Aberrations - Chromatic

Aberrations are caused by…

• Way light is refracted

• Index of medium

• Location of object point

Types of aberrations

• Chromatic: Dependent on lens material

• Monochromatic: Dependent on lens design/form

Chromatic aberration

Refractive indices are wavelength dependent and higher for short wavelengths

Caused by properties of the lens material

Blue will always focus in front of red

If index of refraction for blue and red light are similar…

They focus closer → Good

Longitudinal Chromatic Aberration (LCA)

Dioptric distance between red and blue focus, also called axial chromatic aberration

LCA equation

LCA = P / V

P: Power of lens

V: Abbe value

Duochrome (Red/Green) Test

Makes use of the eye’s LCA

Transverse Chromatic Aberration (TCA)

From differences in prismatic effects of lens for various wavelengths, also called lateral chromatic aberration

Cause colored fringes around borders, blurs image, and can decrease performance

TCA equation

TCA = delta / V

Delta: Prism power

V: Abbe value

What is the TCA at OC

0

No prismatic effects

Achromatic doublet

Each lens of doublet must have diff Abbe value

Not a practical design for spec lenses

Front surface higher power/abbe → Net plus

Back surface higher power/abbe → Net minus

How to control chromatic aberration

• Use material with higher Abbe value

• Frame and pt PD same and centered vertically

• Shorten vertex distance