In this post we want to address see-through Contrast Ratio (CR) particularly in AR smart glasses.
One of the main limitations of Augmented Reality (AR) smart glasses design is the image visibility on a bright background. Try to wear any of the available smart glasses, even the most expensive ones, in most cases you will not be able to use them outdoors since you can hardly see the image at regular day conditions. This limitation of not sufficient contrast reduces the number of use cases where smart glasses can be beneficial, for example in sport activities, for car drivers, jet pilots and more.
See-through Contrast Ratio importance
The image see-through contrast is the major performance driver with a significant impact on the image visibility. This parameter has direct effect on other visibility parameters, such as color discrimination, as well as on other system parameters, such as power consumption, heat dissipation and lifetime. It is possible to simulate see-through contrast performances and perform system design and optimization using non-imaging design software such as LightTools. This way the trade-off between the different techniques can be done by simulation.
The See-through Contrast Ratio Trade-off
There is always a trade-off between a good see-through CR and system size and volume, or in other words, how bulky the smart glasses are. If the smart glasses image source is driven at a very high power in order to see the symbol on a bright background, its power consumption is significant, and it requires a large battery. The system also heats up and heat dissipation needs to be considered at the design phase, and above all, the display technology that is chosen has to be able to provide the required high brightness. This also has a strong impact on the system optical concept, because high optical efficiency is needed in order to deliver the high brightness image to the eye.
See-through Contrast Ratio Requirements
As a rule of thumb, outdoor smart glasses require a see-through CR of more than 1:1.4 so the user will be satisfied with the image visibility at a variety of use cases including outdoor conditions. That means that for a given background luminance, the system internal luminance has to fill the following condition:
This is the basic CR that we always use as a first order requirement, at the beginning of any see-through system development. There are many exceptions, assumptions and rules that should be taken into account in order to evaluate more precisely the image visibility under extreme light conditions. For example, the image color versus the background color. In a color system, where the symbol color coordinates strongly differ from the background color, the image will be highly distinguishable even on a bright background. In this case, the comfortable see-through CR may be lower than 1.4. When using red symbol on a blue background (clear sky), for example, a CR of 1.3 may be satisfactory.
Design Challenges and Considerations
In any case, we can confidently state that smart glasses for outdoor use require high luminance. When developing such a system, several approaches have to be combined to overcome the many design challenges and to produce a good and sustainable system:
1. Optimal display technology
Only a display technology that has the ability to provide high luminance can be used in AR systems in outdoor day conditions. But this is not all. The high luminance has to be achieved with a reasonable lifetime and high efficiency. When we are talking about a full-color system and not monochrome, the technologies that may be considered are very limited and there are many compromises to be made for choosing the right and optimal display technology. For further review on Micro-display technologies, pros. & cons., please read the full article review:
2. Optical concept efficiency
The choice of the system optical concept may have a significant impact on the see-through CR. A non-efficient concept, where significant light losses are incorporated in the optical concept, may require an extremely high display source brightness with high power consumption, thus severely affecting system lifetime. The optical designer shall consider the efficiency issues carefully and pay attention to them when choosing the concept. Overall light efficiency in an AR system may range from a few percent to more than 50%. An efficient optical path may open new degrees of freedom, enabling the developers to choose what would they prefer to gain depending on the application and customer preferences: a lower cost or smaller display size; a significantly high see-through CR and visibility; or maybe small form factor well-designed smart glasses.
3. Variable transmission
One of the straightforward ways to increase the see-through CR is to screen or filter the background luminance from reaching the user's eye. According to the see-through CR formula, CR increases when background luminance decreases. This is why most of the available smart glasses are supplied with a dark or low transmission optical filter or visor. This concept can be seen in HoloLens, Magic leap or other leading AR smart glasses. Currently, the developers are using this concept in order to increase see-through CR and to reduce power consumption. However, at a low light background level, the outside scene can hardly be seen in these systems, and they become more VR systems rather than AR systems.
So, what can be done in order to make those smart glasses useful at various luminance levels? In our opinion, variable transmission glasses are the best available solution to control the external outside scenery light reaching the user’s eyes. The see-through transmission is adjusted according to the external luminance and by doing that, high CR may be maintained without any impact on the required display luminance, power consumption and lifetime. Today there are different variable transmission technologies, such as photo-chromic or electro-chromic materials that may be integrated for this use. These technologies are already used in different markets and applications and are adapted in the AR and HUD systems as well. In our opinion, the technology that will take the lead in AR smart glasses in the future is the one that enables filtering only a small area of the smart glasses, where the user both looks at the projected symbol and see-through. In that case, the overall transmission and the local symbol CR will be high, both being achieved in a very low overall power consumption.
4. Polarization Design
Another approach to increase the overall optical path efficiency is to use polarization as a design degree of freedom in the system. This approach may be very useful when using a Micro-display technology based on polarization, such as LCD or LCoS. The system concept and the optical coatings can be designed and optimized for linear light polarization in the optical path. This design approach will increase the internal efficiency and the see-through CR. In our experience, the see-through CR may be increased by more than 50% using smart polarization system design, without increasing display power consumption or affecting display lifetime.
5. Spectral Design
Similarly to the polarization design approach, the internal efficiency and see-through CR may also be increased by precise matching between the image source spectrum and the end-to-end optical path, including optical components, coatings, filters, multiple reflections etc. all the way to the user's eye. For example, a good matching between the backlight spectral radiance and the LCD RGB color filters may improve LCD transmission by tens of percent. Optimizing the reflecting and transmitting coatings spectral curve applied on optical elements in the system may also improve the internal system efficiency and see-through CR.
The following images show a qualitative comparison between some of the approaches discussed herein, as built in our lab for these effects' demonstration:
Summing-up
Overcoming high ambient light conditions was always a hot topic in the AR smart glasses market. Today, this is one of the main technical bottlenecks in this industry. A system that will provide good CR under a wide range of lighting conditions will make smart glasses useful and valuable to a larger number of users at various use cases. It will probably open new markets for the AR industry and drive the industry growth and expansion.
We would like to acknowledge Mr. Ronen Lin from Smart Films International for using their variable transmission technology in the images published herein.
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