A duochrome test is a test commonly used to refine the final sphere in refraction, which makes use of the
longitudinal chromatic aberration
Longitudinal is a geometric term of location which may refer to:
* Longitude
** Line of longitude, also called a meridian
* Longitudinal engine, an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicl ...
of the eye. Because of the chromatic aberration of the eye, the shorter wavelengths (green) are focused in front of the longer red wavelengths.
Testing
The patient is asked to compare the clarity of the letters on the green and the red side. If the letters of the green side are clearer +0.25 D sphere is added and if the letters on the red side are clearer -0.25 D sphere is added.
With optimal spherical correction, the letters on the red and green halves of the chart appear equally clear.
Because this test is based on chromatic aberration and not on color discrimination, it is used even with people having
color vision deficiency
Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...
.
The eye with overactive accommodation may still require too much minus sphere in order to balance the red and green.
Cycloplegia
Cycloplegia is paralysis of the ciliary muscle of the eye, resulting in a loss of accommodation. Because of the paralysis of the ciliary muscle, the curvature of the lens can no longer be adjusted to focus on nearby objects. This results in sim ...
may be necessary.
The duochrome test is not used with patients whose
visual acuity
Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal ...
is worse than 20/30 (6/9), because the 0.50 D difference between the two sides is too small to distinguish.
See also
*
Eye examination
An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, op ...
*
Subjective refraction
Subjective Refraction is a technique to determine the combination of lenses that will provide the best corrected visual acuity (BCVA). It is a clinical examination used by orthoptists, optometrists and ophthalmologists to determine a patient's ne ...
References
{{Reflist, refs
[{{cite book, last=Myron Yanoff, Jay S. Duker, title=Ophthalmology, year=2009, publisher=Mosby Elsevier, isbn=9780323043328, pages=67, edition=3rd]
Diagnostic ophthalmology
Medical tests