Within the field of optometry, there exist many devices that are used to assess the direction of fixation of an eye. An example of such a device is described in U.S. Pat. No. 6,027,216, the contents of which are hereby incorporated in their entirety. Many such devices utilize either a scanning laser beam to perform measurements or rely on measurements using polarized light sources.
Methods of scanning a laser beam to perform measurements typically involve the mechanical movement of an optical device. For retinal birefringence scanning, there is a mirror that is both tilted and spinning at a high speed (e.g. 12,000 rpm). When utilizing mechanical movements of optical devices, vibrations can present significant complexities to scanning instruments. The vibrations must be kept low enough so as not to impact the measurements intended by the instrument.
There are other complications with using mechanical movements for scanning optical instruments, such as:
Lifetime of the assembly—motors have a shorter life span than virtually all other components.
Fabrication—tight tolerances are required to achieve balance.
Assembly—highly skilled personnel are required to assemble the mechanism and make adjustments to minimize vibrations: this is unlikely to be an automated process.
Noise—even relatively quiet motors will make an audible sound that can be distracting to a patient.
Safety—the failure of a component spinning at 12,000 rpm can pose significant risk to the rest of the instrument if there is a failure of part of the mechanism (due to issues such as fatigue).
Cost—the combination of the above issues generates significant requirements on the design of the instrument that add time and materials to the production process, increasing overall cost.
Furthermore, methods of measuring fixation that use polarization techniques depend heavily on measurements of relative intensity of light. The measurement of a change in polarization is ultimately reduced to a change in intensity of light—the change in the polarization state is measured and estimated proportional to the intensity of the measured signal. Such changes in light intensity must be measured in the environment containing very low background light in order for the result to be accurate. In practice, the signal-to-noise ratio requirements for polarization measurements of signals reflecting from the retina can be challenging to meet.