Initial Duochrome test, also known as Bichrome test, consists of two sides of different colours, red and green. Chromatic aberration, the basis of the test, occurs because different wavelengths of light are bent to a different extent. The longer wavelength (red) is refracted less than the shorter (green).Test tries to determine the correcting spherical lens power monocularly first. The duochrome test should be used after monocular refraction and it represents the endpoint procedure of the tested eye. Prism – dissociated Duochrome test is used to equalize the stimulus to accommodation for the two eyes under binocular conditions. This test is initially used for presbyopes who need near correction.
Equipment and set-up:
When testing monocularly, one eye must be occluded. Ask the client to hold the test at 40 centimetres, wearing full correction for near. The client is asked to compare the two sides for their intensity, either the red/green colours or the letters. If the letters on the red side stand out more, add minus power; if the letters on the green side stand out more, add plus power. Neutrality is reached when the letters on both backgrounds appear equally distinct. When both eyes are tested, have both eyes uncovered. Put 6 prisms base down in front of the left eye, so the client with their left eye see the bottom line and with their right eye the upper line. Repeat the procedure again as was done monocularly.
Interpreting the results:
To check if it was done right, occlude one eye first and ask the client to find at which distance the random text is seen well. Then occlude the other eye, the distance at which the text is seen well should remain about the same.
If more than +/- 0.50 DS is required to balance, this usually indicated the duochrome test is not reliable for this client and should be ignored or the initial refraction is not right.
Prisms can sometimes lead to inaccuracy because the prism can detract from quality of the image. Therefore, it is sometimes recommended instead of putting six prisms up in one eye, use three prisms (RE 3.00 base down, LE 3.00 base up) on both eyes. In either case, the right eye will see the upper image.