Physiological optics4th lecture

Dr. Mohammad Shehadeh

Visual acuity testing

• Visual acuity (VA) is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain

• Visual acuity is a measure of the spatial resolution of the visual processing system and is usually tested in a manner to optimise and standardise the conditions.

Why we chose 6 meter for our tests?

• black symbols on a white background are used (for maximum contrast) and a sufficient distance allowed to approximate infinity in the way the lens attempts to focus.

• Twenty feet, or six metres, is essentially infinity from an optical perspective (the difference in optical power required to focus at 20 feet (6.1 m) versus infinity is only 0.164 diopters)

In the term "20/20 vision",

• the numerator refers to the distance in feet between the subject and the chart.

• The denominator indicates the separation at which the lines that make up those letters would be separated by a visual angle of 1 arc minute, which for the lowest line that is read by an eye with no refractive error (or the errors corrected) is usually 20 feet (6.1 m)

• This means that at 20 feet or 6 metres, a typical human eye, able to separate 1 arc minute, can resolve lines with a spacing of about 1.75mm

In low light vision, there is low resolution despite the high sensitivity?

• . This is due to spatial summation of rods, so 100 rods could merge into many bipolars, in turn converging on ganglion cells, and the unit for resolution is very large, thus acuity being small

• visual acuity, or resolving power, is the property of cones.[5] To resolve detail, the eye's optical system has to project a focused image on the fovea which have the highest density of cone photoreceptor cells (the only kind of photoreceptors existing on the fovea), thus having the highest resolution and best color vision.

• Acuity and color vision, despite being mediated by the same cells, are different physiologic functions that do not interrelate except by position.

• Acuity and color vision can be affected independently.

• The maximum resolution is that 30 seconds of arc, corresponding to the foveal cone diameter or the angle subtended at the nodal point of the eye.

• visual acuity is affected by the size of the pupil. Optical aberrations of the eye that decrease visual acuity are at a maximum when the pupil is largest (about 8 mm), which occurs in low-light conditions.

• When the pupil is small (1–2 mm), image sharpness may be limited by diffraction of light by the pupil (see diffraction limit).

• Between these extremes is the pupil diameter that is generally best for visual acuity in normal, healthy eyes; this tends to be around 3 or 4 mm.

• The visual cortex is the part of the cerebral cortex in the posterior part of the brain responsible for processing visual stimuli is the occipital lobe.

• The central 10° of field (approximately the extension of the macula) is represented by at least 60% of the visual cortex. Many of these neurons are believed to be involved directly in visual acuity processing.

• Proper development of normal visual acuity depends on having normal visual input when it is very young.

• Any visual deprivation, that is, anything interfering with such input over a prolonged period, such as a cataract, severe eye turn or strabismus, or covering or patching the eye during medical treatment, will usually result in a severe and permanent decrease in visual acuity in the affected eye if not treated early in life.

• The decreased acuity is reflected in various abnormalities in cell properties in the visual cortex. These changes include :

• a marked decrease in the number of cells connected to the affected eye

• as well as few cells connected to both eyes, resulting in a loss of binocular vision and depth perception, or stereopsis.

• The period of time over which is highly sensitive to visual deprivation is referred to as the critical period.

Near Visual Acuity Testing

• The near visual acuity is usually tested at a distance of 25-33 cm. Near acuity charts usually comprise unrelated words or passages of text.

• Each point is equal to 1/72 inch and blocks are sized in multiples of this, e.g. the blocks bearing N5 letters measure 5/72 inches in height.

• Times Roman is used as the standard font because the size of printed text depends on the font chosen

Potential Visual Acuity Testing

• These tests may be used to assess the potential visual acuity of eyes in which it is not possible to see the macula e.g. because of a cataract.

• Good potential visual acuity indicates that cataract surgery is likely to be of benefit. The simplest clinical test is the pinhole test.

Pin hole test

• The blue field entoptic phenomenon is the ability to see moving white dots when blue light diffusely illuminates the retina.

• They are thought to represent light transmitted by white blood cells in the perifoveal capillaries. When this phenomenon is present, macular function is grossly intact.

The blue field entoptic phenomenon

• Interferometers project laser light from two sources on to the retina. Interference occurs where the two sources meet and this is seen as a sine wave grating if the macula is functioning.

• The potential acuity meter projects a letter chart on to the retina through a small aperture

Contrast Sensitivity • Tests of visual acuity do not adequately reflect the ability of the

eye to see low-contrast objects such as faces.• In many conditions, e.g. cataract, glaucoma and optic neuritis,

the visual acuity may be almost normal whilst the contrast sensitivity is considerably reduced.

• Contrast sensitivity is measured using a sine wave grating. This is a pattern in which there is a gradual transition between alternating light and dark bands, i.e. the edges of the bands appeared blurred. Narrower bands are described as having a higher spatial frequency.

• A contrast sensitivity curve is constructed by plotting a range of different spatial frequencies against the lowest degree of contrast at which the eye can still detect the grating.

• Low or very high spatial frequencies must have higher levels of contrast in order to be seen.

• In clinical practice, the contrast sensitivity is measured using either a television monitor or a chart. The Pelli-Robson contrast test chart displays letters that have decreasing levels of contrast to their background.

• The VISITECH chart has 40 circles with different sine wave gratings and levels of contrast. The subject must indicate the orientation of the circles.

Pelli-Robson contrast test chart

Glare Testing • Scattered light which reduces visual function is called

glare.• Glare may be the predominant symptom of corneal

oedema or scarring, cataracts or opacification of the posterior lens capsule.

• The effect of a glare source depends on its position and intensity and on the light scattering properties of the ocular media.

• Glare testing refers to the measurement of visual function (e.g. visual acuity, contrast sensitivity, colour vision) in the presence of a source of glare.

Polarisation of Light

• The orientation of the plane of the wave motion of rays comprising a beam of light is random unless the light is polarised

Figure 1.14a shows a beam cut across and viewed end-on: the light is travelling perpendicular to the page. In contrast, Fig. 1.14b shows the cross section of a beam of light in which the individual wave motions are lying parallel to each other. Such a beam is said to be polarized

• Polarized light is produced from ordinary light by an encounter with a polarizing substance or agent.

• Polarizing substances, e.g. calcite crystals, only transmit light rays which are vibrating in one particular plane.

• Thus only a proportion of incident light is transmitted onward and the emerging light is polarised.

• A polarising medium reduces radiant intensity but does not affect spectral composition.

• Light is polarised on reflection from a plane surface, such as water, if the angle of incidence is equal to the polarising angle for the substance.

• The polarising angle is dependent on the refractive index of the substance comprising the reflecting surface

• At other angles of incidence the reflected light is partly polarised, i.e. a mixture of polarised and non-polarised light.

• Furthermore, the plane of polarisation of the reflected light from such a surface is parallel with the surface.

• As most reflecting surfaces encountered in daily life are horizontal, it is possible to prepare polarised sunglasses to exclude selectively the reflected horizontal polarised light .

• Such glasses are of great use in reducing glare from the sea or wet roads.

Stereoscopic Vision

• Stereopsis is the ability to fuse slightly dissimilar images, which stimulate disparate retinal elements within Panum's fusional areas in the two eyes, with the perception of depth.

• It is graded according to the least horizontal disparity of retinal image that evokes depth perception, and is measured in seconds of arc.

• The normal stereoacuity is approximately 60 seconds of arc or better (slightly different values are quoted by different workers).

• An individual with very good stereoscopic vision may have a stereoacuity of better than 15 seconds of arc, which is the smallest disparity offered in the Frisby stereotest (range 600–15 seconds of arc).

Clinical Tests of Stereoacuity

Titmus test,• which includes the Wirt fly test, is in the form of vectographs. • A vectrograph consists of two superimposed views presented in

such a way that the light entering each eye is plane polarised, the light from one view being at right angles to that from the other.

• The composite picture must be viewed through a polarising visor or spectacles.

• The Wirt fly is the largest target in the test, which also includes graded sets of animals and circles, one of which is disparate and appears to stand forward. The test must be viewed at 40 cm, and covers a range of stereoacuity from approximately 3000 to 40 seconds of arc.

The Frisby test• consists of three clear plastic plates of different thicknesses. • On each plate there are four squares filled with small

random shapes. • One square on each plate contains a 'hidden' circle, which is

printed on the back surface of the plate. • The random shapes give no visual clue to the edge of the

'hidden' circle, and the test is purely three-dimensional and does not require polarising or coloured glasses to be worn.

• At a 40 cm viewing distance the plates show a disparity of 340, 170 and 55 seconds of arc,

• and by adjusting the viewing distance the test can be used to give a disparity range from 600 to 15 seconds of arc.

TheTNO test

• comprises computer-generated random dot anaglyphs. • An anaglyph is a stereogram in which two disparate views

are printed in red and green respectively on a white ground.

• Red–green spectacles are worn to view the anaglyph. The eye looking through the red filter sees only the green picture, as black, and the eye looking through the green filter sees the red picture, again as black, and the two views may be fused to give a stereoscopic effect.

• In the TNO test the disparities range from 480 to 15 seconds of arc.

The Lang stereotest

• targets are made up of fine vertical lines which are seen alternately by each eye

• when focused through built-in cylindrical lens elements.

• The displacement of the random dot images creates disparity ranging from 1200 to 550 seconds of arc.

• The test card must be held parallel to the plane of the patient's face to avoid giving uniocular clues.

• The test is viewed at a normal reading distance.