Contrast Sensitivity
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51. Clinical Analogy - The 'Night Driving' Vision
Imagine two patients: both read 6/6 on the Snellen chart, yet one struggles to drive at night, read faces in dim light, or detect a grey tumour on a grey background. Their Snellen acuity is identical, yet their functional vision is worlds apart. This gap is what contrast sensitivity measures. Snellen acuity measures sensitivity largely at higher spatial frequencies using high-contrast optotypes. Contrast-sensitivity function (CSF) testing measures contrast thresholds across a range of spatial frequencies at varying levels of contrast - capturing sensitivity across a range of spatial frequencies. It is the difference between pixel count (Snellen) and the camera's dynamic range (CSF).
2. Definitions & Psychophysical Foundations
Contrast
- Contrast (C) = (Lmax – Lmin) / (Lmax + Lmin)
- This is the Michelson contrast formula, applicable to sinusoidal gratings.
- Values range from 0 (no contrast) to 1 (maximum contrast = 100%).
- L = luminance; max/min = peak/trough of the grating
- Weber contrast = ∆L/L - used for isolated targets on uniform backgrounds
Spatial Frequency
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1- Spatial frequency (SF) is measured in cycles per degree (cpd) of visual angle.
- One cycle = one light + one dark bar pair.
| Spatial Frequency | What It Represents | Clinical Relevance |
|---|---|---|
| Low SF | Coarse features, face recognition | • In glaucoma: high SF affected early; low SF loss may occur with progression. • Optic neuritis/multiple sclerosis (MS): contrast sensitivity (CS) may be reduced across all SF and may be abnormal despite normal visual acuity (VA). |
| Mid SF | Medium detail - PEAK of contrast sensitivity function (CSF) | Optimal for most functional tasks |
| High SF | Fine detail | • Snellen testing measures high-contrast visual acuity. |
| Spatial Frequency | What It Represents | Clinical Relevance |
|---|---|---|
| Low SF | Coarse features, face recognition | • In glaucoma: high SF affected early; low SF loss may occur with progression. • Optic neuritis/multiple sclerosis (MS): contrast sensitivity (CS) may be reduced across all SF and may be abnormal despite normal visual acuity (VA). |
| Mid SF | Medium detail - PEAK of contrast sensitivity function (CSF) | Optimal for most functional tasks |
| High SF | Fine detail | • Snellen testing measures high-contrast visual acuity. |
- Table 1: Spatial frequency ranges and their functional correlates
Contrast Sensitivity (CS) & Contrast Sensitivity Function (CSF)
- Contrast Sensitivity = 1 / Contrast Threshold.
- The contrast threshold is the minimum contrast needed to just detect a grating of a given spatial frequency.
- A higher contrast sensitivity means that lower luminance contrast is required for detection (better performance).
- The Contrast Sensitivity Function (CSF) is a plot of contrast sensitivity versus spatial frequency.
- It is an inverted-U / bandpass curve, peaking at 3–6 cycles per degree in normal subjects.
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Keep reading Contrast Sensitivity
10 more sections are included with the Clinical plan.
- Neurophysiological Basis
- Clinical Testing of Contrast Sensitivity
- Disease-Specific CSF Impairment Patterns
- Contrast Sensitivity in Special Clinical Contexts
- Photopic vs Scotopic vs Mesopic CSF
- Chromatic Contrast Sensitivity