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Fig. 1.13

Offset square wave grating used to measure vernier visual acuity.

which can be detected. It is measured using a square wave grating. An offset of 3–5 seconds of arc is normally discernible. This is less than the limit of Snellen visual acuity and is therefore also called hyperacuity.

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.

The British N system is based on the use of the typesetters' point system to specify the size of the metal block on which letters were traditionally cast. 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. Jaeger text types are a less satisfactory alternative because they do not follow a logical progression in size.

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.

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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. (p. 116).

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.

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 (cf. pin-hole test, p. 117).

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.

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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.

Fig. 1.14

Cross section of beam of light to show plane of wave motion.

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,

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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 (cf Chapters 2 and 3). 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 (see below). Such glasses are of great use in reducing glare from the sea or wet roads.

Birefringence

Some substances have a molecular structure which transmits light waves lying parallel to its structure but which selectively slows and therefore redirects (cf. p. 33, refraction) light waves vibrating in a plane perpendicular to its structure. Crystals of quartz have this property, which is known as birefringence. Because they split incident unpolarised light into two polarised beams travelling in different directions, they have two refractive indices.

Dichroism

The molecular structure of dichroic substances completely blocks transmission of light waves not aligned with its structure by absorption. Thus, only one beam of polarised light emerges, much weakened in intensity compared with the incident non-polarised light. Tourmaline and polaroid (the latter made from fine iodine and quinine sulphate crystals embedded in plastic) are dichroic substances, polaroid being commonly used in sunglasses.

Other examples of the use of polarised light in ophthalmology are the assessment of binocular vision in which polarising glasses may be used to dissociate the eyes, e.g. in the Titmus test (p. 19); in pleoptics to produce Haidinger's brushes; and in the manufacture of optical lenses to examine them for stress.

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