Visual Acuity Chart and Room Lighting in the Refraction Room
Two kinds of light matter in a refraction room: the luminance of the test chart, which international standards specify, and the general room lighting, which changes the result if it is not kept the same from patient to patient.
This page sets out what the standards say about the chart, what to decide about the room light, and how the chart distance fits the room.
The figures on this page come from the published standards cited below. They describe chart presentation, not clinical procedure; how a practice measures visual acuity is the practitioner’s decision.
What the standards say about chart luminance.
ISO 8596 (visual acuity testing) and ISO 10938 (chart displays — printed, projected and electronic) both set the luminance of the chart background between 80 and 320 cd/m², with 200 cd/m² recommended. The optotypes must be dark against it: their luminance no more than 15% of the background.
In practice that means a chart that is lit or self-illuminated to a known level, rather than one that depends on whatever light happens to fall on the wall. Projectors, screens and back-lit cabinets are specified by their makers against these figures; ask for the luminance when you compare them.
Keep the room lighting the same every time.
Room lighting changes the measured result. In one published comparison, acuity measured with the room lights on was slightly better than with the room lights reduced. Whichever condition a practice chooses, the useful rule is consistency: the same lights, in the same state, for every patient.
- Dimmable general lighting — so the room can be darkened for parts of the examination and returned to a fixed level for acuity.
- No light falling on the chart — avoid luminaires that throw light or glare onto the chart surface or into the patient’s eyes.
- Task light at the unit — the lights built into refraction units light the working area and the controls, not the chart. LY-500, for example, has DC 24 V, 3 W lighting.
Chart distance, and what a mirror does.
Every chart is designed for a viewing distance, commonly 6 m (20 ft), with 4 m charts also widely used. The patient’s eyes have to be at that optical distance from the chart.
A mirror on the wall facing the patient, with the chart reversed above or behind the patient, folds the path: the room needs to be roughly half as long. The distance is then measured from the eyes to the mirror and back to the chart. The room layout guide covers the furniture footprint that has to fit alongside it.
Questions about chart and lighting.
How bright should a visual acuity chart be?
ISO 8596 and ISO 10938 specify a chart background luminance between 80 and 320 cd/m², with 200 cd/m² recommended, and optotypes no brighter than 15% of the background.
Should the refraction room be dark during the eye test?
Room lighting affects the measured acuity, so the key is to keep it the same for every patient. Many rooms use dimmable lighting: darker for parts of the examination, a fixed level for acuity, and no light falling on the chart.
What distance should the eye chart be from the patient?
The distance the chart is designed for, commonly 6 m (20 ft); 4 m charts are also common. With a mirror, the room can be roughly half as long because the optical path is folded.
Does the refraction unit light the chart?
No. The lighting built into a refraction unit lights the working area and controls. The chart is illuminated by its own cabinet, projector or screen.
Planning the room around the chart?
Send us the room length, whether you will use a mirror, and the instruments. We reply with the furniture that fits and a quotation.