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Let’s talk about POWER – part II

In previous discussions, I've explored the limited weight budget allocated for smart components in everyday eyewear—approximately 15 grams (0.53 ounces),  review of optical technologies, with free space optics and waveguides emerging as the leading candidates, as well as the power limitations imposed by battery technology, specifically the 180mWh/gram efficiency offered by cutting-edge LiPo batteries.

Today, I aim to dive deeper into the implications of these power and weight restrictions on the overall functionality and performance of the system.

A primary consideration in our exploration is determining the weight allocation for the batteries, as this will directly influence the total power budget we can work with. The choice between optical solutions—whether waveguide or free space optics—plays a significant role in this determination. Assuming the frame, electronics and hinges are similar, the main difference between smartglasses solutions will be in the optical engine, lenses and battery size.

To illustrate this, let's examine a top-level comparison of the weight budget allocated for free space optics versus waveguide solutions. As seen below, the main differences relate to the combiner material (glass versus Polycarbonate), the number of waveguide layers, and the requirements for the ophthalmic solution, with waveguides necessitating additional elements, whereas free space optics designs can incorporate the prescription directly into the lens.

Weight (grams)

Free  Space



Monocular Combiner



Waveguide single layer glass. Free Space PC

Monocular Optical Engine



Frame, Hinges & Electronics



Total Monocular (w/o battery)



Waveguide – requires side balancing due to optical engine on the side

Total Binocular (w/o battery)



Battery weight budget



Glasses limit for all day use is 50 grams

Battery power budget (W/H)



Ophthalmic Correction per side



Free Space – Added Thickness to the combiner

Waveguide – Front & Back Lenses

Battery weight budget with Ophthalmic Correction



Waveguide with Ophthalmic correction weighs above 50 grams

Battery power budget (W/H)



From the analysis presented, it is evident that, under optimal conditions, the power budget (derivate from the weight budget limitations) for Free Space and Waveguide solutions—excluding considerations for the ophthalmic component—stands at 1.8 W/h and 0.9 W/h, respectively. If we base our calculations on a scenario where the system operates continuously for 6 hours, a Free Space System would be able to utilize up to 300mW average power budget for a binocular setup while a  Waveguide-based system would be limited to 150mW (average power budget).

Now let’s add to the discussion the optical efficiency of both the waveguides and the free-space solutions.

For AR combiners to effectively overlay digital images onto real-world views, a minimum contrast ratio of 1.5 is necessary. This means for every nit of brightness from the scenery that reaches the eye, the display must add an additional 0.5 nits to ensure visibility. Given that average sunny conditions are about 10,000 nits, sunglasses which reduce incoming light by approximately 85% would require the display brightness to around 750-1000 nits at the user’s eye to achieve satisfactory visibility.

The following images compare the Free Space and Diffractive Waveguide Optical solutions.

The comparison between Free Space and Diffractive Waveguide optical solutions reveals substantial differences in their efficiency. With an optical path that is roughly 30 times more efficient, Free Space optics are compatible with low-power RGB OLEDs for projecting images. On the other hand, the lower light efficient Diffractive Waveguides require brighter light sources as LCOS, DLP, LBS, and MicroLEDs that consume more power. This contrast highlights the influence of optical efficiency on choosing the appropriate display technology for Augmented Reality (AR) devices, with direct implications for both power consumption and overall device functionality.

The following chart shows several image sources and their ability to meet the thresholds of free space and waveguide combiners. 

Source – Bernard C. Kress – Optical Architectures for Augmented-,Virtual-, and Mixed-Reality Headsets

In our comparative analysis of free-space and waveguide optical solutions, two fundamental distinctions emerge, profoundly impacting their performance and suitability for augmented reality (AR) applications. The first distinction lies in the weight of the materials used—Polycarbonate versus Glass, along with the methods employed to integrate the ophthalmic solution. Free-space optics, utilizing lighter materials and a more efficient integration method, offer a notable advantage in this area. This difference not only affects the comfort and wearability of the AR glasses but also has implications for the device's overall design and functionality.

The second key distinction is in their optical efficiency. The free-space solution boasts a significantly higher optical efficiency compared to waveguide technologies. This efficiency translates into a critical advantage: the ability to use low brightness, power-efficient OLED image sources. The outcome of this efficiency is not merely a reduction in power consumption; it also leads to a dramatic extension of the device's operational life, with potential working times extended to approximately eight times longer than those offered by waveguide-based solutions using the same battery.

   *Monocular Device

These two pivotal differences—the weight of the optical components and the optical efficiency—collectively work in favor of the free-space solution, showcasing its superiority in integrating a larger battery and achieving extended usage durations, thereby enhancing the overall user experience in AR applications.


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