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Fig. 1.2

Spectral sensitivity of cone pigments.

Congenital colour defects characteristically affect particular parts of the colour spectrum. Acquired colour defects occur throughout the spectrum but may be more pronounced in some regions. For example, acquired optic nerve disease tends to cause red–green defects. An exception occurs in glaucoma and in autosomal dominant optic neuropathy which initially cause a predominantly blue– yellow deficit; it has recently been found that visual field loss in glaucoma is detected earlier if perimetry is performed using a blue light stimulus on a yellow background. Acquired retinal disease tends to cause blue–yellow defects (except in cone dystrophy and Stargardt's disease, which cause a predominantly red–green defect).

Clinical Testing of Colour Vision

Clinical tests of colour vision are designed to be performed in illumination equivalent to afternoon daylight in the northern hemisphere.

The Farnsworth–Munsell (FM) hue 100 test is the most comprehensive method. It comprises 84 coloured discs, numbered in sequence on the undersurface and divided into four groups of 21. The colours of each group occupy a portion of the colour spectrum. The colours differ only in hue and have equivalent brightness and saturation. Each group must be arranged in a row with the reference colours at each end and the intervening discs in order of closest colour match. The order of placement indicates the nature of the colour defect.

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The D-15 test uses colours from all parts of the spectrum which must be arranged in order from a single reference colour. The test does not distinguish mild colour defects, but for most purposes those passing the test are unlikely to have problems with hue discrimination.

Ishihara pseudoisochromatic test plates specifically test for congenital red–green defects, the most common abnormality of colour vision. The test plates consist of random spots of varying isochromatic density. Numbers or wavy lines (for illiterates) are represented by spots of different colours. A patient who is colour blind will see only a random pattern of spots or incorrect numbers. The figures can only be distinguished from their background by their colour and not by a difference in contrast.

The Lanthony New Colour Test tests hue discrimination and can be used by children.

Ultraviolet Light

The retinal photoreceptors are also sensitive to wavelengths between 400 nm and 350 nm in the near ultraviolet (UV-A). These wavelengths are normally absorbed by the lens of the eye. In aphakic eyes or pseudophakic eyes with intraocular implants without UV filter, such UV radiation gives rise to the sensation of blue or violet colours. Newly aphakic patients frequently remark that 'everything looks bluer than before the operation'.

Of greater concern is the recent evidence that wavebands between 350 nm in the UV and 441 nm in the visible spectrum are potentially the most dangerous for causing retinal damage under normal environmental conditions. It is therefore desirable that intraocular lenses filter out these wavelengths and protect the retina. Intraocular implant lenses are therefore being produced which incorporate a UV- A absorbing substance.

The bright illumination employed in modern ophthalmic instruments may also cause retinal damage under some circumstances. Prolonged exposure to high intensity indirect ophthalmoscope illumination, intraocular light pipe illumination and operating microscope light is potentially damaging to the retina, which may in many instances

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already be unhealthy. Some instruments have yellow filters built into them to reduce exposure to the most damaging wavelengths.

Fluorescence

Fluorescence is the property of a molecule to spontaneously emit light of a longer wavelength when stimulated by light of a shorter wavelength. For example, the orange dye fluorescein sodium when excited by blue light (465–490 nm) emits yellow–green light (520–530 nm) (see Fig. 1.3).

Fig. 1.3

Absorption and emission spectra of fluorescein.

Fluorescein angiography allows the state of retinal and choroidal circulation to be studied by photographing the passage of fluorescein through the vasculature after it has been administered systemically. White light from the flash unit of a fluorescein camera passes through a blue 'excitation' filter to illuminate the fundus with blue light (Fig. 1.4). The wavelengths transmitted by the excitation filter approximate to the absorption spectrum of fluorescein. Most of the light is absorbed, some is reflected unchanged, and some is changed to yellow–green light by fluorescence. The blue reflected light and yellow–green fluorescent light leaving the eye are separated by a yellow–green 'barrier' filter in the camera. This blocks blue light and exposes the camera film only to yellow–green light from the fluorescein, thereby delineating vascular structures and leakage of dye.

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Fig. 1.4

Filter system for fundus fluorescein angiography.

The phenomenon of pseudofluorescence occurs if there is an overlap in the spectral transmission of the excitation and barrier filters. This allows reflected wavelengths at the green end of blue to pass through the barrier filter and appear as fluorescence.

Other important applications of fluorescein include the staining of ocular surface defects, anterior segment angiography, the measurement of aqueous humour production and outflow, and, in light microscopy, the localisation of tissue constituents using fluorescein bound to specific immunoglobulin.

Indocyanine Green

Indocyanine green (ICG) dye is a fluorescent substance which absorbs 805 nm and emits 835 nm infrared radiation. The retinal pigment epithelium does not absorb these wavelengths, and it is therefore possible to observe fluorescence of the choroidal circulation after ICG is administered intravenously. Only 4% of 805 nm radiation absorbed by ICG is emitted at 835 nm compared with the total fluorescence of fluorescein. ICG angiography is not yet in general clinical use, but it has been shown to delineate occult choroidal neovascularisation not visible with fluorescein. ICG has also been used to photosensitise vascular lesions to diode laser photocoagulation (cf. diode laser, pp. 223–24).

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