27/1/2015

Author: Matt Jaskulski, University of Murcia

Recently it has been proven that spherical aberration has influence over the depth of field (DOFi - our span of sharp vision when looking at a certain distance) and the accommodation lag (Bernal-Molina et al., OVS, 2014). We are currently researching if the longitudinal chromatic aberration (LCA) also plays a role in increasing the DOFi during accommodation, thus effectively extending its range.

We built a custom-designed adaptive optics system that can measure, save, compensate and induce in real time all of the aberrations (imperfections) of a subject's eye. Its three essential components are:

• An infrared diode - whose beam of light is directed into the eye. The light that is reflected from the retina and exits the eye through the pupil. It is then being measured by a Hartmann-Shack sensor.
• The Hartman-Shack type sensor - comprising of an infrared-sensitive camera and an array of over 1000 micro-lenses in front of it. It is used to precisely map the wavefront of infrared light originating from the eye. It is then saved as a Zernike-polynomial expansion.
• A deformable mirror – that is able to deform itself and assume any curved shape by means of 52 piezo-electric actuators beneath it's surface. Working in a closed data loop with the Hartmann-Shack sensor it can adapt itself to the aberrations of the eye contained in the measured wavefront and compensate them completely giving the subject perfect vision. It is also possible to give any subject the vision as seen through optics of an eye of another person or induce any previously stored wavefront aberration maps.

We use this system to research the effects of the longitudinal chromatic aberration (a phenomenon that arises from the fact that the spectrum of light our eyes are sensitive to doesn't focus exactly on the retina – the blue, short-wavelength light focuses in front, and the red, long-wavelength light behind it) on the DOFi and the lag of accommodation (another phenomenon that arises from the fact that our eyes accommodate, or bring objects into focus only the minimum required amount, doing the least possible effort). We discovered that this span is twice as large when considering red, green and blue light separately than when considering white light that we perceive in everyday life. In other words the DOFi due to the chromatic aberration (the dioptric span between the outer limits of the R and B DOFi) was on average 1.9 times larger than the DOFi found for the white light.

Image: Optotipes on a microdisplay as seen by the subject through the adaptive optics system.