Pupil: General information
The IRIS Pupil
The pupil of the iris is the optical aperture of the eye. This opening central and circular edge delimits a disc through which is the path of the light rays refracted by the cornea to the retina destination. The pupil appears as a black disk, diameter variable, generally concentric to the cornea. Changes in the diameter of the pupil IRIS explain the pupil plays the role of a natural optical diaphragm, allowing to control the flow of light incident and affecting the optical properties of the eye (retinal image quality changes).
The color of the iris varies between individuals and gives the "color" of the eyes. No weakly pigmented irises are blue, strongly pigmented irises are brown. Moderately pigmented irises appear gray or green. Remember that the color "blue" is not related to a mechanism for selective absorption of the colourful radiation, but to selective diffraction of short (seen as blue) wavelengths by irien tissue (this mechanism also explains the blue color of the sky).
The size of the IRIS Pupil is under the influence of two muscles whose effects are opposed (antagonist muscles): the pupillary sphincter, and the pupillary dilator. The pupillary sphincter is an annular smooth muscle located around the edge of the IRIS ward. It is innervated by fibers from the 3e cranial nerve (nerve oculo-motor): these fibers make relay in the ciliary ganglion.
The pupillary dilator is constituted by radiaires fibers extending from the pupillary edge to the ciliary body. It is innervated by branches of the sympathetic nerve, making relay in the superior cervical ganglion. The contraction of this muscle cause pupillary dilation.
The edge of the pupil from the front is generally circular: some conditions may change this aspect. The persistence of a coalescence between the iris and the lens (synechiae) for example causes a distortion of the shape of the pupil. There are various forms of pupil in the animal world (see animal wards), also the circular shape of the human eye to is not universal!
Many ophthalmological neuro conditions induce an anomaly of constriction and dilation of the pupil. This page is especially important in physiological optics, and refractive surgery functional aspects.
Pupil of entry, exit pupil
The pupil is usually seen through the cornea; It corresponds to the entrance pupil, which is the image formed by the "anatomical" pupil's cornea This image is earlier and slightly magnified (13%) - see calculation of the diameter of the pupil of the formula of vergence entry. The exit pupil is the image of the anatomical pupil seen through the lens (magnification: 3%).
During accommodation, the IRIS pupil suffered a slight anterior displacement, under the effect of the curvature of the lens: about 0.4 mm.
The IRIS pupil of entry is useful to the definition of the pupillary axis: it connects the center of the pupil with a point on the surface of the cornea, and for which the tangent plane to the cornea and passing by this point is perpendicular to this axis. THEpupillary axis can be identified by setting the subject a point light source, until the image of the reflection of it located in the center of the pupil. The line connecting the source to the center of the pupil then sets the pupil axis.
Position of the pupil
The absence of centering on the optical axis of the eye is one of the peculiarities of the human pupils. On average, the offset of the pupil to the optical axis is 0.5 mm. The limbic circle, or the corneal vertex are markers to locate the center of the IRIS ward. The center of the pupil is located in temporal of the vertex for the vast majority of human eyes (in other words, the vertex is projected in the nasal portion of the pupil). The position of the pupillary Center can vary depending on the size and movements of the pupil; the amplitude of these movements can reach a few hundred microns. The dynamic pupillometry allows to spot the movements of the pupil, which are saved to a fixed reference (: limbe sclero-corneal, or corneal vertex).
Size of the pupil
The diameter of the pupil varies between 2 mm (maximum constriction: myosis) and 8 mm (maximum dilation: Mydriasis). This corresponds to a change of the pupillary area by a factor of 16.
The diameter of the pupil is under the influence of various factors:
-The degree of illumination
This factor is certainly the one that determines the most the pupillary disk size. For a same illumination (the observed scene luminance), pupillary diameter varies significantly from one topic to another. When the luminance increases, the pupillary diameter is reduced, under the effect of a reflex mechanism that involves the cones and rods of the retina. There is a latency in the order of 0.2 to 0.5 seconds before the pupil diameter is reduced under the effect of the illumination. If the luminance decreases, the increase in pupillary diameter is more slowly than the constriction.
When we light the pupil of an eye, the direct reflex constriction is also interested in the pupil of the other eye: we're talking about consensual pupillary reflex.
The study of the diameter of the pupil (pupillometry) in the dark is dependent on the light conditions:
-scotopic when only the sticks are active (there not enough light to activate the cones): this vision is in fact grayscale (no colorful sensation) and can be characterized by a threshold of less than 0.05 illuminance lux
-mesopic: illuminance is between 0.05 and 50 lux, which allows a joint activity of the rods and cones.
The measurements can be made by different instruments. The Colvard pupillometer is an instrument designed to measure the diameter of the pupil through a system of direct visualization and a landmark graduated in millimetres. Measures performed by the aberrometres or topographer (OPDscan, Topolyzer Vario) seem to be more accurate and reproducible. They are heard, well, affected by the used lighting conditions in the room where the measurements are made.
Average values reported for the pupillary diameter in low light condition are generally understood between 4.7 and 5 mm in conditions mesopiques and 5.8 to 7mm in scotopic condition: it is difficult to establish 'standards' values, because measures are influenced by the technique of pupillometry, as well as the light conditions during the measurements.
-Accommodation and binocular vision
The accommodation of the eye to see a close target clearly induced a reflex, pupillary constriction; This reduction of the pupillary diameter promotes the vision closely (increase in depth of field).
The pupillary diameter tends to decrease with age, as well as pupillary reactivity (Kadlhekova 1958 p227). If the pupil can dilate up to 8 mm in teenagers or young adults, it struggles to cross the diameter of 5 mm in their eighties subjects. The permanent reduction of the pupillary diameter to 1.5 or 2 mm observed in some elderly is called "senile myosis.
Products that produce a dilation of the pupil are called "mydriatics. They may act by stimulating the sympathetic system, or by blocking the system friendly para. The dilation of the pupil is required to examine the fundus and the periphery of the retina. It is also required for certain surgeries including cataract. These agents also cause a blockage of the accommodation; This property is used for a measurement of refraction known as 'under cycloplegics", which is important in certain clinical circumstances (review of the refraction of the child, balance sheet prior to refractive surgery). Atropine blocks acetylcholine receptors, and induced (Mydriasis) dilation of the pupil.
Conversely, some agents can induce contraction of the pupils and are called "myotiques" (ex: pilocarpine).
-Psychological and emotional factors.
Joy, surprise, fear or pain serve to dilate the pupil. The faces of women are judged more attractive when their pupils are dilated)Bull and Shead, Pupil dilation, sex of stimulus, and age and sex of observe. Perceptual and Motor Skills, 1979,49 (1); 27-30). Mental math and concentration cause dilatation (pupillary)Hess and Polt, Pupil size as related to interest value of visual stimuli. Science, 1960; 132 (3423): 349-350)
Effects of pupillary diameter.
Humans and primates have a circular pupil of variable diameter. There are forms of different wards in the animal kingdoms: ovals, in slot, or even more complex form.
For optimal optical quality, the pupillary diameter of the eye should be between 2 and 3 mm. In below the retinal image quality at risk related to diffraction degradation. Beyond that, the effect of aberrations of high degree can also reduce the quality of vision.
When the eye has a defective vision (ex: myopia), the size of the pupil also influences the perceived image sharpness. Low myopia (ex:-0.50 D) affect visual acuity when the diameter of the pupil increases: this is mainly related to increased sprawl of the task of defocus in the plan of the retina.
The reduction of the pupillary diameter increases the depth of field, because it decreases the diameter of the retinal blur targets of share and others from the focal plane (and so unfocused). Them animal wards may be asymmetric, i.e. narrower in some directions than others. The vision is favored in directions perpendicular to the narrowest of the pupil.
Pupillary response to light
Contrary to belief, variations in the diameter of the pupil cannot ensure a luminous flux of constant intensity to the retina. Indeed, changes in pupillary surface (slip surface) allow to vary the luminous flux by a factor of 16 (for a variation of 2 to 8 mm radius, since the surface is proportional to the square of the RADIUS). In comparison, changes in luminance to which the human eye is exposed vary in a proportion of 100000, of Moonlight (0,01 cd/m2) on a day sunny (1000 cd/m2). Of course, the ambient light has an effect on the diameter of the pupil, but some work suggests that variations in the diameter of the pupil are essentially intended to optimize visual acuity based on Visual acuity.
Pupil and refractive surgery
The role of the pupil to the risk of undesirable visual symptoms (halos, glare, etc.) is controversial.
During the development of the techniques of photoablation contemporary abandonment of the technique of radiaire keratotomii (PKR) excimer laser, some publications appeared only accredit a discordance between the diameter of the optical box set and the pupil could result in the occurrence of undesirable visual phenomena. At that time (early 90s), optical areas were equipped with often less than 5 mm in diameter. With the progress made in laser delivery systems, programmed optical areas have been expanded, and work has been done in improving the design of transitional areas.
The need to program an optical area at least as wide as the measured pupil in terms mesopiques (optical area is where the cornea is sculpted to correct the optical defect of the eye) is an a priori logic to avoid a too important part of incident light and collected by the pupil not be refracted through an area where the curvature would be inappropriate (fitting with the untreated area see area untreated itself).
However, there are some effects which weigh this concept, as the effect of Stiles-Crawford.
Effect of Stiles-Crawford
This phenomenon is based on a physiological optical model for which the retinal photoreceptors act individually as small "fiber": this model is based on the appearance of the outer segment of the photoreceptors, whose length can reach some tens of microns. The probability of capture of incident photons is higher if the angle of incidence is lower (and maximum if the photons have a route parallel to the axis of photoreceptors). Beyond a certain angle of incidence, the probability of capture is dwindling. Thus, light rays (which represent the path of photons) are most effective when they are refracted by the center of the optical area by the periphery of it. However showed that the rods are extremely sensitive to light (a stick can react to stimulation of a single photon is a quantum of energy Basic!). However, the phenomenon of Stiles Crawford has been verified experimentally (Baron WS, Munnerlyn C. Predicting visual performance following excimer also keratectomy. Refract Corneal Surg, 1992; 8:355 - 362), and modelled as a "filter apodisation", weighting favorably the rays refracted to the center of the pupil towards those refracted by its periphery
Data in the medical literature
A meta analysis (comprehensive analysis of all scientific articles on the same topic in a given period) was published recently)Myung D, Schallhorn S, handle EE. Pupil Size and LASIK: A Review. J Refract Surg. 2013 1; 29 (11): 734 - 41). It establishes that between 2002 and 2013, 12 publications have established clearly thata large diameter of pupil in terms mesopiques or photopiques was not a risk factor for the occurrence of halos. One study compared eyes operated on for a programmed optical area greater than their diameter pupillary mesopic to those who had a lower area (a priori more at risk); It has not been highlighted of difference between these groups)Villa and al. Night vision disturbances after successful LASIK surgery. BR J invest, 2007; 91:1031 - 1037). The degree of correction performed by contrast was a predictor of halos. Similarly, the realization of a LASIK with use of the femtosecond laser and treatment personalized laser reduces the risk of halos to LASIK with microkeratome and conventional treatment)Schallhorn SC et al. Comparison of night driving performance after wavefront-guided and conventional LASIK for moderate myopia. Ophthalmology, 2009; 116:702 - 703).
A single study has found a link between pupil diameter and risk of halos: the optical area used for the correction of the patients included in this study was of 5.5 or 6 mm)Hegelsen has et al. Pupil size and night vision disturbances after LASIK for myopia. ACTA 2004; invest Scand, 82:454 - 460). Another study showed that the unwanted visual symptoms after LASIK can be reduced by the instillation of aceclidine (agent myoptique, which reduces the diameter of the pupil by an average of 2.5 mm) (Randazzo et al. Pharmacological management of night vision disturbances after refractive surgery: results of a randomized clinical trial. J Cataract Refract Surg 2005; 31:1764 - 1772). This is not surprising (reduction of optical aberrations of high degree), and only shows that the reduction of the diameter of the pupil can be useful to reduce the perception of halos or other undesirable visual phenomena (but not that wide pupillary diameter more exposed to these phenomena).
In conclusion, it appears with photoablation techniques used for modern LASIK, and superior optical areas or equal to 6 mm, there is no correlation between the risk of halos and the diameter of the pupil in mesopiques conditions. However, other factors are probably at fault: the quality of balance, of the interface, reducing or maintaining a rate of optical aberrations of high physiological level.