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Optical Metrology

In order to understand how an optical element is affecting performance of an optical system it is necessary to measure optical quality (wavefront aberration) of the element. We developed a high resolution Shack-Hartmann wavefront sensor to measure various optical components including ophthalmic lenses such as contact lenses and intraocular lenses.

  1. High Resolution Shack-Hartmann Wavefront Sensor
  2. Measuring Customized Soft Contact Lenses
  3. Measuring Intraocular Lenses

1. High resolution Shack-Hartmann wavefront sensor

We have designed a high resolution Shack-Hartmann wavefront sensor to characterize the aberrations for both contact lenses and IOLs.


Figure 1: A schematic diagram of the Shack-Hartmann wavefront sensor, illustrating both the contact lens and IOL measurement arms.

A schematic diagram is shown in the above figure for the layout of our high resolution wavefront sensor. The sensor mainly includes wet cells containing either a soft contact lens or an IOL and a high resolution lenslets array. The soft contact lens was immersed in saline solution to prevent lens dehydration and deformation. The IOL was also placed in saline solution to mimic the conditions that the lens would encounter in a living eye.

2. Measuring Customized Soft Contact Lenses

The Yoon lab has been working to develop customized soft contact lenses (CSCL) containing irregular surface profiles. These lenses may be considered a practical, non-surgical correction method to compensate for the eyes' higher order wavefront aberrations (HOA). Conventional methods for measuring contact lens power, such as lensometry, cannot evaluate the HOA generated by the irregular surface profile. We have been reliably measuring the HOA in these custom contact lenses using the high resolution wavefront sensor. Because of the smaller refractive index difference between the soft contact lens (nlens) and lens solution (nsolution) than between the contact lens and air, the measured wavefront (Wwater) will be less than the lens generated aberrations in air. By multiplying the measured wavefront with a conversion factor (CF), the lens generated aberrations in air can be calculated. The conversion factor (CF) can be calculated according to the following equation.

Based on this method, customized soft contact lenses were measured. The figure below illustrates the designed and the measured values for a customized soft contact lens (lens aberrations have been converted to the aberrations in air). The designed and measured aberrations for the lens correlate well.


Figure 2: Designed and measured wavefront aberrations for a customized soft contact lens.

3. Measuring Intraocular Lenses

In addition to measuring customized contact lenses the high resolution wavefront sensor has been used to measure two popular IOL's that are on the market today for the correction of presbyopia; the ReZoom lens from Advanced Medical Optics, and the ReSTOR lens from Alcon. In order to better understand how these lenses are aiding people's vision, we have measured them using the high resolution Shack-Hartmann wavefront sensor. We were able to measure the higher order aberrations in these lenses to see how they increase depth of focus which helps to restore a person's ability to see form far to near. Both lenses use various amounts of higher order spherical aberrations and depth of focus as you can see in the figures below:


Figure 3: Wavefront maps and profiles of measured higher order spherical aberrations for two multifocal IOL's on the market today.

The above images represent the wavefront map, and below it is the profile of the wavefront for the two multifocal lenses. The spherical aberration terms in each lens interact to create modulated wavefront profiles seen below the color maps. The resultant image quality can be seen in the below convolved letters.


Figure 4: The letter E convolved with the higher order spherical aberrations of each multifocal IOL showing the predicted optical quality looking far (left) to near (right).

Both lenses provide good image quality and depth of focus, but each lens differs in what it is trying to optimize with its corrections. It is the goal of the work to further optimize multifocal lens designs using various amounts of higher order spherical aberrations and possible other aberrations in order to maximize the image quality and depth of focus for patients suffering from presbyopia and not wanting to rely on glasses or contacts to improve their vision.

The Shack-Hartmann wavefront sensor is a useful tool for the measurement of optical elements. It is the goal of the Yoon lab to perfect not only the wavefront sensor for more accurate and repeatable measurements but also to improve upon current corrective lens technologies to further advance visual quality and care.

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