Optics: measuring and testing – Focal position of light source
Reexamination Certificate
2003-08-29
2004-11-16
Casler, Brian L. (Department: 3737)
Optics: measuring and testing
Focal position of light source
C356S125000, C359S641000, C351S205000
Reexamination Certificate
active
06819413
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
1. Technical Field
This invention pertains to the field of wavefront sensing methods and devices, and more particularly, wavefront sensing methods and devices used to measure the optical quality of an optically transmissive system or device, for example, an optical component such as a lens.
2. Description
A light wavefront may be defined as the virtual surface delimited by all possible rays having an equal optical path length from a spatially coherent source. For example, the wavefront of light emanating from a point light source is a sphere (or a partial sphere where light from the point light source is limited to emission along a small range of angles). Meanwhile, the wavefront created by a collimating lens mounted at a location one focal length away from a point source is a plane. A wavefront may be planar, spherical, or have some arbitrary shape dictated by other elements of an optical system through which the light is transmitted or reflected.
A wavefront analysis system, including a wavefront sensor, may be used to measure characteristics of an optically transmissive system by detecting the wavefront of light emerging from the system and comparing it to some expected ideal wavefront (planar, spherical, etc.). The optically transmissive system might be a single component or may be very complex, such as a transmissive optics system, (e.g., a collimating lens; ophthalmic lens). The differences between the ideal expected wavefront and the actual measured wavefront are caused by optical aberrations of the system under test (SUT).
A number of different wavefront sensors and associated methods are known. Among these are interferometers and the Shack-Hartmann wavefront sensor. Each of these systems will be described briefly below. A more detailed discussion of wavefront sensing techniques may be found in “Introduction to Wavefront Sensors,” 1995, Joseph M. Geary, SPIE Press.
Interferometers
An interferometer is an instrument that uses interference of light waves to detect the relative wavefront difference between a test light beam and a reference beam. Interferometric methods of sensing a wavefront are highly sensitive but very limited in dynamic range. A typical interferometer can only directly measure optical path differences of less than one wavelength—a 2&pgr; phase ambiguity exists beyond the one wavelength point. If the optical path difference is greater than one wavelength, then the correct phase difference is often inferred computationally using phase unwrapping techniques. However, real optical configurations can be constructed where these techniques are likely to fail. Other limitations of interferometric techniques include the necessity of relative stability of the reference and test beam paths. This means that any vibration in the test instrument leads to a degradation of the measurement accuracy.
Shack-Hartmann Wavefront Sensors
A Shack-Hartmann wavefront sensor is a device that uses the fact that light travels in a straight line, to measure the wavefront of light.
FIG. 2
shows a basic configuration of a Shack-Hartmann wavefront sensor
200
. The Shack-Hartmann wavefront sensor
180
comprises a lenslet array
182
that breaks an incoming beam into multiple focal spots
188
falling on an optical detector
184
. Typically, the optical detector
184
comprises a pixel array, for example, a charge-coupled device (CCD) camera. By sensing the positions of the focal spots
188
, the propagation vector of the sampled light can be calculated for each lenslet of the lenslet array
182
. The wavefront can be reconstructed from these vectors.
However, Shack-Hartmann wavefront sensors have a finite dynamic range determined by the need to associate a specific focal spot to the lenslet it represents. A typical methodology for accomplishing this is to divide the detector surface into regions (called “Areas Of Interest” [AOIs]) where the focal spot for a given lenslet is expected to fall. If the wavefront is sufficiently aberrated to cause the focal spot to fall outside this region, or not be formed at all, the wavefront is said to be out of the dynamic range of the sensor.
FIG. 3
shows an example of a Shack-Hartmann wavefront sensor
300
in an out-of-range condition.
In practice Shack-Hartmann wavefront sensors have a much greater dynamic range than interferometric sensors. This range may be tens to hundreds of waves of optical path difference. However, this dynamic range is still insufficient to characterize many real optics.
Other Wavefront Sensing Technologies
Other sensors such as the Moire Deflectometer have a higher dynamic range, but lack the sensitivity necessary for accurate measurement of most transmissive optical elements.
Both optical and computational methods have been used to extend the dynamic range of wavefront sensing devices. Some example computational methods include Spot Tracking, Phase Unwrapping, and Angular Spectrum Propagator Reconstruction.
Spot Tracking
This method extends the dynamic range of the Shack-Hartmann wavefront sensor in the case where the wavefront being measured starts out within range and then drifts out of range over a period of time. This case exists for many optical configurations where a lens moves within the optical setup or a component changes optical characteristics due to some cause such as material heating or deformation. Spot tracking is accomplished by comparing current positions of focal spots to positions recorded in a previous frame. The previous positions are used as a starting point for locating the spots after an incremental movement. As long as the frames are taken frequently enough, then it is computationally simple to keep track of them. This technique has been known since at least 1993 (A. Wirth, A Jankovics, F. Landers, C. Baird, and T. Berkopec, “Final report on the testing of the CIRS telescopes using the Hartmann technique,” Tech. Rep. NAS-31786, Task 013 (Adaptive Optics Associates, Cambridge, Mass. 1993)). A limitation to this approach is that the incident wavefront must start out within range.
Phase Unwrapping
In this technique the focal spot to lenslet mapping is inferred using techniques similar to those used in interferometry. This technique is described in “Dynamic range expansion of a Shack-Hartmann sensor by use of a modified unwrapping algorithm,” by J. Pfund, N. Lindlein, and J. Schwider, Optical Society of America, 1998.
Angular Spectrum Propagator Reconstruction
Described by “Algorithm to increase the largest aberration that can be reconstructed from Hartmann sensor measurements,” by M. Roggemann and T. Shulz, Applied Optics, Vol 37, No 20, 1998, this technique is computationally expensive and therefore inappropriate for many measurement applications.
While there are means for tracking and adjusting the positions of these AOIs (as described previously), the simplest, most robust calculations are achieved for the case where a single mapping of lenslets onto the pixels can be maintained. For example, U.S. Pat. No. 5,825,476 discloses a method that uses a missing focal spot to identify the central AOI, and then tracks all the other focal spots using this missing data. However, if there is a speck of dust on the part under test, this easily fools the identification of this missing spot, leading to inaccurate results.
U.S. Pat. No. 6,550,917, the entire contents of which are hereby incorporated by reference for all purposes as if fully set forth herein, discloses a means for extending the dynamic range of a sensor by adjusting the spherical radius of curvature of a reference sphere to match the effective defocus of the optical system under test (in that case, an eye). While a similar scheme can be applied to testing transmissive optics, the requirements for testing an eye en vivo are significantly different from those of measuring a fixed lens or optical element. For an intraocular lens, the focal length in air can be as short as 10 mm, necessitating the use of a different optical testing method.
Without some tracking scheme, the focal spot moves either
Copland Richard J.
Hamrick Daniel R.
Neal Daniel R.
Rammage Ron R.
Topa Daniel M.
Casler Brian L.
Sanders John R.
Volentine Francos & Whitt PLLC
Wavefront Sciences Inc.
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