2.1. Custom-Made Head-Mounted Display

The custom-made hybrid video-optical see-through HMD fulfills strict technological and human-factor requirements towards the realization of a functional and reliable aiding tool for high-precision manual tasks. The HMD was assembled by re-working and re-engineering a commercial OST visor (ARS.30 by Trivisio [25]).

As described in [23], the key features of the HMD were established with the aim of mitigating relevant perceptual conflicts typical of commercial AR headsets for close-up activities.

Notably, the collimation optics of the display were re-engineered to offer a focal length of about 45 cm, which constitutes, when used for close-up works, a defining and original feature to mitigate the vergence-accommodation conflict and the focus rivalry. The HMD was incorporated in a 3D printed plastic frame together with a pair of Liquid Crystal shutters and a pair of front-facing USB 3.0 RGB cameras [26]. The stereo camera pair is composed by two LI-OV4689 cameras by Leopard Imaging, both equipped with 1/3″ OmniVision CMOS 4M pixels sensor (pixel size of 2 μm). The cameras were mounted with an anthropometric interaxial distance (∼6.3 cm) and with a fixed convergence angle. In this way, we could ensure sufficient stereo overlap at about 40 cm (i.e., an average working distance for manual tasks). Both the cameras are equipped with an M12 lens support whose focal length (f = 6 mm) was chosen to compensate for the zoom factor due to the eye-to-camera parallax along the display optical axis (at ≈40 cm).

The computing unit is a Laptop PC with the following specifications: Intel Core i7-8750H CPU @ 2.20 GHz with 12 cores and 16 GB RAM (Intel Corp., Santa Clara, CA, USA). Graphic card processing unit (GPU) is a Nvidia GeForce RTX 2060 (6GB) with 1920 CUDA Cores (Nvidia Corp., Santa Clara, CA, USA).