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Reduce measurement time by up to a factor of 3 compared to scanning methods
High accuracy of λ/10 or below
Only method to measure aspheric shapes with a high slope of aspheric departure
Computer Generated Hologram metrology is a promising solution to several of the drawbacks associated with traditional metrology techniques for aspheric lenses. Aspheres are vital to a variety of applications because they allow for a reduction in spherical aberrations and can achieve a low f/#. However, measuring aspheres is much more difficult than standard spherical lenses. Computer Generated Holograms (CGHs) provide an effective means of quantifying the surface irregularity of aspheric lenses by converting spherical reference wavefronts into aspheric wavefronts that match the surface sag of the asphere. Aspheric surfaces are typically measured using scanning equipment, but using CGHs allow them to be measured more quickly and with higher accuracy.
Background information on interferometry and holography is necessary for understanding CGHs metrology:
To measure the surface power and irregularity of a spherical lens with high precision (λ/10 - λ/20), a Fizeau interferometer is used to compare a wavefront reflecting off of the test lens with a wavefront generated by a calibrated reference sphere. The test lens is placed in a position where the incoming wavefront perfectly matches its nominal radius of curvature. If the test lens deviates from the reference sphere, the optical path difference between the two beam paths will form interference fringes corresponding to the error in the lens. When the test lens is perfectly matched to the nominal radius of curvature, no fringes will form.
Figure 1 (left): Fizeau Interferometer for Testing a Spherical Lens
Figure 2 (right): Interference Fringes
A hologram is a recorded interference pattern that produces a desired wavefront or 3D image when illuminated with a suitable reference wave. Analog holograms are made by splitting an expanded laser beam into two paths in an interferometer. One path illuminates the object and the other path serves as a reference wavefront. Interference of these two beams is recorded onto light sensitive film. When this film is developed and illuminated by the reference beam, it will generate light from the object beam.
On the other hand, a Computer Generated Hologram does not require two interfering beams. Instead, the interference pattern is designed on a computer and then directly etched into a substrate using lithographic methods.
Figure 3: Setup for Creating a Conventional Analog Hologram
Computer Generated Holograms for Asphere Metrology
The surfaces of aspheres have departure from their best fit reference sphere and this increased optical path difference creates fringes so dense that they can no longer be resolved by the imaging system of the interferometer. This problem can be solved by using a Computer Generated Hologram, which is an interference pattern that converts the spherical wavefront leaving the reference sphere into an aspheric wavefront that matches the ideal surface sag of the aspheric lens.
Figure 4: CGH turning a spherical wavefront into an aspheric wavefront
Transforming the best-fit spherical wavefront into an aspheric wavefront is extremely beneficial as the calibrated reference wavefront allows the interference fringes to be resolved and the surface irregularity of the asphere to be precisely measured.
Comparison to Other Metrology Methods
While other methods exist for measuring aspheric surfaces, such as contact profilometers, optical profilometers, and aspheric stitching interferometers, CGHs have numerous benefits that outweigh those options including:
Full surface measurement rather than measuring a 2D slice of the asphere, therefore allowing CGH metrology to capture non-rotationally symmetric surface irregularity
Significantly faster surface measurement compared to most scanning methods, reducing measurement time by up to a factor of 3
High measurement accuracy of λ/10 or below
Potential to measure aspheric surfaces with a high slope of aspheric departure that are impossible to measure with other methods
However, CGH’s require a higher up-front cost compared to other metrology methods because a unique CGH is required for each specific aspheric prescription. If small quantities of an asphere are being measured, CGH metrology will not be cost effective, but will lead to significant long-term savings for volume manufacturing. For volume production, the reduced measurement time of CGH metrology makes it the optimal choice for asphere metrology.
Computer Generated Hologram Metrology at Edmund Optics®
Edmund Optics® is taking advantage of Computer Generated Holograms to push the boundaries of our in-house aspheric metrology. CGH metrology gives us the ability to reduce measurement time on our high volume, manufactured aspheres and also make surface irregularity measurements with higher accuracy than conventional asphere metrology. This technique expands our aspheric manufacturing capabilities by allowing us measure aspheric shapes with a high slope of aspheric departure that could not be measured through other metrology methods.
CGH metrology reduces the time spent measuring each part compared to typical scanning techniques for aspheric metrology and can also make more accurate surface irregularity measurements. CGH metrology has the additional benefits of identifying non-rotationally symmetric surface irregularity and being able to measure surfaces with a higher slope of aspheric departure.
There is a higher up-front cost to using a CGH since a unique CGH is needed for a specific aspheric surface. If small quantities of an asphere are being measured then CGH metrology won’t be cost effective, but will lead to significant long-term savings for volume manufacturing. In addition, when using a CGH transmission and reflection of unwanted diffraction orders can cause ghost fringes in the interferogram; care must be taken to tune diffraction efficiency and separate the diffraction orders to avoid this.
CGH metrology accuracy is limited by pattern placement error in etching the interference pattern onto the substrate as well as alignment error of the CGH to the interferometer. Often, a CGH will have an alignment feature or built-in mount to mitigate the alignment error.
A CGH has a minimum feature size in its interference pattern that limits the maximum possible diffraction angle. This is why an aspheric CGH is often used along with a reference sphere in the interferometer to reduce the required diffraction angles.
Links to technical articles appearing in industry publications authored by Edmund Optics® or featuring contributions from EO's engineering team and key management.
The Long and the Short of It: Techniques for Measuring Aspheres Watch
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