OPTIVAR

The project is working on 8 main activities, having obtained the following results so far:

Activity 1: Representation of freeform surfaces with orthogonal polynomials.

The design process of a freeform optical system is divided in three phases: first, a design close to the final objective is developed using SMS (Simultaneous Multiple Surfaces) method, resulting in a cloud of points of the surfaces. Second, that surface is approximated with a set of orthogonal polynomials. Finally, the coefficient of the polynomials are optimized using the commercial software CODE V. The optimization phase depends strongly on the initial solution and the type of polynomials used, affecting the time of resolution an even the achievement of a solution or not. For this phase, OPTIVAR has carried out an exhaustive study comparing the different families of polynomials: XY, Legendre, Zernike and Forbes, to select the most adequate. The results allow the optimization of results using the most appropriate polynomial family, depending on the shape of the aperture. Fig. 1 shows an example of the results obtained.

Fig.1. Example of the results obtained for each orthogonal polynomial family.

Activity 2: Design of systems with diffractive surfaces.

With the SMS method in its current state of development, it has been possible to design systems with up to 4 optical surfaces to perfect focus 4 different points. Theoretically, N points could be focused with N surfaces, although the complexity of the problem grows dramatically with N. The analysis show, as expected, that the more points are focused, the closer is the optical system to an ideal one.

Diffractive lenses add degrees of freedom to the design of the system, allowing to improve it in different aspects. With N surfaces, 2N points could be focused. Also less than 2N points could be focused and use the degrees of freedom to correct aberrations, for example, chromatic ones. At this stage of the project, a synthesis method has been developed to obtain the profile of one diffractive surface to focus 2 points. Also a method to correct chromatic aberration using those surfaces has been developed. Fig. 2 shows an example of design that focuses 2 points with a single diffractive surface.

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Fig.2. Example of a system with one diffractive surface that focuses two points.

Activity 3: Design of optics for multi-channel architecture.

One of the advantages of the multi-channel architecture is the possibility to develop optical designs equivalent in performance to the conventional ones, but with shorter focal distance. This property is fundamental for Virtual Reality technology, because it allows the design of much more compact headsets and, accordingly, more comfortable for the users.

In the case of Virtual Reality, only designs with reduced number of channels have been used (2 to 4). Reducing the distance display-lens is a challenge for optical design, as shown in Fig. 3. This figure also shows two examples of designs developed in OPTIVAR: one with 2 channels (together with the prototype manufactured) and one with 4 channels (to be manufactured).

Fig.3. Example of optical design for VR with 2 and 4 channels

In this activity, we have designed and developed prototypes for a large set of lenses (more than 30). The objective is the application of those designs to VR systems, to improve some of their parameters. As initial activity, we have designed and manufactured some prototypes for lighting applications, particularly, homogenization of LED’s. Fig. 4 shows one example inspired in insects’ eyes.

Fig.4. Prototype of multichannel system and its application as light homogenizer

Activity 4: Design and development of a Light-Field prototype.

One of the objectives of the project is the generation of Light-Fields using arrays of lenses. The development of this objective will be carried out in several phases, first exploring the market and purchasing displays with the highest resolution possible. Second, developing the Light-Field with the help of a mask of points of different size and position. Finally, the mask will be substituted by an array of lenses specifically designed and with the same performance. The main difference is that the mask is not very luminous, while the array should be as luminous as the original display image. As an intermediate phase, commercial arrays of lenses will be used to start the analysis. The objective of the prototype is to see images in 3D using conventional displays without heasets/glasses. Fig. 5 shows the prototype developed so far. 

Fig.5. Array of lenses used for the prototype of Light-Field displays.

Activity 5: Manufacturing of optical components.

In order to manufacture the prototypes of lenses, as the one shown in Fig. 3, different technologies and materials have been analyzed, depending on the necessary quality and cost. The prototypes have been developed using Direct Cut technology by diamond turning. This technology offers results very close to the theoretical design and can be used to verify it. For pre-industrial series, plastic injection molding (with Zeonex material) should be used.

The lenses have been characterized in the optics laboratory at CeDInt-UPM, including mechanical parameters (profile and roughness) and optical parameters (efficiency, aberrations, etc.).

Activity 6: Developement of display control electronics.

We have designed and tested control electronics for two displays for the VR application: one of the operating at 60Hz and the other one, operating at 72Hz. Although a refresh frequency of 60Hz is enough for the response of the eye, it is important to take into account other parameters, in particular, the delay between the moment when the position of the head is obtained and the moment when it is processed. After theoretical analysis of the problem, we concluded that the frequency of the display is very important. Fig. 6 shows the electronics developed working at 72Hz, under testing at this moment.

Fig.6. Testing of control electronics for 72Hz displays. The shape of the electronics is fitted by design to the shape and size of the headset.

Activity 7: Design and development of a prototype headset and integration.

We have developed different designs of the headset, with different materials and colors. Fig. 7 shows two examples, one of them integrating optics, displays and headtracker. The problems of isolation (to avoid dust and light inside the headset), ventilation of circuits and inter-pupillary distance have also been taken into account during the design phase. 

Fig.7. Two examples of headset developed to integrate the optics and electronics for VR.

Activity 8: Functional tests of prototypes

During the development of the project, continuous subjective tests are carried out with end users, to receive feedback about comfort, immersivity, quality of image, chromatic defects, tracking accuracy, etc.