Duochrome test

What this test screens for and who it's useful for

This online red–green duochrome test is a simple screening tool for refractive error. By comparing how sharp black letters look on a red background versus a green background, it can suggest whether your eye is likely nearsighted (myopia), farsighted (hyperopia), or close to well-focused (emmetropia). The test uses the eye’s natural chromatic aberration: red light focuses slightly behind green light. Where the letters look clearer offers a clue about your focus.

This tool may be useful if you’re curious about your distance focus, you wear glasses or contact lenses and want a quick check, or you’ve noticed blur at distance or near. It can also hint at whether your current correction may be slightly too strong or too weak. People with astigmatism can still try the test, but results may be mixed. This is not a diagnosis and does not replace a comprehensive eye exam with an optometrist or ophthalmologist.

If you have sudden vision changes, eye pain, flashes of light, many new floaters, or other urgent symptoms, seek in-person care promptly.

How to take the test correctly

• Lighting and glare: Use bright, even room lighting. Avoid reflections on your screen. Do not use Night Shift, blue light filters, or “dark mode” during the test.
• Screen settings: Set screen brightness to a comfortable, consistent level (not dim). If your device has color temperature settings, choose a neutral setting. Disable “True Tone”–type automatic color adjustments if possible.
• Viewing distance: Sit at about arm’s length from a computer screen (roughly 20–28 inches). If using a phone, hold it at a typical reading distance without leaning in.
• Wear your usual correction: If you normally wear glasses or contact lenses for distance vision, wear them. With progressives or bifocals, look through the distance portion.
• Test one eye at a time: Cover your left eye with your palm (do not press) or a clean tissue and view the chart with your right eye. Then cover your right eye and test the left. You may also look with both eyes together afterward.
• Adjust size if needed: If you can change the letter size or page zoom, set it so the letters are small but still readable—challenging, not oversized.
• What to look for: Focus on the sharpness and crispness of the black letters on the red half versus the green half. Decide which side looks clearer, or if both appear about the same.

Take short breaks if your eyes feel tired or dry. Blink normally. Try to keep your head and screen straight to avoid tilting that could influence clarity.

How to read your results

• Sharper on Red: If the letters look crisper on the red background, your focus may be slightly in front of the retina, which can be consistent with nearsightedness (myopia) or with a distance prescription that is a bit too strong (over-minus). This can also happen if you are accommodating (focusing) more than usual during the test.
• Sharper on Green: If letters look crisper on the green background, your focus may be slightly behind the retina, which can be consistent with farsightedness (hyperopia) or with a distance prescription that is a bit too weak (over-plus). It may also reflect reduced focusing effort.
• About the Same on Red and Green: Similar clarity on both sides suggests you may be near optimal focus for distance (emmetropia) with the eye or correction you used.
• Neither side is clearly sharp, or results are inconsistent: This can occur with uncorrected refractive error, astigmatism, dry eyes, fatigue, or suboptimal test conditions. It does not identify the exact cause.

Small differences between red and green can be normal. The duochrome test is most sensitive to subtle over- or under-correction, not to exact prescriptions. Only a comprehensive eye exam can determine your full refractive status and check eye health.

What to do if your result is poor

Don’t panic—many people experience blur from common refractive errors that are correctable with glasses or contact lenses. If one eye or both eyes consistently see better on red or on green, or if both sides look blurry, consider booking a comprehensive eye exam with an eye doctor. Bring your current glasses or contact lens prescription to the visit and describe your online results.

If you already wear correction and the duochrome suggests a consistent bias (for example, always clearer on red), your prescription may need adjustment. Do not change your lenses based on an online test alone. An optometrist or ophthalmologist will refine your prescription, check for astigmatism, and evaluate eye health conditions that online tools cannot detect.

Seek prompt in-person care if you notice sudden vision loss, new distortion, eye pain, flashes, or many new floaters.

Limitations of online tests

• Not a diagnosis: The duochrome is a screening tool. It cannot diagnose myopia, hyperopia, astigmatism, or eye disease, and it cannot provide an exact prescription.
• Device and color variability: Screen size, brightness, color calibration, and software color profiles vary widely and can shift results.
• Viewing conditions matter: Distance, lighting, fatigue, dry eyes, and even posture can affect clarity and your responses.
• Astigmatism and other factors: Astigmatism can cause mixed or unstable results. The test does not measure astigmatism power or axis.
• Not ideal for all users: Children, people with low vision, or anyone with eye symptoms should have in-person exams. Color-vision differences usually do not prevent using the test because you are judging sharpness, not color names, but they can still influence perception.

Disclaimer: This online duochrome tool is a screening aid only and does not replace a comprehensive, professional eye examination. For personalized advice and an accurate prescription, schedule an in-person eye exam.