Contrast sensitivity (CS) concept

From the perspective of vision, spatial frequency and contrast are reduced to two parameters that affect object recognition. The so-called spatial frequency refers to the number of bars contained in the 1 ° viewing angle, and the unit is cycle / degree (c/d). The contrast is the relationship between the color brightness of the object and the background color brightness of the object.

Contrast = (maximum brightness - minimum brightness) / (maximum brightness + minimum brightness)

The visual function of the human eye includes not only the visual acuity of the visual chart, but also the ability of the visual system to distinguish the brightness difference between the object and its background. The visual acuity chart used to check visual acuity in clinic is made of high contrast visual markers of different sizes, so visual acuity is the eye's resolution ability under high contrast. However, in the actual life and working environment, there are different contrast conditions, and the high contrast visual acuity chart can not reflect the visual resolution under different contrast conditions. Therefore, in the actual work, it is found that many subjects' visual loss and visual changes are not caused by optical defocusing, such as cataract, glaucoma, irregular astigmatism, etc. Therefore, the human eye needs to distinguish not only objects with clear boundaries, but also objects with fuzzy boundaries. The latter kind of resolution ability is called contrast sensitivity (CS).

Contrast sensitivity (CS) is defined as the reciprocal of the contrast threshold that the visual system can perceive.

Contrast sensitivity (CS) =1/ contrast threshold

The minimum contrast that the human eye can recognize is called the contrast sensitivity (CS) threshold. If the contrast threshold is low, the contrast sensitivity (CS) is high and the visual quality is good; On the contrary, if the contrast threshold is high, the contrast sensitivity (CS) is low and the visual quality is relatively poor.

Conventional visual acuity tests detect eye resolution, while contrast sensitivity (CS) measures eye sensitivity to illumination. Taking different spatial frequencies as abscissa and contrast sensitivity (CS) as ordinate, i.e. combining visual angle and contrast, the contrast that human eyes can distinguish from graphs with different spatial frequencies is measured, and the contrast sensitivity (CS) function (CSF) curve is obtained.

Contrast sensitivity (CS) function (CSF) curve