The OPD is a technique that allows to study fine and precise optical properties of the human eye. The OPD is a science derived from optical Metrology (study of the quality of optical systems), and more directly from astronomy. In ophthalmology, the OPD gathers the techniques designed to study the optical quality of the eye and/or the quality of the retinal image. These review techniques to access the detailed study of the optical properties of the eye, refractive surfaces are the cornea and the lens. They are based on the collection of the ocular Wavefront with an aberrometer, and / or the analysis of the optical quality of the cornea with a corneal topographer (see the) more general page dedicated to "eye and mind." If necessary).
OPD and ophthalmology
What is the OPD?
The OPD in ophthalmology applications are many, and in particular interest to all the modern techniques of study and correction of the optical defects of the human eye: refractive surgery appeals to the OPD for the design of personalized treatments, and the study of refractive (optical) consequences of certain complications.
Interest diagnosis of the OPD
Aberrometrique examination is indicated whenever it is suspected the existence of a reduction of the optical quality of the eye: this reduction affects the light path in the eye, and thus the retinal image. Symptoms of appeal are not confined to a reduction in Visual acuity, but most often associate a feeling of degradation of vision that is not correctable by glasses, and is characterized by more specific symptoms such as a duplication of the images, halos around bright light sources, a printing less contrasty image. These symptoms affect central vision a whole and not a particular point in the Visual field. They tend to appear or worsen when the IRIS pupil dilates, which occurs generally when the ambient brightness decreases.
Understanding the founding principles of this relatively new approach for the clinician is a call to some knowledge of the field of physical optics, and suppose the taking into account of the wave nature of light. The ophthalmologist usually receives limited in optics, especially physical training. Despite the versatility of the relevant visual symptoms of an aberrometrique cause, most ophthalmologists experience some difficulty to understand this area. However, especially medical, profession, must know evolve and increase its field knowledge, in the interests of patients first. The eye is the first and the only optical instrument that Nature invented; It would be regrettable that he is the last to benefit from the same power and precision of analysis, as the telescopes, microsopes or photographic objectives.
Learn more about the place of the OPD in ophthalmology
OPD and adaptive optics
The coupling of an aberrometer with a deformable mirror (Adaptive Optics) allows to directly study the retinal structures by overcoming optical aberrations of the eye. In this application, we're close to astronomy where the OPD was born to help reduce the influence of atmospheric turbulence on the quality of the images collected by earthly telescopes. The aberrometer measures the aberrations of the eye and is enslaved to a deformable mirror. Placed on the optical path between the examiner and the retina to observe, this mirror is deformed to compensate local optical path differences, which are related to imperfections in the eye. The image of the retina wins in definition, and it is possible to directly observe the layer of photoreceptors (cones, sticks) of the retina.
Using a deformable mirror can also be used to induce some aberrations and to study the subjective influence on the vision. Download the article «» Optics adataptive for the study and the correction of the optical defects of the human eye « .
The possibility of selectively correcting or inducing certain high-degree optical aberrations and studying the impact on visual function is interesting in the context of the correction of presbyopia by multifocal.
OPD: General principles
The collection and analysis of the wave (OPD) front represent a technological advancement for the study of refraction equivalent to what has been compared to the simple keratometry corneal topography. As the "ray of light", the ocular Wavefront is a ' ' abstraction ' (it's a theoretical surface, an envelope of circular perimeter as bounded by the edges of the pupil, and which is perpendicular to the direction of propagation of the light 'rays'). This abstraction provides a useful model to quantify and qualify the optical aberrations of the eye: the wave front is useful for predicting the retinal image quality. The aberrometer after collection and analysis of ocular wave-front, allows to draw a map of the optical aberrations of low and high level. Aberrations tell of low degree (degree 1: tilt, degree 2: defocus, astigmatism) correspond to aberrations that are correctable by glasses from glasses. Aberrations from top (grade 3: coma, trefoil, degree 4: tetrafoil, spherical aberration, etc.) are not correctable by glasses, and when their rate exceeds a certain value, occaseionnent of the Visual symptoms in type of halos, duplication of the image, loss of contrast printing. "" These aberrations of high degree are sometimes grouped in ophthalmology 'classic' under the term of "irregular astigmatism.
The collection of the wave on a certain pupil diameter front (ideally broad if one wants to study the retinal image quality in conditions mesopiques) allows mathematical decomposition in aberrations of low and high level. allows to predict the "point spread function" ("point spread function"), end to complicated semantics which means however a very simple concept: that of the preservation of the stigma, IE the fidelity of the image of a point light source formed on the retina. If the image of a point source is relatively spot on the retina, then the quality of the retinal image loss is low: the minimum size of the retinal image (about 2 microns in the best conditions) determines the resolving power of the eye, which corresponds to an angle of minimum of resolution (MAR for Minimum Angle of Resolution) of about 30 seconds (close to 20/10 Visual acuity).
OPD and custom laser treatments
"" The coupling of the aberrometric and excimer laser allows the development of 'personalized treatment"(customized), aimed not only to correct the optical defect" basic "(correction glasses) but also the aberrations of high degree, in order to optimize the quality of vision in post operative. Apart from marketing aspects related to the demand of "customization" (or customization), custom treatments are especially indicated in certain circumstances such as the correction of strong astigmatismes or repeatedly them to shift.
Examples of use of the OPD
To illustrate the interest of the OPD in ophthalmology, maps showing abnormalities typically encountered in certain pathologies of emblematic way gathered in the following pages.
Healthy eye – Keratoconus – Split vision – Decentering after LASIK – Implant Center – Early cataract – Corneal scar – Multifocal implant – Pellucid marginal degeneration – Index myopia – Cornea transplant
They were acquired by the topographer-aberrometer OPD SCAN 3 (Nidek).
This instrument allows the acquisition of topographic data (corneal topography Placido), and aberrometrique (ocular Wavefront). The calculation of the internal Wavefront (posterior side of the cornea, lens, or implant) is obtained by subtraction. Unlike certain aberrometres specifically designed as device acquisition for the realization of profiles of ablation guided by the OPD, this instrument is not confined to this application, and has been designed as a diagnostic tool.
The OPD SCAN III allows the collection of the following information:
-refraction (sphere cylinder axis)
-measurement of the corneal Asphericity
-analysis automated anterior corneal topography (corneal navigator, indices of Klyce and Maeda)
-measurement of the Wavefront eye total (entire eye): top of low degree aberrations (RMS - Zernike polynomials)
-calculation of the original corneal aberrations: aberrations from top of low degree (RMS - Zernike polynomials)
-calculation of internal origin aberrations: aberrations from top of low degree (RMS - Zernike polynomials)
-calculation of the optical quality of the retinal image: Strehl ratio, MTF, Visual acuity Snellen, Siemens Star, etc.