From the article:

Currently, clinical static automated perimetry most commonly uses projection-based perimeters where a light source is projected onto a background of uniform intensity. Although this process can assess the visual field, it is tedious and uncomfortable for patients. Costing upwards of $30,000, traditional perimeters are large, bulky, and expensive with poor patient ergonomics.

We have developed a fully open-source, head-mounted display (HMD) perimeter capable of visual field testing that utilizes a Google Daydream VR headset and an Android smartphone (Samsung Electronics Co., Suwon, South Korea). With a cost of around $100, plus the cost of a consumer smartphone and computer, this system is portable and far less costly than traditional perimeters.

The Iowa Virtual Reality HMD Open-Source Perimeter is as repeatable as the Octopus 900 perimeter and is a more portable and less expensive alternative than traditional perimeters.