Chromatic aberration

Recall from Section 4.1 that light energy is usually a jumble of waves with a spectrum of wavelengths. You have probably seen that the colors of the entire visible spectrum nicely separate when white light is shined through a prism. This is a beautiful phenomenon, but for lenses it is terrible annoyance because it separates the focused image based on color. This problem is called chromatic aberration.

Figure 4.17: Chromatic aberration is caused by longer wavelengths traveling more quickly through the lens. The unfortunate result is a different focal plane for each wavelength or color.

Figure 4.18: The upper image is properly focused whereas the lower image suffers from chromatic aberration. (Figure by Stan Zurek, license CC-BY-SA-2.5.)

The problem is that the speed of light through a medium depends on the wavelength. We should therefore write a material's refractive index as $n(\lambda)$ to indicate that it is a function of $\lambda$. Figure 4.17 shows the effect on a simple convex lens. The focal depth becomes a function of wavelength. If we shine red, green, and blue lasers directly into the lens along the same ray, then each color would cross the optical axis in a different place, resulting in red, green, and blue focal points. Recall the spectral power distribution and reflection functions from Section 4.1. For common light sources and materials, the light passing through a lens results in a whole continuum of focal points. Figure 4.18 shows an image with chromatic aberration artifacts. Chromatic aberration can be reduced at greater expense by combining convex and concave lenses of different materials so that the spreading rays are partly coerced into converging [303].