New research published in Optica and conducted by investigators at the Photonics, Numerical and Nanosciences Laboratory (LP2N) in Talence, France, details a new concept for freeform multifocal lenses that offers the potential for clear vision across varying distances and light conditions.
Let’s start with some background.
Multifocality and extended depth-of-field (EDOF) can play a key role in correcting vision in ocular conditions such as presbyopia as well as during intraocular lens (IOL) implantation following cataract extraction.
Researchers noted, however, that optics design in ophthalmic space “is constrained by the size and wearability of contact lenses and intraocular implants.”
As a result, such design limitations can “restrict their use to small diameters and clean environments, and may hinder comfort in contact lens applications due to possible interference with eyelids,” they stated.
Which leads us to…
The need for a display system that adapts to various vision distances—specifically, a projection lens that is more inexpensive, with a larger depth of field, and an easy-to-implement solution, the researchers concluded.
Their solution: a new concept for freeform multifocal lenses that would enable focal point shaping (position and depth of field) to provide the potential for selecting multiple behaviors:
- Multifocal
- EDOF
- Depth of encoding via focal spot shapes, independent of the aperture
Let’s talk lens design.
Advanced digital machining with high precision was used to mold a freeform design approach that involved focal power distribution on the output dipper in a spiral pattern, dividing the surface into two equal zones.
To validate its efficacy, the researcher then used it to image a digital “E”—similar to light-up boards used by optometrists—to ensure image quality was consistent across multiple aperture sizes.
The expectation: for the modified diopter to create two equivalent optical fields that assemble into two different zones, combining to produce new focal zones.
How does this compare to a standard astigmatism lens?
The modified spiral design creates multiple points of focus—with improved image quality—similar to having numerous lenses in one.
This is intended as a unique contrast to the typical symmetrical lens design, which involves various focal regions with both resolution and non-uniform defects.
And a trifocal lens?
Researchers simulated a conventional trifocal lens to match that of the spiral lens.
Their findings: the spiral lens demonstrated advantages over conventional trifocal lenses at larger apertures for most focal points, while preserving the multifocal behavior even at smaller apertures, a feature not exhibited typically in a trifocal lens design.
See here for details, including why the authors ultimately concluded that the comparison was inaccurate.
Gotcha. So what were the conclusions?
The researchers concluded that, by integrating features such as multifocal and high-quality optical vortices onto the diopter of a ‘classical lens,’ they have “created a compact and wearable optical element that produces such vortices with potential applications in wearable optics and ultracompact embedded imaging systems.”
They added that this supports the potential for such freeform multifocal lenses to provide wider depth perception while also reducing the need to rely on larger apertures in real-world scenarios.
What do they recommend?
Future research would focus on utilizing the spiral lens’s design parameters (shape and spiral distribution) to enhance performance across various apertures and focal zones, the authors stated.
Lastly, the significance.
“Spiralized multifocal lenses represent a significant advancement in multifocal optical surfaces,” they stated, “combining the advantages of optical vortices with a compact and wearable design, and hold great promise for further development and application in the field.”