What Are The Key Distance Parameters To Define in The Optics of AR Glasses?

Today, let’s discuss the key distance parameters within the optical systems of AR glasses and the logic behind their definitions.

First, there is a parameter that—strictly speaking—isn’t a "distance" itself, yet nearly all other distance parameters are intrinsically linked to it: the size of the "eyebox." We have previously covered the concept of the eyebox and how its dimensions affect display performance.

Here, we first define the Eyebox dimensions as 12×10 mm.

The first key distance parameter is known as "Eye Relief" (hereinafter referred to as ER). As the name suggests, it refers to the distance between the human eye and the surface of the waveguide. This parameter represents the ideal positioning of the human eye relative to the waveguide—a value that must be accounted for during both the waveguide design simulation phase and the industrial design (ID) process for the eyewear.

Generally speaking, the EB (exit beam/pupil) size we define is based on a fixed ER (eye relief). For instance, if we define an EB size of 12×10 mm assuming an ER of 14 mm, a key point arises: when a user wears the device and their actual ER exceeds the design ER, the effective EB size decreases—and conversely, if the actual ER is smaller than the design ER, the effective EB size increases.

The second important distance parameter is the Pupillary Distance (PD)—the distance between the centers of the left and right pupils. According to empirical data in the optometry industry, the PD for the vast majority of the population falls within the 58–72 mm range. Similar to the vertex distance (the distance between the lens and the eye), a specific PD is selected as the "ideal" value during the design process; given that 64 mm is the midpoint of the 58–72 mm range, it is typically established as the default PD.

Based on the definition of the Exit Box (EB)—where the human eye is positioned at the center—as long as the pupils remain within the boundaries of the EB, the entire image will be visible. Consequently, the compatible PD range for a binocular AR display is defined as the designed ideal PD ± the horizontal dimension of the EB.

The third critical distance parameter is the horizontal/vertical separation between the center of the input grating and the center of the exit pupil (EP). This can also be understood as the path length that light—emitted from the optical engine and coupled into the waveguide via the input grating—travels within the waveguide.

A greater horizontal distance between the input grating center and the EP center results in a longer propagation path for the light within the waveguide, making it increasingly difficult to maintain both optical efficiency and uniformity.However, since the optical engine is typically positioned within the end-piece areas on either side of the front frame—meaning its location is linked to the overall width of the eyewear—and because the EB (exit pupil) center is determined by the interpupillary distance (IPD) and is essentially fixed, the scope for adjusting this distance is quite limited. In conventional eyewear designs, the frame width generally falls within a specific range; consequently, this distance typically measures between 30mm and 40mm (though this may differ for designs utilizing "one-to-two" waveguide configurations).

Another factor is the vertical distance between the in-coupling center and the EB (exit pupil) center. The definition of the K-vector in the optical design depends heavily on this distance; in other words, the specific logic used to define this vertical distance—such as whether the EB center is positioned above, at the same level as, or below the in-coupling center—directly influences the choice of optical architecture.There is no fixed conclusion regarding this effect—for instance, the notion that placing an out-coupling point higher up is always better for design—as it depends on the specific design scheme; therefore, I will not elaborate further.

The above covers three key distance parameters in the definition of optical systems for AR glasses. It is clear that the definition and design of optical specifications cannot be decoupled from industrial design (ID). In most cases, there is an inherent trade-off between optical performance and aesthetic appeal: parameters that enhance optical metrics may not align with ID requirements, while an excessive focus on ID goals can severely constrain the optical design process.

Consequently, a critical aspect of product definition lies in finding an optimal balance—a "best-fit" solution—between ID and optical performance, thereby establishing a unique, differentiated identity for the product.