# Contrast Sensitivity Testing > Contrast sensitivity testing for Optometry: Pelli-Robson scoring, sine-wave grating charts, disease patterns of loss and why acuity alone misleads. - Canonical URL: https://prepelephant.com/topics/allied/optometry/contrast-sensitivity-testing - Exam / course: Allied Health · Subject: Optometry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Contrast Sensitivity Testing", PrepElephant, https://prepelephant.com/topics/allied/optometry/contrast-sensitivity-testing ## Direct answer Snellen acuity measures only the finest resolvable black-on-white detail — the tail of the contrast sensitivity function — while contrast sensitivity testing maps the whole curve: how faint a target the eye still detects across spatial frequencies, from broad gratings to fine letters. The office standards are the Pelli-Robson chart (large letters at 1 metre falling in contrast from triplet to triplet by 0.15 log units; normal performance is about 1.65 to 1.80 log contrast sensitivity, with values below roughly 1.5 considered abnormal in adults) and sine-wave grating charts such as Vistech/FACT, which sample five spatial frequencies and read orientation thresholds. Its clinical force is in the patients with "normal 6/6" who cannot function: cataract (high-frequency loss, worse with glare), glaucoma and optic neuritis (frequency-dependent losses before acuity falls), amblyopia, age-related decline, and driving performance, where contrast predicts hazard detection better than acuity does. ## What you must remember - **The curve:** peak sensitivity sits at low-to-mid spatial frequencies (about 2-5 cycles per degree); the high-frequency cut-off corresponds to conventional acuity — roughly 30 cycles per degree equates to 6/6. - **Pelli-Robson essentials:** 1 metre, letters sized for near-acuity independence, scored in triplets — credit the lowest triplet with at least two of three letters correct; young normal values 1.80 log units, and below about 1.65 is the usual adult abnormal threshold. - **Grating charts:** Vistech/FACT present gratings at five spatial frequencies, the patient reporting tilt left, right or straight; results plot a curve, not a number — low-frequency loss plus preserved high frequencies points neural rather than optical. - **Optical versus neural patterns:** media and refractive blur erode high frequencies first; optic nerve and cortical disease may gut low and mid frequencies while small-print acuity survives; cataract plus glare testing exposes the true disability. - **Glare and mesopic adjuncts:** brightness-acuity testing mimics headlights; a cataract patient with 6/6 room acuity and heavy glare loss is the classic " unfit night driver" case. - **Clinical triggers to test:** functional complaints despite 6/6 (glaucoma suspects, resolved optic neuritis, early cataract, post-LASIK dry eye, amblyopia follow-up, diabetic retinopathy), elderly drivers, and low-vision assessment. - **Conditions discipline:** correct refraction first — uncorrected 0.50 D of astigmatism slashes contrast and masquerades as disease; monocular then binocular testing, best correction, consistent chart luminance. - **Programme note:** Indian driving fitness relies on acuity and Ishihara; contrast is not routine — the optometrist's role is advocacy in cataract and glaucoma follow-up where function outruns acuity charts. ## Interpreting the contrast sensitivity curve A 58-year-old cataract candidate reads 6/6 both eyes in clinic yet refuses night driving. Pelli-Robson scores 1.50 right and 1.55 left — borderline loss; FACT shows a high-frequency droop with preserved low frequencies; glare testing drops acuity two lines under bright light. Interpretation: optical loss (nuclear sclerosis scattering light), not neural — the shape matches media, the glare confirms scatter, and the referral letter quantifies a functional disability the Snellen chart never recorded. Contrast her clinic neighbour: a 34-year-old post-optic-neuritis patient, also "6/6 recovered", scores 1.35 log on Pelli-Robson with a mid-frequency notch on grating testing and no glare drop. The pattern is neural — demyelination's signature of slowed conduction hitting suprathreshold contrast more than resolution. Two 6/6 eyes, two disabled visual systems, two different letters: cataract surgery candidacy on the one hand, rehabilitation and driving counselling on the other. Reading the curve's shape, not its lowest point, is the diagnostic act. ## Where students slip Treating contrast sensitivity as "fuzzy acuity" — it is a separate dimension whose low-frequency arm no Snellen chart touches. Scoring slips: averaging triplets instead of the two-of-three rule, testing at the wrong distance, or forgetting that charts are calibrated for specific luminance so window light corrupts comparisons between visits. Interpretation slips: diagnosing disease from a single abnormal number without refraction control (blur is the great impostor), ignoring glare as a required adjunct in cataract, and overcalling age — sensitivity declines physiologically from the twenties, so a 70-year-old's 1.5 log is not a 30-year-old's 1.5. The viva favourite: name two diseases with normal acuity and abnormal contrast — resolved optic neuritis and early glaucoma. ## Frequently asked questions ### What constitutes normal on the Pelli-Robson chart? Scored in triplets at 1 metre, young adults read about 1.80 log contrast sensitivity; values below roughly 1.65 are treated as abnormal in adults. ### At which spatial frequency does contrast sensitivity peak? Around 2-5 cycles per degree, falling off at both lower and higher frequencies, with the high-frequency cut-off defining conventional visual acuity. ### Which diseases reduce contrast while acuity stays 6/6? Early cataract (especially with glare), resolved optic neuritis, glaucoma, amblyopia and age-related decline — the classic list of function-out-of-proportion-to-acuity. ### Why must refraction precede contrast testing? Even 0.50 D of uncorrected astigmatism or myopia degrades the curve and mimics disease; best correction separates optical blur from true neural or retinal loss. ### Why add glare testing to contrast measurement? Because scattered light in media opacities dominates real-world disability — headlights at night — so a brightness-acuity test reveals the cataract patient whom clinic acuity cleared to drive.