The Optical Knowledge Portal
Information – Education – Certification
Here you will find definitions for relevant terms used in optics and ophthalmology:
Abbe number = The Abbe number measures the colour dispersion as light passes through a transparent optical medium. The lower the number, the greater the colour dispersion. This light dispersion leads to undesired “colour fringing” around objects viewed through a spectacle lens. The Abbe number is important for correcting optical aberrations.
The following rule applies in general: The higher the refractive index, the lower the Abbe number and the greater the colour dispersion.
Absorption = Filtering effect by reducing the radiation intensity when light waves travel through a substance and release energy to this substance. Absorption indicates the percentage of light, e.g. UV radiation, filtered out when travelling through a medium (spectacle or contact lens, crystalline lens).
Acetate = Acetate is the most commonly used plastic material for spectacle frames since the material is extremely skin-friendly. The manufacturing process is complex since the frames are milled from pre-manufactured acetate sheets.
Achromat = An achromat is a lens system composed of a converging lens and a diverging lens with differing Abbe numbers. One lens is usually made of crown glass, and the other of flint glass. This lens system is used for magnifying glasses such as telescopic systems. Due to the differing Abbe numbers of the two lenses, an achromat corrects colour distortion such as chromatic aberration. Consequently, achromatic systems largely eliminate disturbing colour fringing caused by varying wavelengths.
Achromatopsia = Also known as rod monochromacy; inherited colour vision disorder that affects the cone photoreceptors in the retina, eventually resulting in total colour blindness.
Accommodation = Ability of the eye to change its focus from a distant object (e.g. TV) to a near object (e.g. newspaper).
Adaptation = Ability of the eye to adjust to various levels of darkness and light.
Afocal = Pertaining to or having no finite focal point.
Afocal system = An afocal system causes no convergence or dispersion of the light beam. Light rays that enter a system parallel to the axis, exit the system parallel to the axis. They have no finite focal point because the focal length of an afocal system is infinite.
This is due to the fact that the rear focal point of the objective lens coincides with the front focal point of the eyepiece lens. Afocal systems enlarge or reduce the field angle when viewing objects in infinity. Galilean and Keplerian telescopes are examples of afocal systems.
Age-related macular degeneration = Degenerative process leading to the gradual loss of photoreceptors in the central part of the retina (macula). This condition occurs in two forms: dry and wet macular degeneration.
Agglutination of proteins = Clumping of proteins.
Amaurosis fugax = Refers to the temporary, sudden and painless loss of vision caused by a blood clot or piece of plaque that blocks a retinal artery.
AMD = See "age-related macular degeneration"
Amotio retinae = Retinal detachment (also known as amotio retinae) occurs when the light-sensitive photoreceptor layer begins to tear or pull away from the underlying choroid.
Amsler grid test = Screening test to early detect pathological changes of the eye, e.g. age-related macular degeneration.
Aniridia = Rare genetic disorder characterised by the absence of the iris.
Aniseikonia = Aniseikonia is an ocular condition where there is a significant difference in the perceived size of images. This difference in image size can have anatomical causes (retinal aniseikonia), nerve-related causes (functional aniseikonia) or optical causes, e.g. anisometropia.
Anisometropia = The refractive error can vary between the two eyes. For instance, the shortsightedness or astigmatism in one eye can be more pronounced than in the other eye. In extreme cases, one eye can be hyperopic while the other eye is myopic. A difference in power of 2.5 dioptres or more is the accepted threshold for labelling the condition anisometropia.
Aperture stop = Opening which limits the amount of light passing through an optical system.
Aphakia = Aphakia is the absence of the crystalline lens in one eye due to a congenital anomaly, surgical removal of the natural lens or a trauma (rare). Without the focusing power of the lens, the eye cannot adjust to various distances. As a result, the eye becomes hyperopic. This can be corrected by implanting an artificial lens or by wearing special contact lenses.
Apoptosis = Programmed cell death, see also "turnover".
Appositional growth = Growth accomplished by the addition of new layers of tissue to those previously formed.
Aqueous humour = Clear liquid that fills the inside of the eye between the cornea and the lens.
Aspheric lens = Has different refractive powers in every point of the lens regardless of the meridian. The lens curve flattens toward the edge.
Aspheric surface/asphere = Refracting or reflecting surface that departs from the spherical form. The lens surface flattens towards the edge and consequently the curvature decreases As a result, aspheric lenses are thinner and lighter.
Astenopia (eye strain) = Uncorrected or insufficiently corrected heterophoria (eyes are not naturally aligned in parallel) can result in various symptoms. They can primarily be attributed to extreme straining of the eyes’ nerve and muscle apparatus, causing headaches, soreness, itching or burning eyes and often also redness of the eyes. In severe cases, astenopia can lead to nausea and vomiting.
Astigmatic lens = Features two meridians perpendicular to each other that have different refractive powers.
Astigmatism = Astigmatism is usually caused by an irregularly shaped cornea or lens (rare) and results in blurred or distorted images on the retina. The cornea or lens is not evenly curved like a sphere or ball since the two principal meridians have different radii of curvature. Astigmatism is often combined with myopia or hyperopia and can be corrected with toric (cylindrical) lenses or contact lenses.
Axial myopia = The most common form of short sightedness is axial myopia. This is caused by the eyeball being too long, resulting in a blurred image on the retina. Refractive myopia is another type of nearsightedness. It occurs when the refractive power of the cornea or crystalline lens is too high.
Basement membrane = Tissue beneath the basal epithelial cells that enhances wound healing and anchors the corneal epithelium to the stroma.
Bifocal lens = Enables clear vision at two different distances, usually for near and distance vision. The upper part is designed for distance use and the lower part for reading.
Binocular = With both eyes.
Bipolar cells = Second layer of retinal neurons.
Blue blocker = Blue blocking filters are special protective filter lenses that cut off part of the light spectrum at a defined point and absorb all high-energy UV light and the blue portion of visible light below this point.
Blue light = High energy light at the short wavelength end of the light spectrum.
Blurry vision = Loss of sharpness of eyesight, making objects appear out of focus and hazy. The primary causes of blurred vision are refractive errors — nearsightedness, farsightedness and astigmatism — or presbyopia. But blurry vision also can be a symptom of more serious problems, including a potentially sight-threatening eye disease or neurological disorder.
Bowman’s membrane = Membrane located below the basement membrane.
Bruch’s membrane = Thin membrane between the choroid and the retinal pigment epithelium.
Bulbus oculi = Globe of the eye; refers to the eyeball without its appendages.
Carotid artery stenosis = Describes the narrowing of the two main arteries supplying the brain: "arteria carotis communis" (ACC → common carotid artery) and/or the “arteria carotis interna” (→ internal carotid artery).
Cataract = A cataract is the clouding of the lens in the eye.
Choroidal neovascularisation = Creation of new blood vessels beneath the retina.
Chromatic aberration = Also known as "colour fringing"; unequal refraction by a lens of light rays of different lengths passing through it.
Chromophores = Protein complex formed by the agglutination (clumping) of proteins.
Ciliary body = Just like the iris and the choroid, the ciliary body is part of the highly vascularised uvea. It controls the shape of the lens (accommodation) and produces the aqueous humour.
Cloudy media = Clouding e.g. of the cornea, lens or vitreous humour.
Cloudy vision = Objects are obscured and appear "milky"; it is often mistaken for blurry vision. Cloudy vision usually is a symptom of specific conditions such as cataracts but blurry and cloudy vision can both be symptoms of a serious eye problem.
Collagens = Long protein chains with tremendous tensile strength.
Colour blindness = The term is often incorrectly used to describe a colour vision deficiency. The complete absence of colour vision is referred to as "achromatopsia" or rod monochromacy.
Colour vision deficiency = Reduced ability to discriminate between colours, especially shades of red and green or blue and yellow.
Comfort filters = Light protection lenses with a blue attenuating effect. They filter out the blue portion of sunlight, which often superimposes itself on the other colours, to an ideal level.
Concave = Means curving in or hollowed inward.
Cones = The photoreceptor cells in the retina responsible for colour vision.
Cone dystrophy = Refers to a group of eye disorders that lead to the dysfunction or functional loss of the cones located in the macula lutea (point of sharpest vision)
Conjunctiva = Thin membrane, interlaced with blood vessels, that connects the eyeball to the eyebrows.
Convex = Means curving out or extending outward.
Cornea = The cornea is a transparent avascular tissue that functions as a window and allows light to enter the eye. Thanks to its mechanical stability, it counteracts the intraocular pressure and ensures the shape of the eye remains stable.
Corneal endothelium = See "endothelium".
Corneal epithelium = See "epithelium".
Descemet’s membrane = Basement membrane of the corneal endothelium.
Desktop video magnifier = Desktop video magnifiers resemble computers. They are less handy than portable video magnifiers due to their size, but provide greater user comfort thanks to their larger screens. They use a camera in a fixed position to enlarge the desired section. The level of magnification needed and the contrast settings can be individually set and adjusted as required.
Diabetic retinopathy = Complication of diabetes mellitus that is marked by changes in the blood vessels in the fundus of the eye. These changes are caused by blocked vessels decreasing the blood flow within the retina.
Dioptres = Unit of measurement used to indicate the refractive power of an optical system.
Drusen = Hyaline deposits (deposits that appear clear and glass-like under the microscope) beneath the retinal pigment epithelium. They have a circular shape and are perceived as greyish dots with the slit lamp.
Dry macular degeneration = The dry variant of age-related macular degeneration is a gerontological process associated with metabolic disorders that impair the removal of retinal waste products (lipofuscins).
Electronic magnifier = Handheld video magnifier that uses a lens to capture texts and objects and then displays enlarged images of these. Its main benefit is that it provides great flexibility – in addition to the magnification level, the colour and contrast settings can also be varied. Features such as adjustable reading lines make reading even more comfortable.
Endothelium = Layer of hexagonal cells lining Descemet’s membrane. This cell layer is created only once and cannot be renewed. Once endothelium cells are destroyed, they are lost forever.
Epithelium = Outermost layer of the cornea. It is composed of approx. 5-7 layers of cells stacked on top of each other. The epithelium cells are all produced in the underlying basement membrane/layer by means of cell division.
Erecting prism = A right-angled optical prism designed to bend light rays by means of reflection, turning an inverted image upright.
Eyelid = The eyelids (palpebrae) are the most external structures of the eye. They protect the eyeball from harmful contact and ensure the tear film is distributed over the anterior surface of the eye, keeping it moist.
Excavation = Depression or pit in the centre of the papilla.
Eye screening = Tests to check the functionality and physiological intactness of the eye.
Fibroblasts = "Repair cells".
Fluid pressure inside the eye = See "intraocular pressure".
Floaters = Particles that float in the vitreous humour and cast shadows on the retina. A sudden shower of floaters (also known as "black rain") signals a retinal tear or detachment.
Fluorescence = Emission of light by a substance that has absorbed light beforehand.
Fovea = Centre of the macula.
Fovea centralis = Pit in the retina.
Foveola = Depression in the centre of sharpest vision (see also "macula lutea").
Funduscopy = Examination of the back of the eye, the fundus.
Fundus of the eye = Back portion of the interior of the eyeball.
GAGS = See "glycosaminoglycans".
Galilean telescopic system = Telescopic system using a convex lens as the objective and a concave lens as the ocular lens.
Ganglion cells with axons = Third order retinal neurons which transmit converted light stimuli to the brain.
Gerontological changes = Age-related changes.
Glaucoma = Group of eye conditions involving the loss of nerve fibres and eventually also leading to total blindness
Glycosaminoglycans = Group of proteins that form an important component of connective tissues.
Gonioscopy = See"three-mirror lens".
Hazy vision = Also referred to as cloudy vision; a dulling or dimming of vision as if there was a veil over the eye.
Hexagonal cells = Cells that resemble six-sided prisms from an optical point of view.
High addition spectacles = Spectacles with high magnification power integrated into a section of the spectacle lens to leave both hands free e.g. for sewing or craftwork.
Hyaline deposits = Deposits that appear clear and glassy under the microscope.
Hydrogel = The hydrogel ("water with a rubbery consistency") is composed of 80% water, 15% collagen (long protein chains with great tensile strength) and 5% GAGs (glycosaminoglycans).
Hyper-evaporative dry eye = Increased evaporation of tear fluid, which can result from Meibomian gland dysfunction or a deficiency in the tear film composition.
Hyperopia = Farsightedness.
Injection therapy = Treatment for wet age-related macular degeneration.
Intraocular pressure = Pressure in the inside of the eye that helps maintain physiological stability.
Intraocular lens = An intraocular lens is an artificial lens often implanted after cataract surgery to enable the most natural vision possible.
IOL = See "intraocular lens ".
Iris = The iris is the anterior-most part of the uvea (middle layer of the eye) and controls the amount of light entering the eye.
Iris coloboma = An iris coloboma is a hole in one of the eye structures, such as the iris. It can be present from birth (congenital) or result from eye surgery or eye trauma. When congenital, it is caused by failed or incomplete closure of the embryonic fissure, which normally closes around the 6th week of pregnancy.
Iris contraction = Contraction of the circular muscles (musculus sphincter pupillae) closes the pupil, allowing less light to enter the eye and minimising glare.
Iris dilation = Widening of the pupil to allow more light to enter the eye in dim ambient light.
Juvenile macular degeneration = Juvenile macular degeneration (macular dystrophy) is the term for several eye diseases that affect the macula of children and young adults. The juvenile type of macular degeneration is nowadays considered to be a genetic condition and, like most types, leads to a rapid progressive deterioration of central vision. Juvenile macular degeneration does not result in total blindness. Patients usually retain their peripheral (spatial) vision.
Jelly bumps = Whitish gel-like material that can build up on a lens surface, mostly made up of protein, calcium, lipid and mucin deposits.
Keplerian telescopic system = Telescopic system using a concave lens as the objective and a convex lens as the ocular lens.
Lactoferrin = Lactoferrin is a protein with very high iron-binding capabilities. Since bacteria need iron to grow, "lactoferrin" deprives bacteria of nutrients by absorbing iron.
Lens refractive power = 19 D without accommodation.
Light absorption = Describes a process during which a body absorbs light energy.
Lighting needs = As people age, they require more light for daily tasks such as reading or writing. Studies have shown that appropriate lighting can significantly reduce the magnification power required by a visually impaired person.
Light protection spectacles = Commonly known as sunglasses.
Light scatter = Occurs when light encounters particles much smaller than its wavelengths.
Light spectrum = The light spectrum is the many different wavelengths of energy produced by a light source. Visible light (for humans) is the part of the spectrum from 380 nm to 780 nm.
Lipids = Fatty, oily or waxy organic compounds.
Lipid layer = Outer layer of the tear film that primarily serves to reduce evaporation from the underlying aqueous (watery) layer.
Lipofuscins = Metabolic waste products.
Low vision = Uncorrectable vision loss, often caused by disease, that significantly impacts daily activities and can even lead to blindness.
Lucentis (ranibizumab) injections = Injectable drug used in the treatment of wet AMD.
Lutein = Carotenoid pigment that builds up in the retina together with zeaxanthin.
Lysozyme = Lysozyme works as an antibacterial agent. It destroys bacteria by damaging their cell walls and agglutinating them.
Macula = Small, highly sensitive area of the retina responsible for central vision.
Macula lutea = Yellow spot at the centre of the retina, responsible for sharp, detailed central vision. Contains a very high concentration of cones.
Macular degeneration = Eye condition that describes a group of pathological changes affecting the macula.
Magnifying visual aids = Specific products for people with reduced or impaired vision e.g. due to an eye disorder.
Meibomian glands = Sebaceous glands that produce meibum (Meibomian secretion). Meibomian gland dysfunction (MGF) can lead to dry eyes.
Melanin = Broad term for a group of natural pigments.
Membrane = Most commonly means a thin, selective barrier.
Meridian of eye = A circle passing through the anterior and posterior poles of the eyeball.
Metabolic disorder = Condition that interferes with the body’s chemical processes involved in growth, maintenance of healthy tissues, disposal of waste products, and production of energy to fuel body functions. Metabolic disorders that affect the eye may cause significant visual disturbances and sometimes even blindness.
Metamorphopsia = Defect in vision in which lines appear wavy.
Monocular = Use of one eye only.
Monochromacy = More commonly referred to as “total colour blindness”, is caused by the total absence of either 2 or 3 of the pigmented retinal cones, reducing vision to one dimension.
Mucins = Mucins are gel-forming glycoproteins secreted by the goblet cells in the conjunctival epithelium. The mucin layer acts as a moistening agent for the naturally water-repellent surface of the cornea.
Mucin layer = Bottom layer of tear film.
Musculus dilatator pupillae = Radial muscle of the iris that opens the pupil.
Musculus sphincter pupillae = Circular muscle of the iris that closes the pupil.
Myopia = Nearsightedness.
Nervus opticus = Optic nerve, also known as cranial nerve II.
Neurons = Nerve cells.
Neuroretina = Retinal layers without the retinal pigment epithelium.
Night blindness = Inability to see clearly at night or in dim light. It is often caused by eye disorders such as retinitis pigmentosa.
Nucleus lentis = The core or inner dense portion of the eye already present at birth.
Nystagmus = Nystagmus is characterised by involuntary rapid eye movements ("dancing eyes" or "jerking eyes"). Symptoms include vertigo or dizziness as a result of the "jerking/dancing" eyes, which is why patients often acquire a head tilt to counter this side effect.
Ocular albinism = Ocular albinism is characterised by a lack of pigment cells (melanin) in the iris and retina that normally help protect the eye from ultraviolet radiation and provide protection against the development of cancerous cells.
Optic canal = Area between the eye socket and the brain the optic nerve runs through.
Optic nerve head = Also known as the optic disc; this is the point where all optic nerves come together and run into the optic canal.
Optical aberration = In optics, aberration refers to any deviation from the formation of a perfect image (light focuses on a single point; the image is true to colour and scale).
The most common types of optical aberration include:
- Spherical aberration
- Coma
- Astigmatism of oblique incidence
- Field curvature
- Distortion
- Chromatic aberration (different deflection of different wavelengths of light)
Ora serrata = Serrated junction between the retina and the ciliary body.
Papilla = Blind spot.
Parasympathetic nervous system = Subsystem of the autonomic nervous system that controls involuntary movements (smooth muscles).
Pathological changes = Changes caused by disease.
Peripheral zone/Periphery = External boundary of any surface or area.
Photochromic lens = Self-tinting lens that changes its light transmitting properties on exposure to UV radiation.
Photopic vision = Vision of the eye under well-lit conditions (daytime).
Photopsia = Flashes of light in the eye.
Photoreceptors = "Rods and cones", visual cells.
Pigment epithelium = Pigmented cell layer located just outside the retina and firmly attached to the underlying choroid.
Pigmentation = Colouring of the skin, hair, mucous membranes and retina of the eye due to the deposition of the pigment melanin.
PMMA = Poly(methyl) methacrylate (transparent thermoplastic/acrylic glass).
Point of sharpest vision = See "macula lutea".
Polarisation = Describes the ability of waves to oscillate in more than one direction.
Polarising filter = Polarising filters significantly reduce reflections and thus further increase contrast vision and enhance contrast.
Polycondensation = Process used to manufacture plastic spectacle lenses.
Polymegathism = Variation in cell sizes in relation to the original state.
Polymerisation = Process used in the manufacturing of plastic spectacle lenses.
Polymorphism = Variation in cell shapes in relation to the original state.
Posterior chamber of the eye = Part of the eye filled with aqueous humour and located between the iris and vitreous humour.
Presbyopia = Vision condition that occurs as part of the normal aging process and involves the loss of the eye's ability to focus on close objects.
Prism base position = Also known as prism base setting. Defines the position of the thickest part of a prismatic lens.
Prismatic effect = Results from the deviation of light rays towards the base of a prism. Prismatic deviation refers to the change in direction imposed on a ray of light as a result of refraction.
Progression channel = Vision zone between distance zone and near zone of a progressive lens.
Progressive lenses = Offer the required dioptric lens power for every distance. They allow smooth vision at any distance. Progressive lenses are roughly divided into three vision zones: distance zone, progression channel (intermediate zone) and near zone.
Progressive myopia = The continuous growth of the eyeball length results in short or near-sightedness, which is referred to as myopia up to a certain level of refractive error. If this level is exceeded because the eyeball continues to lengthen, this is referred to as “progressive myopia”.
Protein complex = See "chromophores".
To follow
Reading devices = Electronic text-to-speech devices for people who are blind and those with visual impairment
Refraction = Test to determine the visual acuity.
Refractive deficit = Dioptric value that is not present to be considered as emmetropic (normal-sighted).
Refractive error = Optical defect in an unaccommodating eye; parallel light rays are not brought to a sharp focus precisely on the retina, producing a blurred retinal image. Can be corrected by eyeglasses, contact lenses, or refractive surgery.
Refractive power = Ability of an optical system to bend light.
Relative distance magnification = Increases resolution by reducing the distance between the object and the eye.
Retina = The retina is actually an extension of the diencephalon. It absorbs light and converts it into electromagnetic impulses that are sent to the brain, allowing us to see images.
Retinal detachment = See "amotio retinae".
Retinal receptors = See "photoreceptors".
Retinitis pigmentosa = Inherited degenerative eye disease that involves the loss of light-sensitive photoreceptor cells and progresses from the peripheral areas towards the centre of the eye.
Rods = Photoreceptor cells in the retina responsible for vision at low light levels.
Scar tissue = Fibrous connective tissue that contains fibres made of proteins called collagens.
Scotoma = Area of lost or depressed vision within the visual field.
Senile cataract = Age-related cataract that progresses with age.
Silica lenses = Mineral lens made of silica (= mineral substance)
6/6 = 6/6 means that at six metres test distance, the patient could correctly identify a letter that a "normal" sighted person should see at six metres i.e. "normal" vision.
Spectral components = Part (nm) of the light spectrum.
Spherical lenses = Feature the same refractive power in all meridians.
Stroma = Thickest layer of the cornea representing about 80% of total corneal thickness and consisting of hydrogel (hydrogel = "water with a rubbery consistency").
Subretinal haemorrhage = Bleeding from new blood vessels grown under the retina, resulting in retinal detachment.
Tarsal zone = Dense fibrous tissue stretching from the eyelid margin to the upper lid crease.
Telescopic system = Magnification system mounted on spectacles and worn directly in front of the eye. Telescopic systems can be fitted to one or both eyes.
Tension tolerance = Individual threshold below which the eye can withstand heightened intraocular pressure without suffering damage. However, the risk of sustaining damage increases as the pressure rises higher and the longer it remains high.
Terrestrial = Opposite of astronomical; "terrestrial viewing" refers to viewing objects on earth as opposed to celestial bodies.
Three-mirror lens = Diagnostic device used to examine the iridocorneal angle, which can be too narrow.
Tonometry = Measurement of intraocular pressure.
Total refractive power of the eye = About + 59 D.
Trabecular meshwork = Responsible for draining the aqueous humour from the eye via the anterior chamber.
Traffic signal recognition = Ability to distinguish between red, green and yellow light in average daylight
Trifocal lenses = Multi-power lenses with three points of focus. Most have one zone for distance vision, one for intermediate vision and one for near vision.
Turnover = Complete renewal of the epithelium (every 7 days); see also apoptosis.
20/20 = 20/20 means that at 20 feet test distance, the patient could correctly identify a letter that a "normal" sighted person should see at 20 feet i.e. "normal" vision.
UVA radiation = Ultraviolet radiation ranging from approx. 315 – 380 nm.
UVB radiation = Ultraviolet radiation ranging from approx. 280 – 315 nm.
UV blocker = Lenses that absorb high-energy light rays.
UVC radiation = Ultraviolet radiation ranging from approx. 200 – 280 nm.
Uvea = Middle layer of the eye, consisting of iris, ciliary body and choroid.
Video magnifier = Video magnifiers are electronic devices that help enlarge vision. They capture texts and images with a camera and directly enlarge them on a monitor or screen.
Visual field loss = See "scotoma".
Vitreous humour = The vitreous humour is normally a transparent substance that occupies the vitreous cavity. It is the largest structure of the eye in terms of volume.
Virtual image = Light-reflecting object that cannot be projected onto a screen.
Visual acuity = Commonly refers to the clarity of vision.
Water layer = Also known as the aqueous layer; the thickest layer of the tear film.
Wavelength = Light is described as a wave that consists of different wavelengths.
Wet macular degeneration = The wet variant of age-related macular degeneration promotes the production of multiple, porous new blood vessels underneath the retina. This is also known as “choroidal neovascularisation”. These new blood vessels tend to leak (subretinal haemorrhage), causing retinal detachment.
To follow
Yellow spot = See "macula lutea".
Zeaxanthin = Pigment.
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