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​The Pelli-Robson chart consists of a series of large, capital letters (usually Sloan letters) printed in black ink on a white background.

​Fixed Size, Decreasing Contrast: All the letters on the chart are the same large size (low spatial frequency), but their contrast gradually decreases from the top of the chart to the bottom.

​Triplets: The letters are organized into groups of three, called triplets. The three letters within a single triplet all have the same contrast. The contrast decreases from one triplet to the next, even within the same line.

​Procedure: The patient reads the letters aloud, starting from the top. The test is stopped when the patient incorrectly identifies two out of the three letters in a triplet.

​Scoring: The score is determined by the last triplet in which the patient correctly identified at least two of the three letters. The result is expressed as a log contrast sensitivity (e.g., 1.95 log units, with a higher number indicating better sensitivity).
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Clinical Significance

​Measuring contrast sensitivity with the Pelli-Robson chart is considered a vital part of a comprehensive eye exam, as it can provide critical information about a patient's functional vision that is not revealed by a standard visual acuity test alone.
​A person may have 20/20 visual acuity but still report having "foggy," "hazy," or "cloudy" vision if their contrast sensitivity is poor.
​Conditions that often cause a decrease in Pelli-Robson contrast sensitivity include:

​Cataracts: As the lens of the eye clouds, it scatters light and reduces contrast.

​Glaucoma: Damage to the optic nerve can impact contrast perception.

​Optic Neuritis/Multiple Sclerosis (MS): These neurological conditions can affect the visual pathways and significantly reduce CS.

​Age-Related Macular Degeneration (AMD) and Diabetic Retinopathy.

​Post-Surgery Evaluation: It's used to evaluate the quality of vision after cataract or refractive surgery (like LASIK).

​Poor contrast sensitivity can severely affect daily activities such as driving at night (especially with glare from headlights), reading small or poorly printed text, and walking/navigation on stairs or uneven ground.

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