Perceptually Calibrated Scale of Optical Aberrations, Generated by Multifocal Corrective Lenses

CONTENT DESCRIPTION OF THE PROJECT Perceptually calibrated scale of optical aberrations, generated by multifocal corrective lenses Scientific background:As a result of the progressive aging of the population, especially in developed countries, more and more economically active people function after cataract surgery. In Poland, approximately 250,000 patients are eligible for such a procedure annually. During it, the natural lens of the eye with a significantly limited transparency is replaced with an artificial implant, which causes the loss of eye accommodation. In order to ensure full functional vision (near, indirect and far vision), the most commonly used are implants in the form of multifocal lenses, which have become the subject of intensive research in visual optics and optometry in recent years . The search for optimal solutions to compensate for the lack of accommodation is a kind of compromise between the depth of field of view and its resolution and contrast. Often, a side effect of increasing the range of viewing distances turns out to be various types of aberrations causing distortions of the seen scenes in the form of glare and halo The glare and halo phenomena are a natural consequence of the optical properties of multifocal lenses, i.e. lenses that create two or more foci of light focus along the optical axis. Such lenses allow for sharp imaging on the screen of the retina of the eye at the same time of objects from several distances. A focused image of a point object (strictly of small angular size) placed at one of the design distances is blurred in the other. An intense bright spot (glow) appears in the center surrounded by rings of light (halo). In daytime vision, the visual centers in the human brain are shown to be good at suppressing the perception of these disturbances. The problem appears in night vision, however, when the scenes consist of light sources (lanterns, car lights, reflections) with high contrast to the background. Visible glare and halo phenomena significantly increase the threshold brightness of noticeable objects in the immediate vicinity of bright points of the field of view. Therefore, they seriously limit the safety of participation in road traffic and the comfort of vision after dark.

There are many works devoted to the problem of measuring and parametrization of the halo and glow phenomena. The first approach is based on the optical parameterization of the impulse response (PSF) and modulating transport function (MTF) of the lenses and, on their basis, the visualization of undesirable aberrations. This approach requires the use of the eye model, which is always the result of some simplifications and idealizations. It also does not take into account the physiological processes related to the vision process. Nevertheless, it allows to objectively predict the occurrence of glare and halo phenomena and associate them with a specific design of a corrective lens. It also allows to measure the size and intensity of these effects using variously defined metrics.

The second approach is to try to measure the subjective perception of halo and glow phenomena among patients with implanted or inserted multifocal lenses (Halo & Glare Simulator; Eyeland-Design Network GmbH, Vreden, Germany). Such analyzes are most often based on the method of questionnaires in which the respondents themselves determine the type of visual disturbance (from a closed graphic list) and its degree (on a quantitative scale). The task of assessing visual sensations on an interval scale is a difficult and imprecise task. Therefore, it is undoubtedly necessary to develop a more reliable, based on psychophysical methodology, quantification of the size and intensity of undesirable phenomena in the field of view.

One of the methods combining the objectivity of the study with the subjectivity of phenomena available only to the examined person is the commercially available C-quant device by Oculus (Oculus Optikgeräte, GmbH, Wetzlar, Germany), used for the diagnostic measurement of distractions in night vision. The compensatory comparison method implemented in the device consists in finding such the intensity of the flickering area located in the center of the field of view to compensate for the glow caused by the bright ring flickering in the counter-phase. With appropriately selected light intensities in the center of the field of view, the glow from the ring is visible for half a period, and a target of the same brightness for the second half of the period, so the flickering disappears. The disadvantage of this method is the result in the form of a single parameter by means of which it is impossible to precisely and independently parametrize the halo and glow phenomena.

Another method, used in the Vision Monitor device (MonCv3; Metrovision, Pérenchies, France), is a study in which the patient reads optotypes with a specific contrast placed at an increasing distance from a point light source [6]. Since the light source creates a glow and a halo in the field of view of the examined person, the minimum brightness of the identifiable individual optotypes differs depending on the distance from it. Measurement of this brightness allows to parameterize - in the analyzed device with relatively low resolution - the glow and halo phenomena in the form of their profile and intensity depending on the distance from the light source. A similar methodology will be used in the project requested for funding.