Presbyopia, which affects more than 100 million people in only the United States of America, is brought on by the crystalline lens of the eye losing its ability to focus as it becomes stiffer with age. Images of objects positioned at various distances are blurred as a result of the accommodative error or lag. The inability of traditional fixed uniform and graded power eyeglasses to accommodate results in a serious visual impairment. In this essay, we’ll talk about the use of small, auto-focusing eyeglasses that increase accommodative range and either partially or completely restore normal visual function. The development of tunable power eyepieces and the application of accommodation correction algorithms are covered in considerable detail in this study.
Age-related presbyopia, which affects more than 1.7 billion people worldwide, causes progressive loss of focal accommodation in the eye, which leads to the development of refractive vision defects in the majority of people beyond the age of 45. The ability of the eye’s crystalline lens to alter its optical power in focusing on objects placed at various distances is known as focal accommodation. A healthy eye’s optical power accommodation amplitude is about 12 D, but it gradually decreases as we age, leading to hazy vision, to roughly 1 D.
Prescription lightweight glasses are used to create the right optical power when the accommodation loss is substantial. One pair of prescription eyeglasses can create sharp images at one object distance but will produce blurry images at other distances because eyeglasses only give a fixed power shift & lack any adaptation. By zoning or grading the lens into areas of varied power in return for a smaller focused field of view, multifocal and progressive spectacles help to alleviate some of this issue.
The area zoning method causes severe eye strain and visual impairment. Fundamentally, no fixed power eyewear can take the role of the crystalline lens’ accommodating function. In order to make up for the loss of accommodation, the ideal corrective eyewear must have variable focus. In order to partially or completely restore normal vision, the accommodation offered by the variable power eyeglasses is added to the lower accommodation of presbyopic eyes.
ARRANGEMENTS IN THE PRESBYOPIC EYE
An eye that is functioning normally can focus on both nearby and distant objects. Although the mechanism behind accommodation and the variables influencing the accommodating response are not fully understood, age-related accommodation loss is caused by two main mechanisms. The ciliary muscles, a band of smooth muscle fibres that surrounds the crystalline and changes its shape & optical power when the muscular band contracts, experience a practical loss of strength. The second is crystalline stiffening. A typical accommodation loss curve for the human eye is seen in Figure 1 below as a function of age.
TUNABLE-FOCUS LIGHTWEIGHT LIQUID EYEPIECES
The development of variable focus lenses using various technologies, such as mechanically sliding surfaces, deformable lenses, and index-changing liquid crystals, is well documented in the literature. The “in focus” field of view and high aberrations produced by lenses based upon mechanically sliding surfaces result in poor image quality.
On the other side, liquid crystal lenses do not need moving parts, but they are exceedingly challenging to use when the aperture is more than 1 cm because this requires extremely thick LCD layers to provide the necessary high powers. Many LCD Fresnel lens configurations have been used to get around this problem, but these lenses are susceptible to chromatic and step edge effects. There are other liquid deformable tunable lenses in use, however many of these are large and bulky since they need to transport fluid into and out of storage reservoirs.