The use of Fresnel lenses in optical systems VR goggles is not new, but has attracted additional attention in the past year. What are Fresnel lenses and what’s good and not-so-good about using them?

Bestcharge coupled device detector

Weight, thickness and cost are also important in HMDs, and thus many vendors have experimented with such design. Sensics, for instance, has rights to a patent that uses Fresnel lenses in wide-field designs such as the piSight shown below.

There are two problems with Fresnel lenses. The main problem is what happens when light hits the ridges, those peaks in the lens that do not correspond to actual curvature in the original lens. When light hits these points it is scatted, and scattered light in an optical system reduces contrast. Thus, you will often see that a Fresnel lens produces a more “milky” image with lower contrast. The second problem is a more technical one — it is more difficult to simulate a Fresnel lens in an optical design software.

Charge coupled deviceworking principle

Virtual reality pioneer Sensics shares insights on VR technology and market. These were collected over a decade of delivering high-performance VR goggles to customers worldwide. Insights include thought-provoking interviews, opinion pieces and plain-language tutorials on VR.

Charge-coupleddevicediagram

Readout: Once the charge packets have been transferred across the entire array, they are sequentially read out from the CCD. The charge in each pixel is converted into a voltage signal proportional to the amount of accumulated charge. These voltage signals are then amplified, digitized, and processed to form a digital image.

CCD vs CMOS

As readers of this blog already heard many times, optical design is a study in tradeoffs. If weight is key, Fresnel may be a great option. If performance is most important, Fresnel lenses might not be the first choice.

Charge-coupleddeviceRadiology

A Fresnel lens (lens 1 in the figure to the right) achieves this by segmenting the classical lens (lens 2) and bringing together small segments of the right curvature. Notice how pretty much each point of the Fresnel lens has the same curvature as the corresponding point in the classical lens.

CCDDetectorspectroscopy

CCDs offer several advantages for imaging applications, including high sensitivity, low noise, wide dynamic range, and excellent uniformity across the pixel array. They are capable of capturing high-quality images with fine detail and high resolution. However, CCDs also have limitations, such as relatively slow readout speeds and susceptibility to blooming (overflow of charge into neighboring pixels).

A charge-coupled device (CCD) is a type of electronic image sensor used in various imaging devices, including digital cameras, camcorders, and scientific instruments. It consists of an integrated circuit containing an array of light-sensitive pixels that convert photons (light) into electrical signals.

For additional VR tutorials on this blog, click here Expert interviews and tutorials can also be found on the Sensics Insight page here

Charge-coupleddevicePDF

Charge accumulation: When light strikes the surface of the CCD, photons interact with the photodiodes, generating electron-hole pairs. The electrons generated in each pixel are collected and accumulated in potential wells within the photodiodes, proportional to the intensity of the incident light.

Charge transfer: After the accumulation phase, the accumulated charge in each pixel is sequentially transferred to adjacent pixels within the CCD array. This transfer is achieved by applying voltage pulses to the electrodes within the CCD, causing the charge packets to move along the rows and columns of the pixel array.

CCD camera

Makers of specialty VR goggles and the leaders in open-source VR. Read additional VR insights at http://sensics.com/insight/

Fresnel lenses were invented nearly 200 years ago by Augistin-Jean Fresnel, a French physicist. The idea is ingeniously simple: the degree to which a lens bends a light ray that hits it depends on material (and hence the index of refraction) from which the lens is made and on the angle of incidence between the light and the surface of the lens. The problem that a Fresnel lens solves is as follows: a classic spherical lens can get very heavy (and expensive) if the curvature and radius is sufficiently large. Since the light bending is essentially determined by the angle at the surface of the lens, could we make a lens that has the same surface curvature at each point of incidence but is not as thick and heavy?

Having said that, these systems can be designed and simulated. Here are three variations of 90+ degree optical systems that were designed by Sensics:

The original use for which Mr. Fresnel invented the lens was lighthouses (the tall maritime tower, not the Valve tracking system). Focusing the light from the lighthouse into a beam required a very large lens, and using a Fresnel design, this lens could be much thinner, lighter and cheaper than a big chunk of glass. These lenses then also found use a rear window lenses for Minivans (for example, this one) or large lightweight magnifiers.

Despite the emergence of newer technologies like CMOS (complementary metal-oxide-semiconductor) sensors, CCDs continue to be used in various scientific, industrial, and astronomical applications where high image quality and sensitivity are paramount.

Pixel array: The CCD contains an array of pixels arranged in rows and columns. Each pixel consists of a photosensitive region called a photodiode, which generates charge carriers (electrons) when exposed to light.