12.1 Glare Testing, Potential Acuity Meter & Contrast Sensitivity

Why supplemental tests exist

Snellen acuity is measured with maximum contrast, controlled illumination and static high-contrast letters. Real life supplies none of those conditions. A patient with early posterior subcapsular cataract can read 20/25 in a darkened lane and be unable to drive at night. The supplemental tests quantify that gap, and in many health systems they determine whether surgery is justified.

Glare testing

Disability glare is loss of visual function caused by light scattered within the eye — by cataract, corneal oedema, corneal scarring, dry eye, refractive surgery interfaces or intraocular lens edge effects. The scattered light forms a veil over the retinal image, reducing contrast.

Distinguish it from discomfort glare, which is the sensation of dazzle without measurable acuity loss.

The brightness acuity tester (BAT) is the standard instrument: a hemispherical bowl with a central aperture and three brightness settings (high, medium, low) simulating direct sunlight, overcast daylight and bright indoor light.

Technique:

1. Measure best-corrected acuity without glare, one eye at a time.
2. Hold the BAT so the patient views the chart through the central aperture, with the bowl illuminated.
3. Allow several seconds of adaptation at each setting — this is essential, as an immediate reading understates the effect.
4. Record acuity at each brightness setting.
5. Record the difference: "OD 20/25 without glare, 20/80 with high-setting BAT."

Interpretation. A drop of three lines or more is generally considered significant disability glare. Posterior subcapsular cataract is the classic offender because it sits at the nodal point of the eye and scatters maximally with a constricted pupil, which is exactly what happens in bright light.

Other glare methods include a penlight held obliquely off-axis while acuity is measured, and brightness-contrast systems built into some acuity projectors.

Potential acuity meter

The potential acuity meter (PAM) projects a bright Snellen chart through an aperture of about 0.15 mm onto the retina. That pencil of light is small enough to pass through relatively clear windows between opacities, so it estimates the acuity the retina and optic nerve could deliver if the media were clear.

Technique:

1. Dilate the pupil where possible; a clear path is easier to find.
2. Mount the instrument on the slit lamp and align through the clearest area of the media.
3. Move the aperture around to find the best window — this is the operator skill that makes or breaks the test.
4. Record the best acuity achieved and the number of letters read.

Limitations, all examinable:

Alternatives include laser interferometry, which projects interference fringes whose spacing the patient reports, and the simpler bedside tests: a bright light projection and two-point discrimination check, an intact red reflex, an absence of an afferent pupillary defect, and good entoptic phenomena (the patient seeing their own retinal vessels when a light is wiggled against the closed lid) all suggest reasonable macular function.

Contrast sensitivity

Contrast sensitivity measures the faintest contrast the patient can detect, across a range of spatial frequencies (how fine the pattern is). Visual acuity measures only the highest spatial frequency at maximum contrast — a single point on a whole curve.

The result is plotted as a contrast sensitivity function, an inverted-U curve peaking at intermediate spatial frequencies around 3 to 6 cycles per degree.

Common instruments:

Technique points: standardise the illumination (charts are calibrated to specific luminance), use the correct test distance, ensure the patient wears the correct refractive correction for that distance, and encourage guessing to threshold rather than stopping at the first hesitation.

Clinical value. Contrast sensitivity loss with preserved Snellen acuity is the signature of:

• Early cataract, especially posterior subcapsular
• Glaucoma, where it can precede field loss
• Optic neuritis and other optic neuropathies, often with red desaturation
• Amblyopia
• Diabetic retinopathy before acuity changes
• Post-refractive-surgery patients complaining of poor night vision despite 20/20 acuity
• Multifocal intraocular lenses, which trade contrast for depth of focus

This is the test that validates the patient who says "I can read the chart but I can't see." It also has functional meaning: contrast sensitivity correlates better than Snellen acuity with driving performance, mobility, face recognition and fall risk.

Putting the three together

A typical pre-cataract-surgery workup for a patient whose Snellen acuity does not obviously justify surgery:

1. Best-corrected acuity — 20/30 OD.
2. Glare testing — drops to 20/100 with the BAT at the high setting. Confirms disability glare.
3. Contrast sensitivity — reduced across mid and high spatial frequencies. Confirms functional loss.
4. Potential acuity meter — reads 20/25, suggesting the macula can support good vision once the lens is removed.

Those four numbers together make the case that Snellen acuity alone does not. Conversely, a PAM of 20/80 in an eye with 20/60 acuity and drusen suggests the cataract is not the main problem, and the conversation with the patient must change accordingly.

Recording rule: always record the conditions — the BAT setting used, the contrast chart and its luminance, the PAM aperture position — because none of these results is comparable across visits without them.