Chromatic aberration, the basis of the test, occurs because different wavelengths of light are bent to a different extent. The duochrome test involves the projection of black letters or symbols onto a bipartite green (at approximately 535nm) and red (at approximately 620 nm) background. The red and green wavelengths are dioptrically equidistant, approximately 0.25 D, from the yellow wavelength (570 nm). The longer wavelength (red) is refracted less than the shorter (green). It is assumed that best vision is attained when the yellow wavelengths are focused on the retina. During the final sphere adjustment its important to make sure the we find the least minus that a patient will accept to make sure that they are not accommodating in order to prevent them from getting headaches, eye strain, or having uncomfortable near vision. The eye typically focuses near the midpoint of the spectrum, between the red and green wavelengths. With optimal spherical correction, the letters on the red and green halves of the chart appear equally sharp.
The duochrome test is not used with patients whose visual acuity is worse than 20/40, because the 0.50 D difference between the 2 sides is too small to distinguish.
- The patient is first assessed with refraction and given the best correction.
- With that correction in the phoropter or trial frames in place, the duochrome chart can then be projected.
- The test starts by occluding one eye (left eye first by convention)
- Room lights must be turned off in order to allow the pupil to dilate which would allow for increase chromatic abberation of the eye
- Ask the patient to compare the letters on green and red sides of the test
- If the letters on the red side stand out more continue to add more minus power using -0.25 DS steps
- If the letters on the green side stand out more continue to add more plus power using +0.25 DS steps
- Neutrality is reached when the letters on both the red and the green background appear equally distinct.
- IF more than +/- 0.5D is required to balance then the test is unreliable
RAM–GAP mnemonic: Red Add Minus; Green Add Plus
The rule of thumb in these cases is to leave younger patients one step into the green and older patients one step into the red.
Pseudophakic patients should be left one step into the green.
The Red-Green duocohrome test is traditionally recommended for use both prior to determining the cylindrical component of the refraction using a Jackson Cross Cylinder (JCC) and subsequent to this determination
The concept of the duchrome test can also be used as a screening tool to assess an individuals global refractive error. This is a subjective test that in theory would be able to quickly give us information if the patient is hyperopic, myopic or emmetropic, however would not be able to tell us the actual refractive error. The test involves using a green and red filter that will be placed over the patients eyes with a standard Snellen eye chart. One eye is occluded and the green filter is placed over the other eye. The patient then reads the chart from top to bottom. The same is repeated with the red filter over their eye. If they are able to read more letters with the red filter on they are more likely myopic and if they can read more letters with the green filter on they are more likely hyperopic. If they can read the same amount of letters with each filter they are likely emmetropic. Due to other modalities this is not commonly used in the clinical setting.