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neutral density filter test

The neutral density filter test remeasures visual acuity through a neutral density filter to help separate amblyopia from vision loss caused by organic disease.1,2 The test rests on the Ammann phenomenon, the observation that neutral filters profoundly reduce vision in eyes with central retinal lesions and glaucoma but do not reduce the vision of amblyopic eyes.1 The original test used a Kodak Wratten gelatin filter No. 96, N.D. 3.00, to distinguish reversible from organic amblyopia.1

One clinical protocol uses an ND 2 filter in a goggle arrangement so the illumination level is controlled.2 In an eye with normal acuity, the filter lowers acuity by about one line of Snellen letters, and anisometropic or organic amblyopia shows a similar drop.2 The acuity of an eye with strabismic amblyopia is not affected by the filter.2

Acuity loss through the filter overlaps among normal eyes, amblyopic eyes and eyes with organic retinal lesions or optic atrophy.1 The test therefore points to organic disease only when the loss falls outside that overlap.1

More detail

Visual evoked response recordings at various levels of light attenuation agree with the acuity findings.1 Normal subjects and patients with strabismic amblyopia show significantly smaller changes in amplitude and latency than patients with poor vision from organic causes.1

Sources (8)
  1. von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 14
  2. Pickwell's Binocular Vision Anomalies, ch. 13
  3. Barbur, J.L., Hess, R.F. and Pinney, H.D. 1994. Pupillary function in human amblyopia. Ophthalmic and Physiological Optics, 14(2), pp.139-149.
  4. Habeeb, S.Y., Arthur, B.W. and ten Hove, M.W. 2012. Neutral density filters simulate night vision disturbances in patients with mild cataracts. Canadian Journal of Ophthalmology, 47(3), pp.280-284.
  5. Lempert, P. 2000. Optic nerve hypoplasia and small eyes in presumed amblyopia. Journal of American Association for Pediatric Ophthalmology and Strabismus, 4(5), pp.258-266.
  6. Lempert, P. 2004. The axial length/disc area ratio in anisometropic hyperopic amblyopia: a hypothesis for decreased unilateral vision associated with hyperopic anisometropia. Ophthalmology, 111(2), pp.304-308.
  7. Rutstein, R. and Daum, K. 1997. Anomalies of Binocular Vision: Diagnosis & Management. St. Louis: Mosby.
  8. Von Noorden, G.K. and Burian, H.M. 1959. Visual acuity in normal and amblyopic patients under reduced illumination: I. Behavior of visual acuity with and without neutral density filter. Archives of Ophthalmology, 61(4), pp.533-535.

Reviewed October 2026

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