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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). The maximum stereoacuity is achieved when the images fall on the macular area of the retina, where the resolving power of the eye is at its best. Good stereoacuity is therefore a product of central single binocular vision. A stereoacuity of better than 250 seconds of arc is said to exclude significant amblyopia, while a stereoacuity of worse than 250 seconds of arc may be an indicator of amblyopia.
Clinical Tests of Stereoacuity
There are quite a variety of tests of stereoacuity available, but those most commonly used in the UK are the Titmus test, the TNO test, the Frisby test and the Lang stereotest.
The 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
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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.
The TNO 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.
Quantitative Measurement of Light (Radiometry, Photometry)
This topic can be confusing because of the different nomenclatures that are used. Radiometry quantifies radiant energy in all parts of the electromagnetic spectrum as an absolute value, whereas photometry quantifies part of the spectrum in terms of the visual response it produces, i.e. the spectral sensitivity of the eye. Photometric measurements are therefore more commonly employed in visual science.
Radiometry measures light in terms of how much is emitted from a source (radiant flux), its intensity (radiant intensity) and the amount falling on a surface (irradiance) or reflected from it (radiance). The equivalent photometric
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measurements are luminous flux, luminous intensity, illuminance and luminance (Fig. 1.15). Radiometric and photometric units are related by the luminous efficiency of the radiation, a conversion factor specific for each wavelength determined by the sensitivity of the eye to it. The peak photopic sensitivity of the eye is to the wavelength of 555 nm (yellow–green), at which 1 watt of monochromatic light has a photometric equivalent of 685 lumens (see below). This wavelength is therefore said to have maximum luminous efficiency. The eye is progressively less sensitive to wavelengths towards each end of the visible spectrum. In other words, the luminous efficiency of the radiation becomes lower and the same energy flux (radiometric unit) is equivalent to a lower luminous flux (photometric unit). The conversion factor falls towards zero outside the range 400–700 nm (visible light). The photometric equivalent of polychromatic light is calculated by summating the photometric equivalents of the constituent wavelengths. The various units are defined below.
The total flow of light emitted in all directions from a source is termed either the radiant flux, if measured in watts, or luminous flux, if measured in lumens (Fig. 1.15).
The intensity of light emitted from a source is a measurement of the flow of light per unit solid angle of space extending away from it. It is called either the radiant intensity if measured in watts per steradian or luminous intensity if measured in candelas (lumens per steradian). A steradian is the unit of solid angle (resembling a cone) and defined as the angle at the centre of a sphere which subtends an area on the surface of the sphere measuring the square of the radius (r). Since the surface area of a sphere is 4πr2, it follows that a point source whose luminous intensity is one candela emits a total of 4π lumens.
The original unit of luminous intensity was the candle, based on the emission from a wax candle of standard composition. Attempts to produce a more uniform and precise source of light by which others could be measured led to the current standard unit, the candela, whereby the luminous intensity per square centimetre of a black body radiator at the freezing point of platinum is defined as 60 candelas because the black body radiator is 60 times brighter than the standard candle.
When radiant flux is incident on a surface, the surface is
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Fig. 1.15
Radiometric and photometric units of emitted and reflected light.
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