Changes

==Introduction==

==Introduction==

The lenses in a [[virtual reality]] [[head-mounted display]] (HMD) make possible to focus on the images produced by the displays that are very close to a user’s eyes. The lenses are placed between the screens and the viewer’s eyes, giving the illusion that the images are out to a distance where they can be viewed comfortably. This is achieved through the lenses in the VR HMD helping to reduce the least distance of distinct vision. It should be noted that if a user normally needs glasses or contact lenses to clearly see objects far away, he will need to wear the glasses or lenses while using the VR equipment. <ref name=”1”>Doc-Ok.org (2016). Head-mounted displays and lenses. Retrieved from http://doc-ok.org/?p=1360</ref> <ref name=”2”>Santillian, R. DIY virtual reality tutorial: How to Make a VR Headset. Retrieved from https://diyhacking.com/diy-virtual-reality-headset/</ref>

The lenses in a [[virtual reality]] [[head-mounted display]] (HMD) make possible to focus on the images produced by the displays that are very close to a user’s eyes. The lenses are placed between the screens and the viewer’s eyes, giving the illusion that the images are out to a distance where they can be viewed comfortably. This is achieved through the lenses in the VR HMD helping to reduce the least distance of distinct vision. It should be noted that if a user normally needs glasses or contact lenses to clearly see objects far away, he will need to wear the glasses or lenses while using the VR equipment. <ref name=”1”>Doc-Ok.org (2016). Head-mounted displays and lenses. Retrieved from http://doc-ok.org/?p=1360</ref><ref name=”2”>Santillian, R. DIY virtual reality tutorial: How to Make a VR Headset. Retrieved from https://diyhacking.com/diy-virtual-reality-headset/</ref>

==The lenses of the eyes==

==The lenses of the eyes==

It is very hard or even impossible to focus on things that are very close to the eyes. For example, if a person puts a piece of paper with text on it, one inch in front of one of the eyes, it will prove difficult to read it unless that person is extremely near-sighted. <ref name=”1”></ref>

It is very hard or even impossible to focus on things that are very close to the eyes. For example, if a person puts a piece of paper with text on it, one inch in front of one of the eyes, it will prove difficult to read it unless that person is extremely near-sighted. <ref name=”1”></ref>

The lenses in the eyes alter the incoming light in order to focus it on the receptors. The lenses bend according to the distance between the object being focused on and the eyes, converging the incoming light rays on the light-sensitive cells on the retina. When an object is closer to the eye’s lens, the rays coming from that object diverge at a large angle and, consequently, the lens has to adjust in order to focus the rays on the retina (Figure 1). Rays of light coming from an object at medium range will diverge less, and rays from an object infinitely far away are parallel and do not diverge. This means that as the distance between the eye and the object increases, the eye’s lens does not need to bend as much to maintain focus. This process of changing the shape of the lenses is called accommodation, and it is common not only in human but also other mammals, birds, and reptiles. <ref name=”1”></ref> <ref name=”3”></ref> <ref name=”4”>VR Cover (2016). How lenses for virtual reality headsets work [Video]. Retrieved from https://www.youtube.com/watch?v=NCBEYaC876A</ref>

The lenses in the eyes alter the incoming light in order to focus it on the receptors. The lenses bend according to the distance between the object being focused on and the eyes, converging the incoming light rays on the light-sensitive cells on the retina. When an object is closer to the eye’s lens, the rays coming from that object diverge at a large angle and, consequently, the lens has to adjust in order to focus the rays on the retina (Figure 1). Rays of light coming from an object at medium range will diverge less, and rays from an object infinitely far away are parallel and do not diverge. This means that as the distance between the eye and the object increases, the eye’s lens does not need to bend as much to maintain focus. This process of changing the shape of the lenses is called accommodation, and it is common not only in human but also other mammals, birds, and reptiles. <ref name=”1”></ref><ref name=”3”></ref><ref name=”4”>VR Cover (2016). How lenses for virtual reality headsets work [Video]. Retrieved from https://www.youtube.com/watch?v=NCBEYaC876A</ref>

A ring of muscle around the lens - the ciliary muscle - is responsible for making the lens thicker and rounder when viewing close objects; the muscles relax, flattening the lenses when the object is at a distance. There is a limit to how much the muscles can compress the lens. This means that there is a minimum distance on which the eye can focus. Generally, it is around 4 inches for children and young adults with normal vision. This value increases with age. <ref name=”1”></ref>

A ring of muscle around the lens - the ciliary muscle - is responsible for making the lens thicker and rounder when viewing close objects; the muscles relax, flattening the lenses when the object is at a distance. There is a limit to how much the muscles can compress the lens. This means that there is a minimum distance on which the eye can focus. Generally, it is around 4 inches for children and young adults with normal vision. This value increases with age. <ref name=”1”></ref>

[[File:VR headset fresnel lenses 2.png|thumb|Figure 4. A lower number of Fresnel lenses. (Image: VR Cover)]]

[[File:VR headset fresnel lenses 2.png|thumb|Figure 4. A lower number of Fresnel lenses. (Image: VR Cover)]]

Lenses are needed in VR HMDs in order to bend the light, making it easier for our eyes to accommodate the incoming rays and correctly see the images generated on a display that is 1.2 to 3 inches from the user’s eyes. With the lenses, the images are perceived farther away than they really are. The intermediate lens bends light rays, reducing the divergence angle of light coming from the screen to a point where the eye can focus it (Figure 2). <ref name=”3”></ref> <ref name=”4”></ref>

Lenses are needed in VR HMDs in order to bend the light, making it easier for our eyes to accommodate the incoming rays and correctly see the images generated on a display that is 1.2 to 3 inches from the user’s eyes. With the lenses, the images are perceived farther away than they really are. The intermediate lens bends light rays, reducing the divergence angle of light coming from the screen to a point where the eye can focus it (Figure 2). <ref name=”3”></ref><ref name=”4”></ref>

The created illusion of an image that is farther away than in reality is called a virtual image. If the rays of light between the intermediate lens and the eyes are projected backward, they will intersect in a single point behind the VR screen (Figure 2). This is the reason why users can focus on the images from the VR displays that are so close to their eyes. They are not trying to focus on images generated very close to their faces but on virtual images that seems to be at a larger distance. The distance of the virtual images depends on the model of the HMD. In the Oculus Rift DK1, the virtual screens were infinitely far away while in the Oculus Rift DK2, they were about 4.5 feet away. <ref name=”1”></ref>

The created illusion of an image that is farther away than in reality is called a virtual image. If the rays of light between the intermediate lens and the eyes are projected backward, they will intersect in a single point behind the VR screen (Figure 2). This is the reason why users can focus on the images from the VR displays that are so close to their eyes. They are not trying to focus on images generated very close to their faces but on virtual images that seems to be at a larger distance. The distance of the virtual images depends on the model of the HMD. In the Oculus Rift DK1, the virtual screens were infinitely far away while in the Oculus Rift DK2, they were about 4.5 feet away. <ref name=”1”></ref>

The HMDs can also use Fresnel lenses. Indeed, the HTC Vive uses this type of lenses, and the Oculus Rift CV1 has hybrid Fresnel lenses. These lenses are used to make the headset’s lenses thinner and lighter while maintaining the same function as regular lenses. Fresnel lenses have the same curvature than normal lenses but they are segmented (Figure 3). <ref name=”3”></ref> <ref name=”4”></ref>

The HMDs can also use Fresnel lenses. Indeed, the HTC Vive uses this type of lenses, and the Oculus Rift CV1 has hybrid Fresnel lenses. These lenses are used to make the headset’s lenses thinner and lighter while maintaining the same function as regular lenses. Fresnel lenses have the same curvature than normal lenses but they are segmented (Figure 3). <ref name=”3”></ref><ref name=”4”></ref>

However, there are some compromises when using Fresnel lenses: firstly, creating lenses with many segments will result in a sharper image but some light will be lost that gets scattered at the lenses’ peaks (Figure 3). Secondly, Fresnel lenses with fewer segments will result in less scattered light but the image will not be as sharp (Figure 4). <ref name=”4”></ref>

However, there are some compromises when using Fresnel lenses: firstly, creating lenses with many segments will result in a sharper image but some light will be lost that gets scattered at the lenses’ peaks (Figure 3). Secondly, Fresnel lenses with fewer segments will result in less scattered light but the image will not be as sharp (Figure 4). <ref name=”4”></ref>