The Eye Care Advocate

Duochrome Test: A Simple Trick to Aid Your Refraction

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by

Jason Searle
in , , ,

The duochrome test is a simple yet powerful tool in subjective refraction1. It helps fine-tune the spherical endpoint by using longitudinal chromatic aberration1,2. Chromatic aberration is a property within optics where different wavelengths of light focus at slightly different points2.

The test presents black letters or circles against a red-green split background2 (Figure 1). This makes use of the fact that red (longer wavelength) is refracted less than green (shorter wavelength)1. When balanced, letters on both sides appear equally clear, confirming the correct spherical prescription.

An image of the duochrome test display. A rectangle is split into two, with the top half in red and the bottom half in green. In both the top and bottom halves there are two black rings, with a smaller black ring concentrically inside.
Figure 1. An image of a duochrome test display. In this display the targets are black circles that appear on both a red and green background, split so red sits above the green.

Despite its simplicity, the duochrome test plays an important role in refining refraction, ensuring the best visual clarity while minimising unnecessary accommodation. Optometrists can use it before and after applying a Jackson Cross Cylinder (JCC) to ensure the circle of least confusion is correctly positioned1.

It’s fast, effective, and when used correctly, helps prevent over-minused or under-plussed prescriptions. Let’s first explore the optics behind the duochrome test and why red and green are used instead of other colour combinations.

Understanding the Optics of the Duochrome Test

As mentioned above, the duochrome test relies on a fundamental optical principle known as chromatic aberration1,2,4. This occurs because different wavelengths of light refract at different angles when passing through a lens (Figure 2)2,3. This includes the human eye. The test uses this effect to refine the spherical correction during subjective refraction4.

A ray diagram used to explain how chromatic aberration occurs and aiding the post on the duochrome test. There are two black beams (representing incident white light) entering a light purple lens, with the beam breaking into red, green and blue rays, each forming their own focal point. The blue focuses closer tot he lens and the red focuses further from the lens. The green focuses between the blue and the red.
Figure 2. A ray diagram demonstrating chromatic aberration. The individual wavelengths that make up the incident white light all refract at different rates as they pass through the lens. Each of these wavelengths then form their own focal points.

Why Red and Green and Not Red and Blue?

At first glance, it might seem logical to use red and blue, as they are the most dioptrically distant wavelengths in the visible spectrum. However, the duochrome test uses red (around 620nm)4 and green (around 535nm)4 for several key reasons:

Brightness Perception: Blue light appears darker to the human eye than green (Figure 3)5, making it harder for patients to accurately compare clarity between the two sides. Green is perceived as significantly brighter, and comparable to red, allowing a more reliable subjective response.

Figure 3. Whilst all three squares above have the same luminance, the human visual system perceives the blue to be darker than both the red and the green.

Minimising Colour Interpretation Errors: Since the test is based on longitudinal chromatic aberration, not colour perception, red-green contrast works best for maintaining clarity while reducing potential patient confusion.

Midpoint Focus: The yellow wavelength (around 570nm)4 sits at the dioptric midpoint between red (620nm) and green (535nm) (Figure 4), with a focal difference of approximately 0.44D between them. Since 0.25D is roughly half of this gap, adjusting by +0.25D or -0.25D brings the focus closer to balance in a predictable way. The yellow midpoint will indicate a circle of least confusion, whereby the blur of both the red and green will be equally minimal.

The colour spectrum of visible light. An arrow highlights yellow as being the midpoint between red and green which also corresponds to a midpoint focus of the light.
Figure 4. The colour spectrum of visible light. An arrow highlights yellow as being the midpoint between red and green. This corresponds to a midpoint focus of the light.

If the patient is near the correct spherical endpoint, small adjustments allow the midpoint/circle of least confusion to settle precisely on the retina. When the patient sees the targets on the green clearest, adding +0.25D moves the focal point forward. This ensures that the CLC sits correctly on the retinal plane. If red appears clearer, adding -0.25D shifts it backward, achieving the same goal. This makes the duochrome test both practical and efficient for finalising subjective refractions.

Simplified:

Whilst we have explained in detail above, this section breaks down how the duochrome test works in a simpler form.

In a well-corrected eye, the circle of least confusion should be on the retina. This will create small, yet equal amounts of blur on both the red and green targets. The targets will therefore be described as equally in focus. You can see this below in Figure 5.

Example of the duochrome test when the circle of least confusion sits on the retina. On the left, the duochrome appears clear. To the right is a schematic of the retina, with a green focus in front and a red focus behind. The circle of least confusion (in yellow) sits at the retina.

Figure 5. When the circle of least confusion sits on the retina, there is equal blur due to the blur of the green focus in front of the retina matches the blur of the red focus behind the retina. Note the duochrome test shows circle targets that are of equal clarity.

If the green background appears sharper, it suggests the green focal point is on the retina and that the circle of least confusion is behind the retina (Figure 6). This indicates the need for additional plus power to bring the circle of least confusion anteriorly onto the retina.

Figure 6. When the green focus sits on the retina, the circle of least confusion sits behind the retina. This suggests more plus is needed in the refraction to bring the circle of least confusion back on to the retina. Note the duochrome test shows that the circles are clearer on the green.

If the red background appears sharper, it suggests the red focus is on the retina and the circle of least confusion sits in front of the retina (Figure 7). This means a minus lens will be required to push the circle of least confusion back towards the retina.

Figure 7. When the red focus sits on the retina, the circle of least confusion sits in front of the retina. This suggests more minus is needed in the refraction to bring the circle of least confusion back on to the retina. Note that the duochrome circles are clearer on the red.

The duochrome test is a quick, effective way to ensure that the spherical correction is precisely calibrated – reducing accommodation stress and preventing over-minusing, particularly in myopic patients.

The Duochrome Test Procedure

Essentially, the duochrome test is a useful confirmation test to ensure that the circle of least confusion is positioned on the retina. Many optometrists tend to use this prior to Jackson Cross Cylinder (JCC) to ensure a good starting point to refine the astigmatic axis, but many also include it following the astigmatic power correction to ensure the circle of least confusion remains on the retina prior to the final power check6.

In most cases, you will want to use the duochrome test with what you believe to be the best vision sphere during a monocular refraction – as this test will help to confirm that result.

Step-by-Step Guide

  1. Occlude one eye
    The test must be performed monocularly to assess each eye individually.
  2. Reduce ambient light
    Lowering room illumination dilates the pupil, enhancing chromatic aberration for a clearer red-green distinction1,7.
  3. Use the duochrome test chart/area of the test chart
    Each test chart is different and some systems have a separate screen/projection for the duochrome test and others have a region on the chart with the duochrome test target. It is often easier for patients to determine clarity of letters over circles, but essentially you are looking for a chart that is split into red and green with bold, black targets of equal size to compare.
  4. Ask the patient:
    “Are the letters/circles clearer on the red or the green side, or are they the same?”

    If the patient has abnormal colour vision, the test can still work as the optics will be unaffected1. However, you will need to ask if the targets are clearer on the top/bottom or left/right of the screen as they will not be able to perceive the colour of the background the targets are on. That said, the results may be less reliable and, if protanopic, they will likely report the red side to be duller1.
  5. Adjust spherical power based on the response:
    – If green appears clearer, add +0.25D to move the focal point forward.
    – If red appears clearer, add -0.25D to shift the focus backward.
  6. Continue adjustments:
    If clarity continues to favour one background, repeat +0.25D or -0.25D steps until balance is achieved1.
  7. Consider the reliability of results:
    If more than ±0.50D is required for balance, the test may not be dependable for this patient. (More information on why this may occur during the duochrome test can be found below).
  8. Final verification:
    Use the plus-minus technique to confirm whether the adjusted spherical power offers the best subjective visual clarity. Do not overrule subjective response to clarity based on the final duochrome test result1!
  9. Final Notes:
    If there is no clear end-point where the targets look the same (i.e. introducing a ±0.25 lens flips between red and green being clearer) then you can use the following as guidance.

    In younger patients, mild over-minusing may not always need correction, as their accommodation can compensate.

    However, in presbyopic and pseudophakic patients, it’s better to leave them slightly towards the red, ensuring comfort and reducing unnecessary accommodative effort

Record Keeping

In many cases, optometrists do not record the results of the duochrome test prior to moving on to Jackson Cross Cylinder. However, some may choose to record the result after completing their monocular subjective refraction.

If you wish to record your duochrome result (or if you need to for your particular optometry course/exams) then the following method is often recorded:

FINAL OPTICAL PRESCRIPTION : ACUITY : DUOCHROME TEST RESULT1

For example:

Right Eye: -2.00 / -0.50 x 45 = 6/4 Duochrome: R = G

or

Right Eye: +2.00 / -0.25 x 25 = 6/6 Duochrome: R>G

Where the equals (=) sign indicating the duochrome test colours were balanced and the more than (>) sign indicating that one colour was left clearer than the other. In this case “R>G” indicates red was clearer than the green1.

Having the results of the duochrome test recorded can significantly help you understand how the patient will get on with their optical prescription and offer insights into the next steps should they not tolerate the prescription.

Common Errors When Performing the Duochrome Test

Even though the duochrome test is a quick and handy tool for refining spherical correction, several factors can lead to misinterpretation or inaccurate adjustments. Recognising these pitfalls will ensure your end point will be optimal for your patient.

Excessive Adjustments Beyond ±0.50D
If the patient requires more than ±0.50D to achieve neutrality, it suggests the test is unreliable for this individual. Excessive power changes indicate either a poor initial refraction or an accommodation-related discrepancy.

Misleading Results in Patients with Ocular Pathology2
The duochrome test relies on chromatic aberration, but in cases of brunescent cataracts1,2, corneal opacities2, or dyschromatopsia2 (abnormal colour vision), the ability to discern differences in clarity may be impaired. In bruenscent cataracts, green and blue wavelengths are absorbed1, often making the red be perceived as clearer, even when it may not be. Post-corneal transplant patients may also struggle with the test2, making results unreliable.

Neglecting Patient Preference / Subjectivity
Some patients may not be able to perceive a difference, even if a difference is apparently. Similarly, they may have a preference to one colour or another (often red4) and as such still repeat that one colour is clearer, even if it is more blurred1. If the test isn’t making sense compared to your other refraction tests, don’t rely on the duochrome test results.

Over-Minusing Due to Accommodation2
In patients with strong accommodative response, particularly younger individuals, the eye may still attempt to focus dynamically2. This can result in over-minus prescriptions, as the patient may keep compensating, making the red background appear clearer than it actually is. Cycloplegia may be required to ensure accurate results.

Relying on the Duochrome Test Alone1
Whilst the duochrome test is a quick and effective test to help identify the endpoint of the best vision sphere, it should not be used alone. Utilise this result alongside the other subjective tests performed, using the duochrome test as additional evidence/justification to your end result.

Not Keeping Balance2
In patients who are binocular, you will want to ensure that the prescription is balanced between the two eyes2; else you will find that one eye will accommodate or blur more than the other. If deciding to leave a patient slightly on the green, ensure the other eye matches, else this could lead to binocular discomfort and may lead to a non-tolerance to your prescription.

Advantages and Limitations

The duochrome test is widely used in subjective refraction, but like any method, it has its advantages and limitations. Understanding its strengths and weaknesses ensures optometrists use it effectively within the broader refractive process.

Advantages

  • Fast & Efficient: Quickly refines the final spherical correction.
  • Minimises Accommodation Influence: Helps prevent over-minusing, especially in younger patients.
  • Works Well Across Most Refractions: Effective for both myopes and hyperopes.
  • Simple Adjustments: The +0.25D / -0.25D shifts correspond practically to the 0.44D difference between red and green focal points1.
  • Useful in Routine Practice: Applied towards the end of subjective refraction to confirm the spherical endpoint.

Limitations

  • Unreliable in Low Visual Acuity: Patients with VA worse than 6/9 (20/30) may struggle to distinguish clarity differences. This is due to the reduced acuity making it more difficult to distinguish subtle blur from clear lines.
  • Affected by Ocular Pathology: Cataracts, dyschromatopsia, and corneal irregularities can make results misleading.
  • Accommodation Instability Can Skew Results: If the patient is still accommodating, the test may suggest excessive minus correction. Cycloplegic refraction may be needed.
  • Lens Coatings Impact Perception: Certain anti-reflective coatings and filters alter chromatic aberrations3 and contrast sensitivity8, affecting test reliability. Be aware if your trial lenses/phoropter have additional coatings.

While highly effective, the duochrome test is best used as a refinement tool rather than a standalone method. Combined with other refractive techniques, it ensures precise spherical correction without reliance on a single test.

Conclusion

The duochrome test is a valuable tool in subjective refraction, allowing optometrists to refine the final spherical correction with precision. By leveraging chromatic aberration, it provides a straightforward way to ensure the circle of least confusion (CLC) sits exactly on the retinal plane, minimising over-minus and under-plus prescriptions.

Understanding the optics behind the test, particularly the 0.44D separation between red and green focal points, ensures optometrists can apply +0.25D or -0.25D adjustments efficiently. While the test is fast, practical, and widely used, it’s essential to recognise its limitations, especially in cases of low visual acuity, excessive accommodation, or ocular pathology.

By using the duochrome test alongside other refraction techniques, optometrists can achieve the most accurate correction, ensuring optimal comfort and clarity for patients.

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Further Reading/Sources

  1. Elliott DB (2007). Determination of the refractive correction. Clinical Procedures in Primary Eye Care: 3rd Edition. Philadelphia: Butterworth Heinemann pp. 111-112.
  2. Chukwuyem EC, Musa MJ, and Zeppieri M (2023). Subjective refraction technique: duochrome test. StatPearls [Online]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK592409/ [Accessed: April 7th 2025].
  3. Manion GN, and Stokkermans TJ (2023). Chromatic aberrations. StatPearls [Online]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK597386/ [Accessed: April 7th 2025].
  4. Rolandi R, Zeri F, Duse A, Rizzo GC, Ponzini E, and Tavazzi S (2024). Red and green defocus curves and duochrome test in different age groups. Journal of Optometry 17(3): 100497.
  5. Olguntürk, N. (2021). Psychological color effects. In: Shamey R (eds) Encyclopedia of Color Science and Technology. Berlin, Heidelberg: Springer. pp 1-4.
  6. Gantz L, Schrader S, Ruben R, and Zivotofsky AZ (2015). Can the red green duochrome test be used prior to correcting the refractive cylinder component? PLoS One 10(3): e0118874.
  7. Rosenfield M, Aggarwala KR, Raul C, and Ciuffreda KJ (1995). Do changes in pupil size and ambient illumination affect the duochrome test? Journal of the American Optometric Association 66(2): 87-90.
  8. Citek K (2006). Contrast sensitivity function. In: Duckman RH (eds) Visual Development, Diagnosis, and Treatment of the Pediatric Patient. Philadelphia: Lippincott Williams & Wilkins. pp 52-68.

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