Wednesday, 6 May 2015

What is Spherical Aberration?

October 5, 2011 By Nasim Mansurov18 Comments

Spherical Aberration is an optical problem that occurs when all incoming light rays end up focusing at different points after passing through a spherical surface. Light rays passing through a lens near its horizontal axis are refracted less than rays closer to the edge or “periphery” of the lens and as a result, end up in different spots across the optical axis. In other words, the parallel light rays of incoming light do not converge at the same point after passing through the lens. Because of this, Spherical Aberration can affect resolution and clarity, making it hard to obtain sharp images. Here is an illustration that shows Spherical Aberration:

As shown above, light rays refract or change their angle when passing through the lens. The ones closer to the top and the bottom of the illustration end up converging at a shorter distance along the optical axis (black/red dotted line), while the ones closer to the optical axis converge at a longer distance, creating different focal points along the same axis. The point of best focus with the “circle of least confusion” is illustrated as the thick green line. Spherical Aberration is not just caused by lens design, but also by the quality of the lens material. Lenses made of poor quality material and large bubbles can drastically impact light refraction.
A perfect lens would have all light rays converge in a single focal point, as illustrated below:

The best focus point with the circle of least confusion is therefore located right on this focal point. A normal spherical lens design would not allow the above to happen though, so specialized precise methods by manufacturers have been developed over the years to reduce the effect of spherical aberration.

Ways to Reduce Spherical Aberration

Modern lenses employ different techniques to dramatically reduce spherical aberration. One of the methods employs using a specialized apsherical (meaning non-spherical) lens surface, which is curved outwards on one side for the sole purpose of converging light rays into a single focal point, as illustrated below:

Spherical aberration is most pronounced when the diaphragm of the lens is wide open (maximum aperture). Stopping down the lens even by a single stop dramatically reduces spherical aberration, because aperture blades block the outer edges of spherical lenses. A clear example of this can be found in the focus shift article.

centring

Interactive demo: Fitting ZEISS lenses

1. Anatomic Fitting

After the preliminary anatomic fitting of the spectacles, the selected frame should be

fitted to a corneal vertex distance of between 12 and 16 mm and
display a (tilt) pantoscopic angle of between 8° and 12°.

If not specified individually for customised lenses, ZEISS lenses are designed for these parameters. These recommended values cannot always be observed due to special anatomic conditions or to an unusual frame shape. The optician should then take this into account in refraction and the lens order.

2. Lateral Centration

Lateral centration of ZEISS lenses without prismatic power

Lateral Centration for ZEISS Lenses without Prismatic Power

The centration distance should correspond to the interpupillary distance when the patient is looking into the distance (distance PD).
This applies to all ZEISS lenses:

Single vision lenses for distance and near
Officelenses, the proximity lens
Progressive lenses
Bifocal and trifocal lenses it is standard practice to use the segment tops as reference points in lateral centration. These are each marked at a distance of 2.5 mm nasally from the distance PD.

3. Vertical Centration

1. Natural head posture
2. Head posture for marking the vertical centration according to the "centre-of-rotation requirement" (frame plane perpendicular)

Vertical Centration of ZEISS Lenses

Single vision lenses and Office lenses
The "centre of rotation requirement", i. e. the lens is centred when its optical axis runs through the eye’s optical centre of rotation, should be met in the fitting of single vision lenses and office lenses. There are two ways of determining the vertical centration:

The patient raises his head until the frame plane is perpendicular. The pupil centres are then marked, with the patient’s eyes in the zero visual direction (looking straight-ahead).
With the patient’s head and body in a natural posture, the positions of the pupil centres are marked for the zero visual direction (looking straight-ahead).

Learn more

4. ...with Prismatic Power

Centration of ZEISS Lenses with Prismatic Power

In prismatic prescriptions the centration points determined must be displaced by 0.25 mm per 1 cm/m in the opposite direction to the prism base. This takes into account the movement of the eyes in prismatic lenses.

Learn more

5. ...with i.Terminal 2

Centration with i.Terminal 2

The computer-assisted i.Terminal 2 fitting system can be used to determine all centration data with extreme precision using two video images, with the patient’s head and body in a natural posture. The correct position of the centration points is calculated and displayed on the basis of the applicable centration requirement, the dioptric power and the lens type.

Measurement of centering errors, automated adjustment and mounting of lenses

Josef Heinisch, Trioptics GmbH

Classical optical measurement techniques combined with modern PC technology provide accurate alignment, bonding and cementing of optical components and systems. These processes are fully automated and significantly reduce production time and cost.

The processes involved in manufacturing optical lenses – grinding, polishing and coating – have become increasingly automated in the past few decades. Computer numerical controlled grinding and polishing machines, as well as computer-controlled interferometers for the examination of the surface profile, are in widespread use.

However, many lens manufacturers still use simple optical or tactile measurement instruments for determining centering errors. More often than not, the lenses are still adjusted manually. The adjustment and centering errors of the optical components have a decisive influence on the image quality of the lens. The use of electronic autocollimators and automated adjustment equipment makes the mounting process quicker and more precise – for example, when cementing achromatic lenses or bonding lenses into a mount.

Centration error measurement

When measuring a centering error using the reflection mode (Figure 1), an electronic autocollimator, read via a PC, is at the core of the measurement. An illuminated target (usually a reticule) is projected at the center of curvature of the spherical or almost spherical lens surface under investigation. In this instance, the rays of light meet the surface at an almost perpendicular angle. Some of the light is reflected back along the exact path it came (a condition of autocollimation) and displays the target on a CCD camera. A lateral shift of the center of curvature creates a direct lateral shift of the reticule image.

Figure 1. This diagram demonstrates the principle of measuring centering errors with an electronic autocollimator using the reflection mode.

If the surface under investigation is rotated around a reference axis, a corresponding circular movement of the reticule image is created on the CCD sensor. The radius of this circular path is directly proportional to the position of the center of curvature in relation to the reference axis. The live reticule image depicts the exact position of the center of curvature in the X-Y axis, whereby the center of the circular path represents a reference point in the overall space. Powerful light sources and light-sensitive CCD sensors ensure that even test items that are well antireflection-coated produce a sufficient autocollimation image.

In extreme cases, it is also possible to carry out the measurement using near-infrared (750- to 1000-nm) illumination. In this spectral range, the antireflection coatings optimized for the visible range have enough reflectivity. As an alternative to incoherent light sources, laser autocollimators are available, although these can produce images that are difficult to interpret due to speckling.

Centration error measurement of objectives

The effectiveness of measuring centering errors with an electronic autocollimator in combination with computers is apparent when measuring multiple lenses.¹ This method identifies the centering error of each individual lens surface in a mounted optical assembly. Any selected axis of rotation serves as a reference axis. Suitably precise air bearings with radial and axial runout errors of <0.05 μm are available.

First, the centering error of the outermost optical surface is measured in relation to the axis of rotation. The next step is to focus in to the center of curvature of the second optical surface. For the calculation of the position of the center of curvature (Z), the optical properties of the first outermost surface must be taken into account. When evaluating the true centering error (X, Y) of the second surface using optical calculations, it is also necessary to take into consideration both the optical properties and the centering error of the first surface previously measured. This calculation simply requires the design data (radius, center thickness, refractive index) of the item under investigation. When the exact centering error of the second surface has been identified, the centering error of the third surface can be measured, and so on. This measurement process has proved itself to be extremely robust.

Using design data alone is more than suitable for the evaluation, as it is unusual for a significant difference to exist between that data and actual lens measurements. In practice, it is possible to gauge with a single autocollimator the positioning errors of 20 or more surfaces to an exactness of <1 μm. To clarify the effectiveness of this measurement process, Figure 2 shows the measurement results of an optical assembly. The sample assembly consists of two identical achromatic lenses with surface radii in the region of 40 to 120 mm. The multiple-lens measurement provides the exact position of all the centers of curvature in relation to the rotational axis. Since an achromatic system consists of three optical surfaces, the optical axis of a single achromatic lens can be displayed via a regression line through the three centers of curvature. This is indicated by the dotted blue and green lines in Figure 2.

Figure 2. Measurement of the angle and the distance of the optical axes of the lenses to each other.

If the optical axes are known, it is possible to define the angle and distance between both optical axes (here in the plane of the upper vertex). These calculations are automatically executed as part of the measurement process. The results are shown in Figure 2. The multiple-lens measurement of the objective has been carried out four times. After each measurement, the item under investigation was removed from the bearing and, without any particular provisions, replaced for the next (measurement at a 90° azimuth angle). The result is a standard deviation of 0.16 μm in the distance measurement of both optical axes.

If the exact positions (X, Y, Z) of all centers of curvature are known in a fixed system of coordinates, this information may be used to optimize the optical assembly. In the example given here, the upper achromatic lens can be realigned by the previously measured magnitude of 8.8 μm in the direction of the optical axis of the lower lens (green line).

Automated cementing of achromatic lenses

There is a new process for adjusting two single lenses of an achromatic lens.² Based on the multiple lens method, it does away with the precise initial adjustment of both lenses. Instead, the exact position of each of the three centers of curvature is identified in its uncemented state. The axis of rotation of a highly accurate bearing serves as a reference axis for the measurement of the centering error, but not as a reference axis for the adjustment. Hence there is no need for precise, self-centering mechanical holders. Both lenses are fixed to the rotor of the bearing, and their center remains unaltered throughout the measurement process. The cement between the lenses is still fluid at this point.

Figure 3. The multilens measurement method is used for the automated adjustment of achromatic lenses in the cementing process.

Once the centering error has been measured, the center of curvature of the upper sphere is adjusted according to the optical axis of the lower lens. A ring-shaped support, fitted with an X-Y piezo actuator, is placed on the upper lens. It moves the upper lens so that all three centers of curvature eventually lie on the one line, the optical axis (Figure 3). The cement is then hardened using UV light. The entire measurement and adjustment cycle lasts only 10 to 15 seconds. This method is five to 10 times more accurate than the manual process and is particularly effective in the production of small optical components with 1-mm diameters – endoscopes, for example.

Automated bonding of lenses into a mount

When bonding lenses and optical elements into a mount, the optical axes of these elements must be brought into line with the axis of the mount itself. The axis of the mount can be given in terms of the symmetrical axis of a cylindrical mount. An instrument for the precise alignment and bonding is shown in Figure 4.

Figure 4. The frontal view is shown of an automated centering and bonding station for the assembly of lenses into a mount.

Figure 5 is a schematic representation of the basic process. The workpiece (mount with unglued lens) is positioned on the rotor of a rotational axis. The mount’s support has previously been aligned to the axis of rotation to a degree of accuracy of 2 μm. A so-called hydraulic expansion chuck is used as the support, which can transfer the center accuracy almost perfectly to the lens mount.

Before the centering error is measured, a robot arm moves in with a dispenser of UV-curable glue. The glue is applied to the lens and the mount by rotating the test item 360°. The centering error of the upper lens surface is now determined using the autocollimator.

Figure 5. Panels A to D show the step-by-step process of automatically bonding a lens into a mount.

Since the mount and therefore also the ring-shaped face of the lower lens surface are centered perfectly with respect to the axis of rotation, it is sufficient to center the upper surface. To achieve this, a manipulator is introduced on another robot arm. The manipulator may be fitted with either a single-axis piezo actuator (Figure 4) or with three single-axis piezo actuators (Figure 6), which are fixed at 120° from one another on a ring. In the instance where only one actuator is used, the test item must be rotated before the final adjustment in such a way that the axis of rotation, the center of curvature and the piezo axis are all on the same line. If three actuators are used, the lens can be adjusted on the center of rotation without rotating the sample again.

Figure 6. Pictured is an automated bonding station with three piezo actuators.

The accuracy of the adjustment can be increased by taking an additional measurement of the position of the mount’s axis. A linear measuring sensor (resolution 0.1 μm) is placed on the edge of the mount (Figure 7) and takes measurements over a 360° rotation. The results are presented using a sine curve that identifies the X-Y coordinates (centering error) of the mount. Using the piezo actuators, the lens can now be adjusted to the direct center of the mount (green cross in Figure 7).

Once the lens has been successfully adjusted, the glue can be hardened by switching on a UV light source. The advantage of such a process is that the lens can be adjusted with great exactness on the mount, without the mount itself having to be perfectly adjusted to the axis of rotation. The multiple-lens measurement described here, the cementing of achromatic lenses and the gluing of lenses are perfect examples of the advantages of a computer-aided measurement of centering errors.

Figure 7. The center of the mount’s circumference can be identified using an additional measurement sensor. The lens is positioned using three needles (set apart from one another at a 120° angle) and adjusted to the center of the mount.

The previously laborious and therefore expensive form of adjustment has been simplified by intelligent measurement technology. The consistent use of this technology enables a greater degree of accuracy in the production of optical systems and therefore new design opportunities for more compact and higher-quality lenses

Monday, 4 May 2015

Visual acuity, lens and fundus oculi

Transcript

1. VISUAL ACUITY , LENS AND FUNDUS OCULI
2. THE REDUCED OR SCHEMATIC EYE • In the eye, light is actually reflected at the anterior surface of the cornea and at the ant. and post. surfaces of lens. • However, in order to represent the process of reflection in a more simplified way an imaginary concept the reduced or schematic eye has been evolved. • In this concept, the lens is not considered to be present; the ant. surface of the cornea is assumed to possess the whole of the refractive power of the eye with a power of +60 diopters.
3. THE REDUCED OR SCHEMATIC EYE • The whole eye is assumed to have the refractive index of 1.333. the total axial length of the eyeball is about 24mm. • The center of curvature for the imaginary corneal surface having a power of +60 D is 5.5 mm behind the principal plane. • Light will therefore pass undeviated through the center of curvature which is therefore the optical center or the nodal point of the reduced eye.
4. THE VISUAL ACUITY (ACUTNESS OF VISION) • Visual acuity is defined as the shortest distance by which two lines can be separated and still be visualized as two lines. • If two black objects on a white background are separated by a space subtending an angle of less than 1 minute then the two objects are not seen separated but are seen fused with each other. • When the visual angle is increased to 1 minute, the objects are seen separately. Thus 1 minute is the minimum angle for the normal eye.
5. THE VISUALACUITY (ACUTNESS OF VISION) • Clinically visual acuity for distant vision is measured with SNELLEN’S test types. These are a series of letters of varying sizes so constructed that the top letter is visible to the normal eye from 60 meters and the subsequent lines from 36,24,18,9,6 and 5 meters respectively. • The height of each letter, when seen from its corresponding distance, subtends a visual angle of 5 minutes.
6. THE VISUALACUITY (ACUTNESS OF VISION) • Each line in the letter subtends 1 minute of arc. • Visual acuity for near vision is measured with the help of words printed in small type. • In doing the test, the subject is made to sit at distance of 6 meters from the chart. The eyes are tested one by one.
7. THE VISUALACUITY (ACUTNESS OF VISION) • If he can only see the top line, he has a VA of 6/60.This means that the subject can read that letter from 6 meters which a normal eye should be able to read from 60 meters. • If he can read even the smallest line, then his VA is 6/5; this means that the subjects VA is better than the normal average eye which should read the lowest line from 5 meters.
8. THE VISUALACUITY (ACUTNESS OF VISION) • Thus the numerator corresponds to the distance in meters b/w the subject and the chart. This is kept as 6 meters (20ft) as mentioned above. • The denominator corresponds to the distance in meters at which the smallest row of letters read by patient should be read by normal eye. • Thus VA decreases in the order, 6/5,6/6,6/9,6/12,6/18,6/36 and 6/60.
9. LENS • The lens of eyeball is crystalline in nature. It is situated behind the pupil. • It is biconvex, transparent and possesses the elastic property. • The lens is an avascular structure and it receives its nutrition from the aqueous humor. • Lens refracts light rays and helps focus the image of the objects on retina. • The focal length of human lens is 44mm and its refractory power is 23 D. • Lens is supported by suspensory ligaments (zonular fibers) which are attached with ciliary bodies.
10. STRUCTURE OF THE LENS • The lens is formed of three components. 1) The capsule It is highly elastic membrane covering the lens. 2) The anterior epithelium • It is a single layer of cuboidal epithelial cells situated beneath the capsule. • At the margins, the epithelial cells are elongated. • The epithelial cells give rise to the lens fibers present in the lens substance.
11. 3) The lens substance • Lens is formed by long lens fibers derived from the ant. epithelium. • The lens fibers are prismatic in nature and are arranged in concentric layers.
12. CHANGES IN THE LENS DURING OLD AGE • After 40 t0 45 years, the lens looses its elastic property. So, the amplitude of accommodation is decreased. • The person cannot see the near objects clearly. This condition is known as presbyopia. • In old age the lens becomes opaque and this condition is called cataract.
13. CATARACT • It is opacity or cloudiness in the natural lens of the eye. It is the major cause of blindness worldwide. • When the lens becomes cloudy, light rays cannot pass through it easily, and vision is blurred. • Cataract develops in old age after 55 to 60 years.
14. CATARACT • The lens is situated within the sealed capsule. The old cells die and accumulate within the capsule. • Over the years, the accumulation of cells is associated with accumulation of fluid and denaturation of the proteins in the lens fibers causing cloudiness of lens and blurred image.
15. CAUSES OF CATARACT 1. Age 2. Eye injuries 3. Previous eye surgery 4. Diseases like diabetes, Wilson's disease and hypocalcaemia. 5. Long term use of drugs such as steroids diuretics and tranquilizers 6. Long term unprotected exposure to sun light 7. Alcoholism 8. Family history 9. Diet containing large quantity of salt
16. SYMPTOMS 1. Painful blurred vision. 2. Poor night vision. 3. Diplopia in affected eye. 4. Need for bright light while reading. 5. Fading of colors.
17. TREATMENT • Surgery is the only treatment for cataract. • During surgery, the cloudy lens is removed from eye through a surgical incision. • The natural lens is replaced with a permanent clear and plastic intra ocular lens (IOL) implant. • Different procedures were followed to remove the cloudy lens. The common methods are;
18. 1) Extra capsular extraction • This is a rather old technique. • A 12 mm incision is made in eye under an operating microscope to remove the lens as a hole. • The post. capsule of the lens is left in place to hold the IOL implant. • Multiple sutures are required to seal the eye after surgery. The sutures must be perfect otherwise astigmatism may develop.
19. 2) Phacoemulsification • This is current technique. • It is the procedure in which ultrasonic vibrations are used to break the cataract into smaller fragments. It is done through a small (3mm) incision. • An u/s (or laser) probe is used to break the lens material without damaging the capsule. • A foldable IOL is then introduced through the incision. Once inside the eye, the lens unfolds to take position inside the capsule. • No sutures are needed, as the incision is self- sealing.
20. FUNDUS OCULI • The post. part of interior of the eyeball is called fundus oculi or fundus. • Fundus is examined by ophthalmoscope. • The fundus has two parts; 1. optic disk 2. macula lutea
21. FUNDUS OCULI
22. OPTIC DISK ( OPTIC PAPILLA) • Situated near the center of the post. wall of eyeball. • It appears like pale disk. • Formed by the convergence of axons from ganglion cells, while forming the optic nerve. • It contains all layers of retina except rods and cones. • It is insensitive to light, in other words; it is the blind spot. • The central blood vessels of the retina are located right in the center of the optic disk.
23. The retinal arteries are end-arteries except at the optic disk. • It is important to know that ophthalmoscope examination of the eye enables one to see into the eye through the pupil to the retina. • The most obvious feature of retina viewed through ophthalmoscope is the blood vessels on its surface.
24. • The retinal blood vessels originate from the optic disk; where the optic nerve fibers exit the retina. • The examination of optic disk (also called optic nerve head) provides valuable and important information about the state of optic nerve head, arteries and veins of the retina, disorders of retina, pigment epithelium and choroids.
25. Shape of optic disc The normal disc is rounded or slightly oval. Colour of optic disc • Pale-pink. It is distinctly pale than the surrounding fundus. • Temporal side of the disc is paler than the nasal side. • In optic atrophy, the disc becomes more pale than normal. • In inflammation of the optic nerve or raised intra- cranial pressure, the disc becomes hyperaemic and more pink.
26. Physiological cupping • It is the depression in the central part of optic disc. • The cup is paler than the surrounding disc, where from retinal vessels enter and leave the eye. • In glaucoma (increased IOP ) the cupping is greatly increased and the retinal vessels get kinked as they cross the edge of the rim.
27. Blood vessels of the optic disc • Radiate from the disc, dividing into many branches. • The retinal arteries are narrower than the veins and bright red in colour. • It is the artery that crosses the vein. • Spontaneous retinal artery pulsation is an abnormal finding which may occurs if IOP is high while venous pulsation is seen frequently in normal eye and is absent in papilloedema.
28. MACULA LUTEA • M. Lutea or yellow spot is small yellowish area of retina. • Situated a little lateral to the optic disk. • The yellow color of macula lutea is due to the presence of a yellow pigment. • Fovea centralis; is a minute depression in the center of macula lutea where all layers of retina are very thin.
29. MACULA LUTEA • Diameter of fovea is only about 0.5 mm. • Fovea is the region of most acute vision because it contains only the cones which number about 35,000. • The part of the macula lutea surrounding the fovea has rods also which however are much less in number. • Degeneration of macula lutea is common cause for blindness.
30. CAUSES OF ABNORMAL FUNDUS 1) PAPILLOEDEMA • This is a swelling of the optic nerve head due to raised intracranial pressure (ICP). • Absence of inflammatory changes. • Little or no disturbance of visual function. • There is an increased redness of the disc with blurring of its margins in initial stages of papilloedema. • The blurring appear first at the upper and lower margins, particularly in the upper nasal quadrant.
31. PAPILLOEDEMA • The physiological cup becomes filled in and disappears. • Retinal veins are slightly distended. • Spontaneous pulsation of the retinal veins is usually absent. • The disc becomes definitely swollen. • If papilloedema develops rapidly, there will be marked engorgement of the retinal veins with haemorrhages and exudates on and around the disc; but with slow onset there will be little or no vascular change.
32. Causes of papilloedema. 1) Space-occupying lesions particularly. liable to occur in children with tumors of cerebellum and 4th ventricle. 2) Malignant hypertension. • Arterial changes characteristic of this condition. • Haemorrhages, exudates and cotton wool spots extending far beyond the region of the disc.
33. 2) OPTIC NEURITIS • Inflammatory or demyelinating disease may affect any part of the optic nerve, producing an optic neuritis. • The characteristic symptom is loss of vision. • There is a pain on moving the eye. • The pupil on the affected side shows a diminished and ill-sustained contraction to a bright light.
34. 3) RETINAL HAEMORRHAGES. These occur in a number of different conditions and are due to one or more of the following factors; a. Hypertension in which there is increased blood pressure within the retinal vessels. b. Decreased retinal pressure due to proximal large vessel disease. c. Abnormalities in the walls of the retinal vessels, as in diabetes or occlusion of the retinal veins. d. Abnormalities in the circulating blood, as in severe anaemia, leukaemias, and bleeding diatheses. e. Haemorrhages are elongated and flame shaped when superficial, within the nerve fiber layer of the retina, whereas when deep they are round blotches or spots.
35. 4) RETINAL ARTERIOSCLEROSIS. This occurs either as an exaggeration of the general ageing process of the body or in association with hypertension. It is characterized by;

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Friday, 1 May 2015

Afferent pathway - optic disc/nerve, optic chiasm, optic tract and the pretectal nucleus (lying in the dorsal midbrain).
Efferent pathway - this passes through the Edinger-Westphal nucleus before travelling along with the third cranial nerve out to the iris (with a synapse at the ciliary ganglion which sits in the orbit).

There are several reflexes subserved by this pathway and each should be examined in turn. Damage at any level of the pathway or abnormalities of the iris itself can cause abnormalities of pupil shape or function.

Examining the pupils

General examination of the patient

This often-missed component of the examination may provide helpful clues as to what is going on, particularly where there is an underlying neurological cause. There may be a telltale neck scar and associated ptosis in patients with Horner's syndrome or a neurosurgical scar in patients with a third nerve palsy. It may be necessary to go back to the general observation and carry out a further examination depending on the pupillary findings (eg, check for the absent ankle or knee jerk in Adie's pupil).

Pupillary observation

Start by a general observation, noting the shape and size of the pupil in ambient bright light. Size is measured in millimetres and the normal pupil ranges from 1-8 mm. Next, dim the light and have the patient fixate on the far wall. You can then observe the pupils closely by shining a bright light on the patient's face from below (minimise the shadow cast by the nose by placing the light in the midline). If you think there is size asymmetry, a good trick is to stand back and observe the red reflex of both eyes simultaneously with the ophthalmoscope. A slight difference will then become more apparent. If you have access to a slit lamp, use it as a lot of more detailed information can be gauged about the abnormally shaped pupil.

Assessing pupillary reflexes

There are essentially three reflexes to specifically test for:^[1]
Light reflex test

What it assesses - the integrity of the pupillary light reflex pathway.
How to perform it - dim the ambient light and ask the patient to fixate a distant target. Illuminate the right eye from the right side and the left from the left side. (Make sure you do not stand in front of the patient, as their pupils will accommodate to focus on you.) Record whether there is a direct pupillary response (the pupil constricts when the light is shone on it) and a consensual response (the fellow pupil constricts too).
Normal test - there should be a brisk, simultaneous, equal response of both pupils in response to light shone in one or the other eye.

Swinging flashlight test

What it assesses - compares direct and consensual responses of each eye (as opposed to seeing whether they are there or not).
How to perform it - use the same conditions as for the light reflex test and check this reflex first. Then, move the beam swiftly and rhythmically from one eye to the other, making sure that you allow the same amount of light exposure on each eye and that each is illuminated from the same angle. You should note the pupillary constriction of both eyes when the beam is maintained. However, when it is swung, look at what happens to the pupil of the eye you are concerned about and compare this with what is happening to the fellow eye.
Normal test - the pupil should constrict or stay the same size. If it dilates when light is shone on it, then this means that the light reflex is weaker than theconsensual reflex (produced by withdrawing light from the unaffected eye), suggesting optic nerve pathology. This abnormal response is known as a relative afferent pupillary defect (RAPD) and is a very important sign. Note that this is a comparative test: you cannot have a bilateral RAPD.

Near reflex test

What it assesses - this assesses the miosis component of near fixation, otherwise known as accommodation. (The other two components of accommodation are increased lens thickness and curvature, and convergence of the eyes.)
How to perform it - in a normally lit room, instruct the patient to look at a distant target. Bring an object (a toy, the patient's thumb) into their near point (about an arm's length away) and observe the pupillary reflex when their fixation shifts to the near target.
Normal test - there should be a brisk constriction. A near-light dissociation describes the situation where the patient has a significantly better pupillary near reflex than light reflex.

Abnormal pupils

Anisocoria^[2]

This refers to unequal pupils. This is physiological in about 20% of people.^[3] However, if this is a new complaint, the steps to the underlying diagnosis lie in determining which of the pupils is abnormal and then looking for associated signs. The first step is to compare the pupils in light and dim conditions:

If there is a poor reaction to light in one eye and the anisocoria is more evident in a well lit room, the affected pupil is abnormally large.
If there is a good reaction to light in both eyes but a poor dilation in the dark (ie the anisocoria is enhanced), the affected pupil is abnormally small.

The large pupil^[4]

Features - there is poor constriction in a well lit room.
Differential diagnosis - traumatic iris damage, third cranial nerve palsy, pharmacological dilation (ie dilating drops), Adie's pupil, iris rubeosis.

The small pupil^[4]

Features - there is poor dilation in a dim room.
Differential diagnosis - physiologically small pupil, pilocarpine drops, uveitis with synaechiae, Horner's syndrome.

The abnormally shaped pupil

Features - a pupil should be round. Deviation from this suggests abnormalities.
Differential diagnosis - congenital defects (eg, coloboma), iris inflammation or trauma, Argyll Robertson pupils. A fixed oval pupil in association with severe pain, a red eye, a cloudy cornea and systemic malaise suggests acute angle-closure glaucoma which warrants immediate referral.

The abnormally reacting pupil

Light reflex test - abnormalities arise as a result of severe optic nerve damage (eg, transection) - the patient will be blind in that eye, neither pupil reacts when the affected side is stimulated but both pupils react normally when the fellow eye is stimulated.^[5]
Swinging flashlight test^[2] - when the pupil exhibits an RAPD, it is described as a Marcus Gunn pupil. It suggests optic nerve disease, central retinal artery or vein occlusions (see the separate article on Non-diabetic Retinal Vascular Disease). A mild RAPD may also occur in amblyopia, with vitreous haemorrhage, retinal detachment or advanced macular degeneration.
Near reflex test - there are several causes of light-near dissociation which can be grouped according to whether the problem is unilateral or bilateral:^[5]
- Unilateral light-near dissociation - afferent conduction defect, Adie pupil, herpes zoster ophthalmicus, aberrant regeneration of the third cranial nerve.
- Bilateral - neurosyphilis, diabetes, myotonic dystrophy, Parinaud's dorsal midbrain syndrome, familial amyloidosis, encephalitis, chronic alcoholism.

Diseases affecting the pupils

Congenital abnormalities^[5]

Aniridia - this is a bilateral condition arising from the abnormal neuroectodermal development secondary to genetic mutation. It is associated with glaucoma and a number of serious, systemic abnormalities.
Coloboma - this is an uncommon, congenital condition characterised by a unilateral or bilateral partial iris defect. There may be other associated defects both within the eye (eg, the retina) and in the adnexial structures (eg, the lids).
Leukocoria - this refers to a white pupil and may be due to a number of conditions.^[6] Congenital cataracts are generally easily identified but all patients must be assessed for the possibility of retinoblastoma. Other conditions causing a white pupil include persistent fetal vasculature syndrome, Coats' disease andretinopathy of prematurity.

Acquired structural abnormalities

Pseudoexfoliation syndrome - this is a condition characterised by the presence of a grey-white fibrogranular extracellular matrix material deposited on the anterior lens. It is seen, on slit-lamp examination, as a fine grey dusting around the pupil. Pupil shape and function are not affected - it is clinically significant due to its association with glaucoma and its potential to make cataract surgery more tricky.
Sphincter tear - iris tear can occur as a result of blunt or penetrating trauma and can also occur during intraocular surgery. Tears may be associated with glaucoma and, if large, visual problems. All tears, however small, need ophthalmological assessment.
Synechiae - this refers to adhesions between the lens and the iris (posterior synechiae) or the iris and the peripheral cornea (peripheral anterior synechiae). These adhesions will give rise to an abnormally shaped pupil; treatment depends on the underlying cause. Uveitic posterior synechiae are broken with mydriatics whereas glaucomatous anterior synechiae may be managed with miotics.

Neurological abnormalities

Horner's syndrome - see link for separate article.

Causes of Horner's syndrome^[7]
Central (first-order) nerve lesions	Preganglionic (second-order) nerve lesions	Postganglionic (third-order) nerve lesions
Cerebrovascular accidents.	Apical lung tumours (eg, Pancoast's tumour).	Cluster headaches or migraine.
Multiple sclerosis.	Lymphadenopathy (lymphoma, leukaemia, tuberculosis, mediastinal tumours).	Herpes zoster infection.
Pituitary or basal skull tumours.	Lower brachial plexus trauma or cervical rib.	Internal carotid artery dissection.^[8]
Basal meningitis (eg, syphilis).	Aneurysms of the aorta, subclavian or common carotid arteries.	Raeder's syndrome (paratrigeminal syndrome).
Neck trauma (eg, cervical vertebral dislocation or dissection of the vertebral artery).	Trauma or surgical injury (neck or chest).^[9]	Carotid-cavernous fistula.
Syringomyelia.	Neuroblastoma.
Arnold-Chiari malformation.	Mandibular dental abscess.
Spinal cord tumours.

Confirmation of Horner's syndrome is with instillation of a drop of 4% cocaine: in physiological anisocoria, this results in dilation whereas it doesn't where there is a Horner's syndrome.

Further localisation of the problem is carried out with 1% hydroxyamfetamine. Instillation is done >48 hours after the cocaine test as cocaine affects the hydroxyamfetamine. Pupillary dilation suggests a central or preganglionic Horner's syndrome whereas if dilation does not occur, the lesion is likely to be postganglionic.

Midbrain pupils - this refers to the bilateral mid-dilated pupils associated with dorsal midbrain lesions. There is a light-near dissociation but a good response to miotics and mydriatics.
Pupil-involving third cranial nerve (CN III) palsy:^[2]
- When the pupil is involved in a CN III palsy, it is fixed and dilated (or minimally reactive). A partially dilated pupil which reacts sluggishly to light suggests a relative pupil-sparing CN III palsy:
  - CN III palsies with involvement of pupil: requires urgent neuro-imaging with MRI (with magnetic resonance angiography - MRA) or high resolution CT angiograpy (CTA), with or without a lumbar puncture if MRA or CTA are normal.
  - Relative pupil-sparing CN III palsy: tends to be ischaemic in nature and does not need such urgent investigations unless there is progression.
- Other signs of a CN III palsy include features of external ophthalmoplegia: limitation of extraocular movements in all fields of gaze except temporally and a ptosis.
- There may also be evidence of aberrant regeneration where there has been previous CN III damage: as the patient looks down, their lid goes up.
- The differential diagnosis of a CN III palsy includes:
  - Myasthenia gravis
  - Thyroid eye disease
  - Chronic progressive external ophthalmoplegia
  - Orbital inflammatory pseudotumour
  - Internuclear ophthalmoplegia
  Parinaud's syndrome and giant cell arteritis can also cause CN III palsies. An extradural haematoma causes a progressively dilating pupil, due to gradual compression of the third nerve and similarly, a unilateral dilated pupil in a comatose patient should prompt the thought of a subarachnoid haemorrhage.
(Adie's) tonic pupil:^[1]
- This describes a unilateral (80% of cases), mydriatic pupil in otherwise healthy patients (typically, young adults, especially women).
- Over months to years, the pupil diminishes in size to eventually become miotic. There is sluggish, sectoral or no reaction to light but a normal near reflex. Re-dilation after the near response is slow.
- Slit-lamp examination may reveal slow, vermiform contractions of the iris but ultimately, the diagnosis is confirmed by the pupil's hypersensitivity to weak miotic drops (eg, 0.05-0.125% pilocarpine) which causes the abnormal pupil to contract vigorously and the normal pupil minimally.
- Occasionally, it is associated with diminished deep tendon reflexes (Holmes-Adie syndrome) ± autonomic nerve dysfunction.
- The exact cause is not known but it often occurs after a viral illness (eg, herpes zoster ophthalmicus) and denervation of the postganglionic supply to the sphincter pupillae is described.
- This tends to be a benign condition and the patient is simply observed. However, infants <1 year old should be referred to a paediatric neurologist to rule out familial dystonias (Riley-Day syndrome).^[2]
Argyll Robertson pupils - these are caused by neurosyphilis. Although usually asymptomatic, they have characteristic features on examination. These include bilateral (usually asymmetrical) small, irregular pupils showing a light-near dissociation. They are difficult to dilate. Management is related to the underlying disease.

Drugs affecting the pupils^[3]

Topical drugs

Dilating - sympathomimetics (eg, phenylephrine, adrenaline) and antimuscarinics (eg, cyclopentolate, tropicamide, atropine).
Constricting - muscarinic agonists (eg, pilocarpine).

Systemic drugs

Dilating - sympathomimetics (eg, adrenaline (epinephrine)) and antimuscarinics (eg, atropine). Think also of tricyclic antidepressants, amfetamines and ecstasy.
Constricting - opiates (eg, morphine and organophosphates).

Apply fluorescein stain to look for defects (if you suspect corneal perforation, perform a Seidel's test).
If you have access to a slit lamp, assess the cornea from the anterior (epithelial) surface, through the stroma and to the posterior (endothelial) surface by gently moving the focus backwards by a few millimetres. Look for defects (fluorescein uptake), oedema (area of haziness) and infiltrates (a well-demarcated white lesion within the stroma). Vascularisation may occur over the surface or through the stroma, indicating more long-standing disease.
Examine the rest of the globe and its adnexae. If the symptoms warrant it, do a full systemic examination.

Further assessment of the cornea in a specialist unit

Pachymetry - this is the measurement of corneal thickness. It is a painless investigation involving placing a measuring probe lightly on the surface of the anaesthetised cornea.
Specular microscopy - this is a photographic investigation that enables the corneal endothelial cells to be accurately assessed.
Corneal topography - this is another painless investigation which maps the surface of the cornea rather like an ordnance survey map, showing the gradient at each spot and therefore highlighting asymmetries, such as are found in the dystrophic conditions, for example.
Microbiological investigations - a corneal scrape (clinic) or biopsy (theatre) may need to be done.