Question

What image does the Hartmann-Shack sensor make with light reflected through the eye? How does the shape of wave-fronts, (or degree of parallelism of the rays) exiting the eye affect the image? How can one tell the difference between a perfect eye and an abnormal eye by looking at the image?

Answer 1

Hartmann-Shack-Analyser determines the shape and distribution of distorted wavefront. Consider a beam of parallel light passing through a vacuum - ideally, it consists of a pattern of phases moving uniformly at speed of light along the beam. When this beam passes through uniform medium i.e. change in refractive index - its speed will be reduced but phase pattern still moves uniformly. When moving in non-uniform medium, parts of the beam will be slowed more than others, leading to the distorted wavefront. Light reflected back out of the eye as a wave is capture and measured.

The lens focuses that light into a sharp image on the retina, which relays messages through the optic nerve to the brain. If the lens is cloudy from a cataract, the image you see will be blurry. Other eye conditions, such as myopia, cause blurry vision, too, but cataracts produce some distinctive signs and symptoms.

Signs and Symptoms of Possible Vision Problems

1. Severe, sudden eye pain.
2. Recurrent pain in or around the eye.
3. Hazy, blurred, or double vision.
4. Seeing flashes of light or sudden bright floating spots.
5. Seeing rainbows or halos around lights.
6. Seeing floating "spider webs"
7. Seeing a "curtain coming down" over one eye.
8. Sensing a "cup filling up with ink" in one eye
9. Unusual, even painful, sensitivity to light or glare
10. Swollen, red eyes
11. Changes in the color of the iris
12. White areas in the pupil of the eye
13. Sudden development of persistent floaters
14. Itching, burning, or a heavy discharge in the eyes
15. Any sudden change in vision
16. See also Normal Vision Changes to help you understand normal age-related changes in the eyes and your vision.

Waves change speed when they pass across the boundary between two different substances, such as light waves refracting when they pass from air to glass. This causes them to change direction and this effect is called refraction. Water waves refract when they travel from deep water to shallow water (or vice versa).