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Haidinger's brush, more commonly known as Haidinger's brushes is an image produced by the eye, an entoptic phenomenon, first described by Austrian physicist Wilhelm Karl von Haidinger in 1844. Haidinger saw it when he looked through various minerals that polarized light.
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent. The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress.
Esotropia is a form of strabismus in which one or both eyes turns inward. The condition can be constantly present, or occur intermittently, and can give the affected individual a "cross-eyed" appearance. It is the opposite of exotropia and usually involves more severe axis deviation than esophoria. Esotropia is sometimes erroneously called "lazy eye", which describes the condition of amblyopia; a reduction in vision of one or both eyes that is not the result of any pathology of the eye and cannot be resolved by the use of corrective lenses. Amblyopia can, however, arise as a result of esotropia occurring in childhood: In order to relieve symptoms of diplopia or double vision, the child's brain will ignore or "suppress" the image from the esotropic eye, which when allowed to continue untreated will lead to the development of amblyopia. Treatment options for esotropia include glasses to correct refractive errors, the use of prisms and/or orthoptic exercises and/or eye muscle surgery. The term is from Greek eso meaning "inward" and trope meaning "a turning".
In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Neurological researcher Manfred Fahle has stated six specific advantages of having two eyes rather than just one:
- It gives a creature a "spare eye" in case one is damaged.
- It gives a wider field of view. For example, humans have a maximum horizontal field of view of approximately 190 degrees with two eyes, approximately 120 degrees of which makes up the binocular field of view flanked by two uniocular fields of approximately 40 degrees.
- It can give stereopsis in which binocular disparity provided by the two eyes' different positions on the head gives precise depth perception. This also allows a creature to break the camouflage of another creature.
- It allows the angles of the eyes' lines of sight, relative to each other (vergence), and those lines relative to a particular object to be determined from the images in the two eyes. These properties are necessary for the third advantage.
- It allows a creature to see more of, or all of, an object behind an obstacle. This advantage was pointed out by Leonardo da Vinci, who noted that a vertical column closer to the eyes than an object at which a creature is looking might block some of the object from the left eye but that part of the object might be visible to the right eye.
- It gives binocular summation in which the ability to detect faint objects is enhanced.
Peripheral vision, or indirect vision, is vision as it occurs outside the point of fixation, i.e. away from the center of gaze or, when viewed at large angles, in the "corner of one's eye". The vast majority of the area in the visual field is included in the notion of peripheral vision. "Far peripheral" vision refers to the area at the edges of the visual field, "mid-peripheral" vision refers to medium eccentricities, and "near-peripheral", sometimes referred to as "para-central" vision, exists adjacent to the center of gaze.
Strabismus is a condition in which the eyes do not properly align with each other when looking at an object. The eye that is focused on an object can alternate. The condition may be present occasionally or constantly. If present during a large part of childhood, it may result in amblyopia or lazy eyes and loss of depth perception. If onset is during adulthood, it is more likely to result in double vision.
Amblyopia, also called lazy eye, is a disorder of sight in which the brain fails to fully process inputs from one eye and over time favors the other eye. It results in decreased vision in an eye that otherwise typically appears normal. Amblyopia is the most common cause of decreased vision in a single eye among children and younger adults.
Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal image within the eye, (2) the health and functioning of the retina, and (3) the sensitivity of the interpretative faculty of the brain.
Scanning laser ophthalmoscopy (SLO) is a method of examination of the eye. It uses the technique of confocal laser scanning microscopy for diagnostic imaging of the retina or cornea of the human eye.
The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.
Anisometropia is when two eyes have unequal refractive power. Generally, a difference in power of two diopters or more is the accepted threshold to label the condition anisometropia.
Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally or vertically in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often voluntary. However, when occurring involuntarily, it results in impaired function of the extraocular muscles, where both eyes are still functional, but they cannot turn to target the desired object. Problems with these muscles may be due to mechanical problems, disorders of the neuromuscular junction, disorders of the cranial nerves that innervate the muscles, and occasionally disorders involving the supranuclear oculomotor pathways or ingestion of toxins.
An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, ophthalmologist, orthoptist, or an optician. Health care professionals often recommend that all people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are asymptomatic.
Infant vision concerns the development of visual ability in human infants from birth through the first years of life. The aspects of human vision which develop following birth include visual acuity, tracking, color perception, depth perception, and object recognition.
The red reflex refers to the reddish-orange reflection of light from the back of the eye, or fundus, observed when using an ophthalmoscope or retinoscope. The reflex relies on the transparency of optical media and reflects off the fundus back through media into the aperture of the ophthalmoscope. The red reflex is considered abnormal if there is any asymmetry between the eyes, dark spots, or white reflex (Leukocoria).
Scanning laser polarimetry is the use of polarised light to measure the thickness of the retinal nerve fiber layer as part of a glaucoma workup. The GDx-VCC is one example.
Perifovea is a region in the retina that circumscribes the parafovea and fovea and is a part of the macula lutea. The perifovea is a belt that covers a 10° radius around the fovea and is 1.5 mm wide. The perifovea ends when the Henle's fiber layer disappears and the ganglion cells are one-layered.
Bagolini striated glasses test, or BSGT, is a subjective clinical test to detect the presence or extent of binocular functions and is generally performed by an Optometrist(O.D) or orthoptist or ophthalmologist. It is mainly used in strabismus clinics. Through this test, suppression, microtropia, diplopia and manifest deviations can be noted. However this test should always be used in conjunction with other clinical tests, such as Worth 4 dot test, Cover test, Prism cover test and Maddox rod to come to a diagnosis.
Microperimetry, sometimes called fundus-controlled perimetry, is a type of visual field test which uses one of several technologies to create a "retinal sensitivity map" of the quantity of light perceived in specific parts of the retina in people who have lost the ability to fixate on an object or light source. The main difference with traditional perimetry instruments is that, microperimetry includes a system to image the retina and an eye tracker to compensate eye movements during visual field testing.
The FourPrism Dioptre Reflex Test, is an objective, non-dissociative test used to prove the alignment of both eyes by assessing motor fusion. Through the use of a 4 dioptre base out prism, diplopia is induced which is the driving force for the eyes to change fixation and therefore re-gain bifoveal fixation meaning, they overcome that amount of power.
References
- ↑ Reed M. Jost; Joost Felius; Eileen E. Birch (August 2014). "High sensitivity of binocular retinal birefringence screening for anisometropic amblyopia without strabismus". Journal of American Association for Pediatric Ophthalmology and Strabismus. 18 (4): e5–e6. doi:10.1016/j.jaapos.2014.07.017.
- ↑ Gramatikov BI (2014). "Modern technologies for retinal scanning and imaging: an introduction for the biomedical engineer". BioMedical Engineering OnLine. 13: 52. doi:10.1186/1475-925X-13-52. PMC 4022984 . PMID 24779618.
- ↑ "Optical Birefringence". Olympus Microscopy Resource Center. Olympus America Inc. Retrieved 2015-12-06.
- ↑ Gramatikov B (2013). "Detecting fixation on a target using time-frequency distributions of a retinal birefringence scanning signal". BioMedical Engineering OnLine. 12: 41. doi:10.1186/1475-925X-12-41. PMC 3661397 . PMID 23668264.
- ↑ Loudon SE, Rook CA, Nassif DS, Piskun NV, Hunter DG (2011). "Rapid, high-accuracy detection of strabismus and amblyopia using the pediatric vision scanner". Investigative Ophthalmology & Visual Science. 52 (8): 5043–8. doi: 10.1167/iovs.11-7503 . PMID 21642624.
- ↑ GDx-MM: An Imaging Mueller Matrix Retinal Polarimeter. 2007. p. 56. ISBN 978-0-549-27120-8.
- ↑ Issues in Biomedical Engineering Research and Application: 2013 Edition. ScholarlyEditions. 1 May 2013. p. 297. ISBN 978-1-4901-0871-1.