) from the incident light direction by diffuse reflection and transmission from a surface or optical element. It is equal to the ratio of diffuse reflected and transmitted optical power to the incident optical power when light is incident upon a surface.

There is a direct correlation between BSDF and TIS; TIS can be computed by integrating the BSDF in the whole angular space. TIS can also be measured directly using the ²ÝÁñÉçÇø TIS Pro instrument.

Application Example: Head-Up Display

Head-up display systems can include a combiner, which is an optical element that combines two different images that are reflected from an emitter and transmitted from the ambient scene. A challenge for a head-up display design is to characterize how much light is reflected or transmitted from any surface. This parameter is also a function of the wavelength.

When an OLED display is used with a combiner, designers should ensure that the spectral transmission of the combiner fits with the ray emission of the OLED.

In this example, we compare two windshield coatings that can be used as beam splitter in the head-up display. To characterize the coatings, we use the TIS value obtained from the ²ÝÁñÉçÇø TIS Pro.

When we measure these two coatings, the global TIS is around 14%. However, they don¡¯t reflect in the same wavelength. It is possible to measure this data using ²ÝÁñÉçÇø TIS Pro.

Spectral TIS of two different beam splitters

When we replace the blackbody source with an RGB OLED source, then the reflection and transmission values are different. As shown in the following figure, Beamsplitter 2 produces poor surface reflectance. It should not be used.

The ²ÝÁñÉçÇø TIS Pro can help designers quantify beam splitter spectral reflectance as shown in this example and use the data in design simulations.

Application Example: VR System

This example shows a very simple optical VR system. The assembly includes two displays, two lenses, and a housing. In a VR system, it is important to perform a stray light analysis to check for any glare or visual effects that could impact the user¡¯s experience. An effective stray light analysis begins with an accurate simulation model.

The optical property of each surface is needed to correctly simulate stray light effects and evaluate the light power transmitted in every light path. The optical properties are usually dark and diffusive and their bidirectional reflectance distribution function (BRDF) values can be measured with ²ÝÁñÉçÇø Mini-Diff V2 or ²ÝÁñÉçÇø REFLET 180S instruments. The TIS value, which completes the BSDF description, can be measured with the ²ÝÁñÉçÇø TIS Pro.

We performed a stray light analysis using the VR system¡¯s BRDF and TIS data in LightTools software.

The angular directivity of the display is also important for the simulation. It gives the amount of light emitted outside of the field collected by the lens. The light that is not collected by the lens system will be reflected by the housing and could generate unwanted artifacts on the final image observed by the user. The following figure illustrates angular emissions for two different displays. On the left side, the beam is narrower than the right side. On the right side, more light will be reflected by the housing and will produce more stray light.

Once the simulation model is correctly set up, it is easier to find solutions to correct stray light issues.

Application Example: AR System

AR systems can also require stray light simulations as shown in the previous example. In addition, some optical systems use multiple gratings to inject and extract the light from a light guide.

The gratings generate a diffusion or diffraction pattern that can be characterized with the ²ÝÁñÉçÇø Mini-Diff instrument. To obtain the BSDF measurement, the ²ÝÁñÉçÇø Mini-Diff V2 is positioned on the product¡¯s diffractive zone and scans the diffraction pattern. This characterization can be used to compare real products with simulations to detect production defects.

Conclusion

AR/VR systems are challenging because they mix multiple technologies. We have illustrated a few of the challenges in prototyping and simulating AR/VR systems depending on the materials and surfaces needed.

²ÝÁñÉçÇø optical scattering measurement equipment can be essential for designing AR/VR optics with precision and accuracy. As we¡¯ve shown, BSDF and TIS measurements will help you validate the optical behavior of individual components.