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Eye examination
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Eye examination
Medical diagnostics
Traditional Snellen chart used for visual acuity testing
MedlinePlus
003434
LOINC
29271-4
[edit on Wikidata]
An eye examination is a series of tests performed by an ophthalmologist (medical doctor), optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes. 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.
Eye examinations may detect potentially treatable blinding eye diseases, ocular manifestations of systemic disease, or signs of tumours or other anomalies of the brain.
Ideally, the eye examination consists of an external examination, followed by specific tests for visual acuity, pupil function, extraocular muscle motility, visual fields, intraocular pressure and ophthalmoscopy through a dilated pupil.
A minimal eye examination consists of tests for visual acuity, pupil function, and extraocular muscle motility, as well as direct ophthalmoscopy through an undilated pupil.
Contents
[hide]
1 Basic examination
1.1 Visual acuity
1.2 Refraction
1.2.1 Objective refraction
1.2.2 Subjective refraction
1.2.3 Cycloplegic refraction
1.3 Pupil function
1.4 Ocular motility
1.5 Visual field (confrontation) testing
1.6 External examination
1.7 Slit-lamp
1.8 Intraocular pressure
1.9 Retinal examination
2 Eye exams for children
3 Conditions diagnosed during eye examinations
4 Specialized eye examinations
5 See also
6 References
7 External links
Basic examination[edit]
Visual acuity[edit]
Main article: Visual acuity
Visual acuity is the eye's ability to detect fine details and is the quantitative measure of the eye's ability to see an in-focus image at a certain distance. The standard definition of normal visual acuity (20/20 or 6/6 vision) is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc. The terms 20/20 and 6/6 are derived from standardized sized objects that can be seen by a "person of normal vision" at the specified distance. For example, if one can see at a distance of 20 ft an object that normally can be seen at 20 ft, then one has 20/20 vision. If one can see at 20 ft what a normal person can see at 40 ft, then one has 20/40 vision. Put another way, suppose you have trouble seeing objects at a distance and you can only see out to 20 ft what a person with normal vision can see out to 200 feet, then you have 20/200 vision. The 6/6 terminology is more commonly used in Europe and Australia, and represents the distance in metres.
This is often measured with a Snellen chart or with a logMAR base Velorum Visual Acuity System.
Refraction[edit]
In physics, "refraction" is the mechanism that bends the path of light through the eye. In an eye exam, the term refraction is the determination of the ideal correction of refractive error. Refractive error is an optical abnormality in which the shape of the eye fails to bring light into sharp focus on the retina, resulting in blurred or distorted vision. Examples of refractive error are myopia, hyperopia, and astigmatism. A refraction procedure consists of two parts: objective and subjective.
Objective refraction[edit]
An objective refraction is a refraction obtained without receiving any feedback from the patient, using a retinoscope or auto-refractor.
To perform a retinoscopy, the doctor projects a streak of light into a pupil. A series of lenses are flashed in front of the eye. By looking through the retinoscope, the doctor can study the light reflex of the pupil. Based on the movement and orientation of this retinal reflection, the refractive state of the eye is measured.
An auto-refractor is a computerized instrument that shines light into an eye. The light travels through the front of the eye, to the back and then forward through the front again. The information bounced back to the instrument gives an objective measurement of refractive error without asking the patients any questions.
Subjective refraction[edit]
A subjective refraction requires responses from the patient. Typically, the patient will sit behind a phoropter or wear a trial frame and look at an eye chart. The eye care professional will change lenses and other settings while asking the patient for feedback on which set of lenses give the best vision.
Researchers at the MIT Media Laboratory have proposed an eyepiece and software for use with a smartphone as a cheaper alternative to traditional subjective refraction equipment. The system is called NETRA ("near-eye tool for refractive assessment"). A patent was filed for the system in 2010, and the primary audience for the device was projected to be the developing world where access to eye care professionals is limited. The system uses an array of tiny lenses and a grid of pinholes that require the observer to visualize depth in what is otherwise a 2-dimensional surface (the smartphone screen). Rather than determining which of two images looks clearer, patients need only line up an array of dots and lines until they overlap. The degree of adjustment the patent uses on the handheld optical device to accomplish these tasks becomes a measure of visual acuity which can then be translated into an eyeglass prescription. The binocular auto-refractor measures sphere, cylinder, axis and pupillary distance. EyeNetra, the company marketing the devices, expects to bring them to market in February 2016 at a price of just over US$1,000 per device.[1]
Cycloplegic refraction[edit]
Sometimes, eye care professionals prefer to obtain a cycloplegic refraction, especially when trying to obtain an accurate refraction in young children who may skew refraction measurements by adjusting their eyes with accommodation. Cycloplegic eye drops are applied to the eye to temporarily paralyze the ciliary muscle of the eye.
Pupil function[edit]
Main article: Pupil
Medical student giving eye exam at ITESM-Campus Ciudad de México.
An examination of pupilary function includes inspecting the pupils for equal size (1 mm or less of difference may be normal), regular shape, reactivity to light, and direct and consensual accommodation. These steps can be easily remembered with the mnemonic PERRLA (D+C): Pupils Equal and Round; Reactive to Light and Accommodation (Direct and Consensual).
A swinging-flashlight test may also be desirable if neurologic damage is suspected. The swinging-flashlight test is the most useful clinical test available to a general physician for the assessment of optic nerve anomalies. This test detects the afferent pupil defect, also referred to as the Marcus Gunn pupil. It is conducted in a semidarkened room. In a normal reaction to the swinging-flashlight test, both pupils constrict when one is exposed to light. As the light is being moved from one eye to another, both eyes begin to dilate, but constrict again when light has reached the other eye.
If there is an efferent defect in the left eye, the left pupil will remain dilated regardless of where the light is shining, while the right pupil will respond normally. If there is an afferent defect in the left eye, both pupils will dilate when the light is shining on the left eye, but both will constrict when it is shining on the right eye. This is because the left eye will not respond to external stimulus (afferent pathway), but can still receive neural signals from the brain (efferent pathway) to constrict.
If there is a unilateral small pupil with normal reactivity to light, it is unlikely that a neuropathy is present. However, if accompanied by ptosis of the upper eyelid, this may indicate Horner's syndrome.
If there is a small, irregular pupil that constricts poorly to light, but normally to accommodation, this is an Argyll Robertson pupil.
Ocular motility[edit]
Main article: Extraocular muscles
Ocular motility should always be tested, especially when patients complain of double vision or physicians suspect neurologic disease. First, the doctor should visually assess the eyes for deviations that could result from strabismus, extraocular muscle dysfunction, or palsy of the cranial nerves innervating the extraocular muscles. Saccades are assessed by having the patient move his or her eye quickly to a target at the far right, left, top and bottom. This tests for saccadic dysfunction whereupon poor ability of the eyes to "jump" from one place to another may impinge on reading ability and other skills, whereby the eyes are required to fixate and follow a desired object.
The patient is asked to follow a target with both eyes as it is moved in each of the nine cardinal directions of gaze. The examiner notes the speed, smoothness, range and symmetry of movements and observes for unsteadiness of fixation. These nine fields of gaze test the extraocular muscles: inferior, superior, lateral and medial rectus muscles, as well as the superior and inferior oblique muscles.
Visual field (confrontation) testing[edit]
Main articles: Visual field and Visual field test
Testing the visual fields consists of confrontation field testing in which each eye is tested separately to assess the extent of the peripheral field.
To perform the test, the individual occludes one eye while fixated on the examiner's eye with the non-occluded eye. The patient is then asked to count the number of fingers that are briefly flashed in each of the four quadrants. This method is preferred to the wiggly finger test that was historically used because it represents a rapid and efficient way of answering the same question: is the peripheral visual field affected?
Common problems of the visual field include scotoma (area of reduced vision), hemianopia (half of visual field lost), homonymous hemianopsia and bitemporal hemianopia.
External examination[edit]
External examination of eyes consists of inspection of the eyelids, surrounding tissues and palpebral fissure. Palpation of the orbital rim may also be desirable, depending on the presenting signs and symptoms. The conjunctiva and sclera can be inspected by having the individual look up, and shining a light while retracting the upper or lower eyelid. The position of the eyelids are checked for abnormalities such as ptosis which is an asymmetry between eyelid positions.
Slit-lamp[edit]
Slit lamp examination of the eyes in an ophthalmology clinic
Close inspection of the anterior eye structures and ocular adnexa are often done with a slit lamp which is a table mounted microscope with a special adjustable illumination source attached. A small beam of light that can be varied in width, height, incident angle, orientation and colour, is passed over the eye. Often, this light beam is narrowed into a vertical "slit", during slit-lamp examination. The examiner views the illuminated ocular structures, through an optical system that magnifies the image of the eye and the patient is seated while being examined, and the head stabilized by an adjustable chin rest.
This allows inspection of all the ocular media, from cornea to vitreous, plus magnified view of eyelids, and other external ocular related structures. Fluorescein staining before slit lamp examination may reveal corneal abrasions or herpes simplex infection.
The binocular slit-lamp examination provides stereoscopic magnified view of the eye structures in striking detail, enabling exact anatomical diagnoses to be made for a variety of eye conditions.
Also ophthalmoscopy and gonioscopy examinations can also be performed through the slit lamp when combined with special lenses. These lenses include the Goldmann 3-mirror lens, gonioscopy single-mirror/ Zeiss 4-mirror lens for (ocular) anterior chamber angle structures and +90D lens, +78D lens, +66D lens & Hruby (-56D) lens, the examination of retinal structures is accomplished.
Intraocular pressure[edit]
Intraocular pressure (IOP) can be measured by Tonometry devices. The eye can be thought of as an enclosed compartment through which there is a constant circulation of fluid that maintains its shape and internal pressure. Tonometry is a method of measuring this pressure using various instruments. The normal range is 10-21 mmHg.
Retinal examination[edit]
Fully dilated pupil prior to ophthalmoscopic examination
Examination of retina (fundus examination) is an important part of the general eye examination. Dilating the pupil using special eye drops greatly enhances the view and permits an extensive examination of peripheral retina. A limited view can be obtained through an undilated pupil, in which case best results are obtained with the room darkened and the patient looking towards the far corner. The appearance of the optic disc and retinal vasculature are also recorded during fundus examination.
A red reflex can be seen when looking at a patient's pupil through a direct ophthalmoscope. This part of the examination is done from a distance of about 50 cm and is usually symmetrical between the two eyes. An opacity may indicate a cataract.
Using a phoropter to determine a prescription for eyeglasses
Eye exams for children[edit]
See also: pediatric ophthalmology
Children should have their first eye exam at 6 months old. If a parent suspects something is wrong an ophthalmologist can check even earlier.
Early eye exams are important because children need the following basic visual skills for learning:
Near vision
Distance vision: Tumbling E chart, Landolt C chart
Eye teaming (binocularity)
Eye movement
Accommodation (focusing skills)
Peripheral vision
Eye–hand coordination
Conditions diagnosed during eye examinations[edit]
Main article: List of eye diseases and disorders
Myopia
Hyperopia
Presbyopia
Amblyopia
Diplopia
Strabismus
Specialized eye examinations[edit]
Color vision
Stereopsis
Near point of convergence
Keratometry
Cycloplegic refraction
Accommodative system
Amplitude of accommodation
Negative relative accommodation
Positive relative accommodation
Vergence system
Optokinetic system
Amsler grid
Gonioscopy
Corneal topography
Corneal pachymetry
Scheimpflug ocular imaging
Retinal tomography
Ocular computed tomography
Scanning laser polarimetry
Electrooculography
Electroretinography
Ultrasound biomicroscopy
See also[edit]
Binocular vision
Eyeglass prescription
Orthoptics
Retinoscopy
Stereopsis
Subjective Refraction
Vergence
References[edit]
Jump up ^ David L. Chandler (23 June 2010). "In the World: Easy on the eyes".
Jump up ^ Emmett T. Cunningham; Paul Riordan-Eva. Vaughan & Asbury's general ophthalmology (18th ed.). McGraw-Hill Medical. ISBN 978-0071634205.
External links[edit]
Wikimedia Commons has media related to Eye examinations.
A simulator for eye movements and pupil function tests
eMedicine article on Neuro-ophthalmic examination
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Electric power
From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
Electric power is transmitted on overhead lines like these, and also on underground high-voltage cables.
Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.
Electric power is usually produced by electric generators, but can also be supplied by sources such as electric batteries. It is usually supplied to businesses and homes by the electric power industry through an electric power grid. Electric power is usually sold by the kilowatt hour (3.6 MJ) which is the product of power in kilowatts multiplied by running time in hours. Electric utilities measure power using an electricity meter, which keeps a running total of the electric energy delivered to a customer.
Electrical power provides a low entropy form of energy and can be carried long distances and converted into other forms of energy such as motion, light or heat with high energy efficiency.[1]
Contents
[hide]
1 Definition
2 Explanation
2.1 Passive sign convention
2.2 Resistive circuits
2.3 Alternating current
2.4 Electromagnetic fields
3 Generation
4 Electric power industry
5 Use
6 See also
7 Notes
8 References
9 External links
Definition[edit]
Electric power, like mechanical power, is the rate of doing work, measured in watts, and represented by the letter P. The term wattage is used colloquially to mean "electric power in watts." The electric power in watts produced by an electric current I consisting of a charge of Q coulombs every t seconds passing through an electric potential (voltage) difference of V is
P = work done per unit time = V Q t = V I {\displaystyle P={\text{work done per unit time}}={\frac {VQ}{t}}=VI\,}
where
Q is electric charge in coulombs
t is time in seconds
I is electric current in amperes
V is electric potential or voltage in volts
Explanation[edit]
Animation showing electric load
Electric power is transformed to other forms of energy when electric charges move through an electric potential (voltage) difference, which occurs in electrical components in electric circuits. From the standpoint of electric power, components in an electric circuit can be divided into two categories:
Passive devices or loads: When electric charges move through a potential difference from a higher to a lower voltage, that is when conventional current (positive charge) moves from the positive (+) terminal to the negative (−) terminal, work is done by the charges on the device. The potential energy of the charges due to the voltage between the terminals is converted to kinetic energy in the device. These devices are called passive components or loads; they 'consume' electric power from the circuit, converting it to other forms of energy such as mechanical work, heat, light, etc. Examples are electrical appliances, such as light bulbs, electric motors, and electric heaters. In alternating current (AC) circuits the direction of the voltage periodically reverses, but the current always flows from the higher potential to the lower potential side.
Animation showing power source
Active devices or power sources: If the charges are moved by an 'exterior force' through the device in the direction from the lower electric potential to the higher, (so positive charge moves from the negative to the positive terminal), work will be done on the charges, and energy is being converted to electric potential energy from some other type of energy, such as mechanical energy or chemical energy. Devices in which this occurs are called active devices or power sources; such as electric generators and batteries.
Some devices can be either a source or a load, depending on the voltage and current through them. For example, a rechargeable battery acts as a source when it provides power to a circuit, but as a load when it is connected to a battery charger and is being recharged, or a generator as a power source and a motor as a load.
Passive sign convention[edit]
Main article: Passive sign convention
Since electric power can flow either into or out of a component, a convention is needed for which direction represents positive power flow. Electric power flowing out of a circuit into a component is arbitrarily defined to have a positive sign, while power flowing into a circuit from a component is defined to have a negative sign. Thus passive components have positive power consumption, while power sources have negative power consumption. This is called the passive sign convention.
Resistive circuits[edit]
In the case of resistive (Ohmic, or linear) loads, Joule's law can be combined with Ohm's law (V = I·R) to produce alternative expressions for the amount of power that is dissipated:
P = I V = I 2 R = V 2 R , {\displaystyle P=IV=I^{2}R={\frac {V^{2}}{R}},}
where R is the electrical resistance.
Alternating current[edit]
Main article: AC power
In alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow. The portion of power flow that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as real power (also referred to as active power). That portion of power flow due to stored energy, that returns to the source in each cycle, is known as reactive power. The real power P in watts consumed by a device is given by
P = 1 2 V p I p cos  θ = V r m s I r m s cos  θ {\displaystyle P={1 \over 2}V_{p}I_{p}\cos \theta =V_{rms}I_{rms}\cos \theta \,}
where
Vp is the peak voltage in volts
Ip is the peak current in amperes
Vrms is the root-mean-square voltage in volts
Irms is the root-mean-square current in amperes
θ is the phase angle between the current and voltage sine waves
Power triangle: The components of AC power
The relationship between real power, reactive power and apparent power can be expressed by representing the quantities as vectors. Real power is represented as a horizontal vector and reactive power is represented as a vertical vector. The apparent power vector is the hypotenuse of a right triangle formed by connecting the real and reactive power vectors. This representation is often called the power triangle. Using the Pythagorean Theorem, the relationship among real, reactive and apparent power is:
(apparent power) 2 = (real power) 2 + (reactive power) 2 {\displaystyle {\mbox{(apparent power)}}^{2}={\mbox{(real power)}}^{2}+{\mbox{(reactive power)}}^{2}}
Real and reactive powers can also be calculated directly from the apparent power, when the current and voltage are both sinusoids with a known phase angle θ between them:
(real power) = (apparent power) cos  θ {\displaystyle {\mbox{(real power)}}={\mbox{(apparent power)}}\cos \theta }
(reactive power) = (apparent power) sin  θ {\displaystyle {\mbox{(reactive power)}}={\mbox{(apparent power)}}\sin \theta }
The ratio of real power to apparent power is called power factor and is a number always between 0 and 1. Where the currents and voltages have non-sinusoidal forms, power factor is generalized to include the effects of distortion
Electromagnetic fields[edit]
This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2012) (Learn how and when to remove this template message)
Electrical energy flows wherever electric and magnetic fields exist together and fluctuate in the same place. The simplest example of this is in electrical circuits, as the preceding section showed. In the general case, however, the simple equation P = IV must be replaced by a more complex calculation, the integral of the cross-product of the electrical and magnetic field vectors over a specified area, thus:
P = ∫ S ( E × H ) ⋅ d A . {\displaystyle P=\int _{S}(\mathbf {E} \times \mathbf {H} )\cdot \mathbf {dA} .\,}
The result is a scalar since it is the surface integral of the Poynting vector.
Generation[edit]
Main article: electricity generation
The fundamental principles of much electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday. His basic method is still used today: electricity is generated by the movement of a loop of wire, or disc of copper between the poles of a magnet.
For electric utilities, it is the first process in the delivery of electricity to consumers. The other processes, electricity transmission, distribution, and electrical power storage and recovery using pumped-storage methods are normally carried out by the electric power industry.
Electricity is mostly generated at a power station by electromechanical generators, driven by heat engines heated by combustion, geothermal power or nuclear fission. Other generators are driven by the kinetic energy of flowing water and wind. There are many other technologies that are used to generate electricity such as photovoltaic solar panels.
A battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy.[2] Since the invention of the first battery (or "voltaic pile") in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power source for many household and industrial applications. According to a 2005 estimate, the worldwide battery industry generates US$48 billion in sales each year,[3] with 6% annual growth. There are two types of batteries: primary batteries (disposable batteries), which are designed to be used once and discarded, and secondary batteries (rechargeable batteries), which are designed to be recharged and used multiple times. Batteries come in many sizes, from miniature cells used to power hearing aids and wristwatches to battery banks the size of rooms that provide standby power for telephone exchanges and computer data centers.
Electric power industry[edit]
Main article: electric power industry
The electric power industry provides the production and delivery of power, in sufficient quantities to areas that need electricity, through a grid connection. The grid distributes electrical energy to customers. Electric power is generated by central power stations or by distributed generation. The electric power industry has gradually been trending towards deregulation - with emerging players offering consumers competition to the traditional public utility companies.[4]
Use[edit]
Electric power, produced from central generating stations and distributed over an electrical transmission grid, is widely used in industrial, commercial and consumer applications. The per capita electric power consumption of a country correlates with its industrial development. [5] Electric motors power manufacturing machinery and propel subways and railway trains. Electric lighting is the most important form of artificial light. Electrical energy is used directly in processes such as extraction of aluminum from its ores and in production of steel in electric arc furnaces. Reliable electric power is essential to telecommunications and broadcasting. Electric power is used to provide air conditioning in hot climates, and in some places electric power is an economically competitive source of energy for building space heating. Use of electric power for pumping water ranges from individual household wells to irrigation projects and energy storage projects.
See also[edit]
Energy portal
EGRID
Electric energy consumption
Electric power system
High-voltage cable
Power engineering
Power station
Rural electrification
Notes[edit]
Jump up ^ Smith, Clare (2001). Environmental physics. London, United Kingdom: Routledge. ISBN 0-415-20191-8.
Jump up ^ "battery" (def. 4b), Merriam-Webster Online Dictionary (2009). Retrieved 25 May 2009.
Jump up ^ Power Shift: DFJ on the lookout for more power source investments Archived 2005-12-01 at the Wayback Machine.. Draper Fisher Jurvetson. Retrieved 20 November 2005.
Jump up ^ The Opportunity of Energy Group-Buying EnPowered, April 18, 2016,
Jump up ^ Ignacio J. Pérez-Arriaga (ed), Regulation of the Power Sector, Springer Science & Business Media, 2014 ISBN 1447150341, page 8
References[edit]
Reports on August 2003 Blackout, North American Electric Reliability Council website
Croft, Terrell; Summers, Wilford I. (1987). American Electricians' Handbook (Eleventh ed.). New York: McGraw Hill. ISBN 0-07-013932-6.
Fink, Donald G.; Beaty, H. Wayne (1978). Standard Handbook for Electrical Engineers (Eleventh ed.). New York: McGraw Hill. ISBN 0-07-020974-X.
External links[edit]
Wikimedia Commons has media related to Electricity.
U.S. Department of Energy: Electric Power
GlobTek, Inc. Glossary of Electric power Power Supply Terms
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