what is aberration
Chromatic aberration comes in two types: axial and longitudinal.
To reduce chromatic aberration, a higher quality optical device would use a special combination lens called an achromatic lens or achromat for short. The simplest such system consists of two lenses made of two different glasses: a converging lens made of crown glass (the very common kind of glass used in windows) and a diverging lens made of flint glass (the slightly fancier kind of glass used in chandeliers and crystal decanters).
An apochromatic lens corrects for both chromatic and spherical aberration.
chromatic aberration in the eye
The apparent three dimensional appearance of this album cover is an optical illusion caused by chromatic aberration in the eye.
- The change in focal length from 390 nm to 760 nm is almost 0.7 mm, or more than twice the thickness of the retina. Monochromatic illumination gives higher visual acuity than white light. The best results are obtained with yellow (cf. BluBlocker).
- Corrective lenses are optimized for yellow light, but at night the eye is optimized for blue light. This produces a refractive error of about 1 D, which means that at night one should wear glasses with an additional correction of −1.0 D.
- The red-green duochrome test is based on a monocular endpoint in which each eye is tested separately. It is a subjective test that requires responses from the patient and is used to refine the spherical endpoint. 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). 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. http://www.ncbi.nlm.nih.gov/pubmed/1469739
- Rabbetts (1998) estimated that yellow light with a wavelength of 570nm was preferred by the eye. If this wavelength is used as a reference point, as it often is for tungsten light, green light with a wavelength of 535nm focuses 0.25 D in front of the retina and red light with a wavelength 620 nm focuses 0.25 D behind it. So, by using appropriate filters a test may be constructed that, by comparing the clarity of targets presented on red and green backgrounds, allows the practitioner to focus the yellow refer- ence wavelength accurately on the retina and achieve maximum acuity. Such a test is known as the duochrome test (Figure 10.3). The exact filters used with the duochrome test are specified in BS 3668:1963. Subjective Refraction: Principles and Techniques for the Correction of Spherical Ametropia, Andrew Franklin
- The duochrome test also works for color blind subjects.