Contrast Sensitivity and the Contrast Sensitivity Function

An underappreciated variable in measuring visual function is the degree of contrast between the optotype and its background. In general, the higher the contrast, the easier the optotype is to decipher. Good illumination makes it easier to read a book, for example, because the additional light creates a brighter background and therefore a higher contrast against the black letters on the page. If the brightness of an object (Imin) and the brightness of its background (Imax) are known, the following formula can be used to measure the degree of contrast between the object and its background:

Thus, for letters printed with perfectly black ink (ie, 100% nonreflecting) on perfectly white paper (ie, 100% reflecting), the contrast would be 100%. Snellen visual acuity is commonly tested with targets, either illuminated or projected charts, that approximate 100% contrast. Therefore, when we measure Snellen visual acuity, we are measuring, at approximately 100% contrast, the smallest optotype that the visual system can resolve. In everyday life, however, 100% contrast is rarely encountered, and most visual tasks must be performed in lower-contrast conditions.

To take contrast sensitivity into account when measuring visual function, we can use the modulation transfer function (MTF). Consider a target in which the light intensity varies from some peak value to zero in a sinusoidal fashion. The contrast is 100%, but instead of looking like a bar graph, it looks like a bar graph with softened edges. The number of light bands per unit length or per unit angle is called the spatial frequency and is closely related to Snellen acuity. For example, the 20/20 E optotype is composed of bands of light and dark, where each band is 1 arcmin. Thus, for a target at 100% contrast, 20/20 Snellen acuity corresponds roughly to 30 cycles per degree of resolution when expressed in spatial frequency notation. If we take sine wave gratings with various spatial frequencies and describe how the optical system alters the contrast of each of them, we have a set of information that constitutes the MTF.

In clinical practice, the ophthalmologist presents a patient with targets of various spatial frequencies and peak contrasts. A plot is then made of the minimum resolvable contrast target that can be seen for each spatial frequency. The minimum resolvable contrast is the contrast threshold. The reciprocal of the contrast threshold is defined as the contrast sensitivity, and the manner in which contrast sensitivity changes as a function of the spatial frequency of the targets is called the contrast sensitivity function (CSF) (Fig 2-7). A typical contrast sensitivity curve obtained with sinusoidal gratings is shown in Figure 2-8. Contrast sensitivity can also be tested with optotypes of variable contrast (such as the Pelli-Robson or Regan charts), which may be easier for patients to use. Which of these approaches is clinically more useful remains controversial.

Figure 2-7 Contrast sensitivity grating. In this example, the contrast diminishes from bottom to top, and the spatial frequency of the pattern increases from left to right. The pattern appears to have a hump in the middle at the frequencies for which the human eye is most sensitive to contrasts. (Courtesy of Brian Wandell, PhD.)

Figure 2-8 A typical contrast sensitivity curve is noted as x-x-x. The shaded area represents the range of normal values for 90% of the population. Expected deviations from normal due to specific diagnoses are noted in the text discussion.

(Developed b y Arthur P. Ginsb urg, PhD. Courtesy of Stereo Optical Company, Inc, Chicago.)

It is important to perform contrast sensitivity testing with the best possible optical correction in place. In addition, luminance must be kept constant when CSF is tested, because mean luminance has an effect on the shape of the normal CSF. In low luminance, the low spatial frequency falloff disappears and the peak shifts toward the lower frequencies. In brighter light, there is little change in the shape of the normal CSF through a range of luminance for the higher spatial frequencies. Generally, contrast sensitivity is measured at normal room illumination, which is approximately 30– 70 lux.

Contrast sensitivity is affected by various conditions of the eye, both physiologic and pathologic. Any corneal pathology that causes distortion or edema can affect contrast sensitivity. Lens changes, particularly incipient cataracts, may significantly decrease CSF, even with a normal Snellen visual acuity. Retinal pathology may affect contrast sensitivity more (as with retinitis pigmentosa or central serous retinopathy) or less (certain macular degenerations) than it does Snellen visual acuity. Glaucoma may produce a significant loss in the midrange. Optic neuritis may also be associated with a notch-type pattern loss. Amblyopia is associated with a generalized attenuation of the curve. Pupil size also has an effect on contrast sensitivity. With miotic pupils, diffraction reduces contrast sensitivity; with large pupils, optical aberrations may interfere with performance.

Miller D. Glare and contrast sensitivity testing. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology [CD-ROM]. Vol 1. Philadelphia: Lippincott Williams & Wilkins; 2006:chap 35.

Refractive States of the Eyes

In considering the refractive state of the eye, we can use either of the following concepts:

1.The focal point concept: The location of the image formed by an object at optical infinity through a nonaccommodating eye determines the eye’s refractive state. Objects focusing at points anterior or posterior to the retina form a blurred image on the retina, whereas objects that focus on the retina form a sharp image.

2.The far point concept: The far point is the point in space that is conjugate to the fovea of the nonaccommodating eye; that is, the far point is where the fovea would be imaged if the optics were reversed and the fovea became the object.

Emmetropia is the refractive state in which parallel rays of light from a distant object are brought to focus on the retina in the nonaccommodating eye (Fig 2-9A). The far point of the emmetropic eye is at infinity, and infinity is conjugate with the retina (Fig 2-9B). Ametropia refers to the absence of emmetropia and can be classified by presumptive etiology as axial or refractive. In axial ametropia, the eyeball is either unusually long (myopia) or short (hyperopia). In refractive ametropia, the length of the eye is statistically normal, but the refractive power of the eye (cornea and/or lens) is abnormal, being either excessive (myopia) or deficient (hyperopia). Aphakia is an example of extreme refractive hyperopia unless the eye was highly myopic (>20.00 D) before lens removal. An ametropic eye requires either a diverging or a converging lens to image a distant object on the retina.

Figure 2-11 Hyperopia with accommodation relaxed. A, Parallel light rays from infinity focus to a point posterior to the retina, forming a blurred image on the retina. B, Light rays emanating from a point on the retina are divergent as they exit the eye, appearing to have come from a virtual far point behind the eye. (Illustration b y C. H. Wooley.)

Astigmatism (a = without, stigmos = point) is an optical condition of the eye in which light rays from an object do not focus to a single point because of variations in the curvature of the cornea or lens at different meridians. Instead, there is a set of 2 focal lines. Each astigmatic eye can be classified by the orientations and relative positions of these focal lines (Fig 2-12). If one of the focal lines lies in front of the retina and the other is on the retina, the condition is classified as simple myopic astigmatism. If both focal lines lie in front of the retina, the condition is classified as compound myopic astigmatism. If, in an unaccommodated state, one focal line lies behind the retina and the other is on the retina, the astigmatism is classified as simple hyperopic astigmatism. If both focal lines lie behind the retina, the astigmatism is classified as compound hyperopic astigmatism. If, in an unaccommodated state, one focal line lies in front of the retina and the other behind it, the condition is classified as mixed astigmatism.

Figure 2-12 Types of astigmatism. The locations of the focal lines with respect to the retina define the type of astigmatism. The main difference between the types of astigmatism depicted in the illustration is the spherical equivalent refractive error. All of the astigmatisms depicted are with-the-rule astigmatisms—that is, they are corrected by using a plus cylinder with a vertical axis. If they were against-the-rule astigmatisms, the positions of the vertical and horizontal focal lines would be reversed.

If the principal corneal or lenticular meridians of astigmatism (or axes, which are 90° to the meridians) have constant orientation at every point across the pupil, and if the amount of astigmatism is the same at every point, the refractive condition is known as regular astigmatism. This condition is correctable by cylindrical spectacle lenses. Note that the axis of the cylindrical correction is perpendicular to the axis of the corneal or lenticular astigmatism. Regular astigmatism may itself be classified into with-the-rule or against-the-rule astigmatism. In with-the-rule astigmatism (the more common type in children), the vertical corneal meridian is steepest (resembling an American football lying on its side), and a correcting plus cylinder axis should be used at or near 90°. In against-the- rule astigmatism (the more common type in older adults), the horizontal meridian is steepest (resembling a football standing on its end), and a correcting plus cylinder axis should be used at or near 180°. The term oblique astigmatism is used to describe regular astigmatism in which the principal meridians do not lie at, or close to, 90° or 180° but instead lie near 45° or 135°.

In irregular astigmatism, the orientation of the principal meridians or the amount of astigmatism changes from point to point across the pupil. Although the principal meridians are 90° apart at every point, it may sometimes appear by retinoscopy or keratometry that the principal meridians of the cornea, as a whole, are not perpendicular to one another. All eyes have at least a small amount of irregular astigmatism, and instruments such as corneal topographers and wavefront aberrometers can be used to detect this condition clinically. These higher-order aberrations in the refractive properties of the cornea and lens have been characterized by Zernike polynomials, which are mathematical