One of the startups in HTC’s Vive X accelerator is releasing an upgrade kit that will add eye tracking to the Vive virtual reality headset. The Beijing-based company 7invensun will open preorders for its aGlass lenses next month, selling them in China for the equivalent of around $220; according to UploadVR, they’ll be sold internationally around the third quarter of 2017.

The aGlass consists of two eyepieces that the “average VR user” can fit inside the HTC Vive’s face mask, including three pairs of interchangeable lenses. The eyepieces are connected to the Vive via USB, which will allow the headset to track pupil movement via sensors and infrared lights. It’s an example of HTC’s plan to expand the Vive ecosystem with third-party accessories, including custom controllers and other upgrade kits.

Several companies are working with eye tracking and VR, including headset maker Fove; major eye tracking company Tobii; and the startup Eyefluence, which was acquired by Google last year. This is the first time we’ve seen an eye tracking system made expressly as an easy upgrade to a major headset like the Vive, however.

Having not tried aGlass, we couldn’t say how well it works, but eye tracking is a potentially very useful addition to VR. You can use it to dial back the graphical quality of images when people aren’t looking at them — a technique called foveated rendering — to wring better performance out of lower-powered computers. Tobii has used non-VR eye tracking to enhance the experience of playing games like Assassin’s Creed Syndicate, and Fove has used it to play some clever narrative tricks. If aGlass works well, it will make it easier for more people to experiment with the technology on a major VR platform.