A practical instrumentation configuration for measuring focal length of cell-phone camera lenses is presented that uses a custom autocollimator, a grating, an autostigmatic microscope, and a precision stage. Uncertainty in LED wavelength is reduced by using a white LED and a set of narrow-band filters. The autocollimator is designed to allow for rapid focus adjustment at each test wavelength. Examples of the measurement technique and an error analysis are provided.
There is a long history of non-contact profilometry of mirrors performed using an autocollimator. An autocollimator is scanned over a mirror resulting in a set of local slope measurements that are then integrated to produce a measurement of the mirror profile. Such profilometers are often associated with the testing of x-ray optics. In this application a small beam is usually created by placing an aperture near the mirror under test. The W2-AM uses a reticle with a circular target. The noise, sensitivity and error characteristics of the W2-AM instrument is assessed relative to the needs of profilometry of x-ray optics.
The addition of a piezo-electric focusing stage and phase retrieval algorithms to a compact, adaptable autostigmatic microscope provides for both improved focus sensitivity during optical system alignment as well as the ability to measure low-order aberrations for system qualification. A description of the instrument and initial results are reported.
The design and performance of a compact, adaptable autostigmatic alignment modulate that can be used as an autocollimator or autostigmatic microscope is described. The new instrument utilizes LED illumination and reticle projection and is compared with a laser diode based point source microscope. Measurement characteristics and practical differences and similarities between the instruments are described.
The unsteady motion of a solar simulator was simulated using dynamic mesh technology in Fluent software. The dynamic irradiation characteristics of the simulator were studied under various conditions. Mesh updates were achieved using a dynamic layering method, and the periodic lifting motion of the simulator was defined using user-defined functions (UDF). Detailed dynamic irradiance characteristics were obtained for comparison with experimental results. The results showed that the simulator height and the number of light sources used were the main factors that affected the irradiance. The irradiance has a linear relationship with the simulator height, which means that the irradiance nonuniformity decreases with decreasing solar height; in addition, the sum of the irradiances under the various operating conditions matches the superposition of the irradiance. The dynamic irradiation numerical results are consistent with the experimental results at typical points, which verifies the reliability of the moving mesh numerical model. The validated model can be used to study various simulator conditions and provides forecast data for diurnal variation simulation of solar radiation.
A custom optical microscope with an integrated liquid lens to enable an extended depth imaging in biological tissue has
been fabricated and shown to provide subcellular resolution using Gabor image fusion in an optical coherence
tomography setup. In this paper, we report on the development and assessment path of this technology over the last few
years. Specifically, we report on the optical design as well as the MTF prediction and experimental measurements of the
new optical microscope head. The microscope head was integrated in a custom optical coherence microscopy setup that
is presented. Images of an African frog tadpole, ex vivo human breast excised tissue, and in vivo skin images, acquired
with the integrated instrument, are presented.
A contactless fingerprint sensor provides deformation-free, high-quality fingerprint images and offers users a cleaner and
more comfortable measurement environment. Here we propose and design an innovative prototype optical, contactless,
compact, fingerprint sensor that quickly produces high-quality, high-contrast interoperable fingerprint images. A proofof-
concept contactless, aliveness-testing (CAT) fingerprint sensor, which is connected to a PC via a firewire cable, was
constructed and is currently operating in our laboratory. The CAT sensor affords a more user-friendly interface
compared to existing contactless fingerprint sensors and also provides robust aliveness testing and spoof detection. In
this paper, we present the imaging system design concepts, finger aliveness detection techniques, and the user-friendly
interface approach. Various fingerprint matching results using the CAT sensor device are also presented and discussed.
A new Time Delayed Source (TDS) has been designed to work with a transmitted wavefront
interferometer. The TDS splits a short-coherence wavelength source into two halves, introduces a time
delay between the two halves and then recombines the two halves. The recombined wavefronts are fed into
a transmitted wavefront interferometer to measure the optical path difference (OPD) between the two
surfaces of a part. The time delay is adjusted to match the separation between the two surfaces of the part
allowing phase shifting techniques to be used to measure the OPD. The measurement approach can be
adapted to measure nominally constant thickness windows, spherical domes and tangent ogives through the
design of suitable wavefront matching optics. Producing the wavefront to measure a tangent ogive is
accomplished by a series of sub-apertures, each measuring a section of a tangent ogive. The tangent ogive
is placed on a rotary stage, and as it rotates, sub-aperture measurements are taken and later stitched together
to generate a complete measurement of the part. Alignment of each sub-aperture instrument is essential to
obtain an accurate measurement. The setup and alignment of a tangent ogive transmitted wavefront
interferometer is also described.
A time-delayed source interferometer manipulates the output of a short-coherence length source so that light
reflected from the two surfaces of a nominally constant-thickness optical component interfere. The interference pattern is
a measure of optical thickness variation and can be phase-shifted. The approach is well suited to optical components that
are nominally constant thickness over some portion of the surface. Interferometers suited to the measurement of
windows, hemispherical domes and tangent ogives have been built. Data acquisition, calibration tooling and processing
methods are described for the stitching of phase data.
Achieving high lateral resolution still remains a challenge for in vivo Optical Coherence Microscopy (OCM) biological imaging. While to address this challenge, the numerical aperture (NA) of the microscope objective in the sample arm of the OCM interferometer may be increased, it introduces trade-offs in terms of loss in the depth of focus over which lateral resolution can still be maintained. As a critical step to offset this problem, we recently presented the optical system design of a dynamic focusing (DF) optical coherence microscope with a built-in liquid lens for re-focusing through the sample depth with no moving parts at in vivo speeds. We present experimental measurements of the modulation transfer function (MTF) acquired from the fabricated research prototype. The measurements were obtained though the edge detection method as a function of the voltage applied and at various positions in the field of view (FOV) within a 2mm cubic sample. Results demonstrate a resolution of 2 µm across the voltage range and the FOV, which validates the expectation by design of a quasi-invariant resolution of less than 3μm over a 2mm×2mm lateral cross-section across the 2mm depth of skin-equivalent tissue. Images of a tadpole sample acquired with the probe at different focal depths are also shown to demonstrate gain in resolution with focusing through different depth zones.
Alignment telescopes and interferometers are commonly used for the alignment of an optical system. Although alignment telescopes quantify angles, they are not particular helpful for quantifying wavefront quality. Interferometers by comparison are often used for alignment, but are most useful for quantifying wavefront quality. However, an optical system must be fairly well aligned before one can even use an interferometer. Many optical systems require the sensitivity and accuracy of an interferometer for final alignment. However, there are many optical systems where visual inspection of a star test would be adequate for system qualification, except for the fact that a visual test is qualitative. An autostigmatic or point source microscope (PSM) is a convenient tool for alignment and performance of a star-test. Like an alignment telescope, an autostigmatic microscope does not conveniently quantify the wavefront quality. Once a focused spot is obtained with an autostigmatic microscope a plane-parallel plate inserted into the converging beam path may be used to introduce a known focus shift. The resulting image may be used to estimate low order-aberrations. Experimental results are presented using very simple hardware.
A variety of tools, such as alignment telescopes and interferometers, are used in the alignment of optical systems.
Alignment telescopes quantify angles, and connect an optical axis to a mechanical axis, but they are not particularly
helpful for quantifying errors in a wavefront. Interferometers, which have exquisite sensitivity and accuracy for
wavefront measurement, are often used for the final qualification of an optical system. However, an interferometer is
not the most convenient tool for alignment. An alternative tool for alignment is the point source microscope (PSM),
which is an example of an autostigmatic microscope. The PSM is a flexible, convenient tool, but like an alignment
telescope, it does not quantify the wavefront. On the other hand, the PSM does provide real-time feedback to an
operator in a compact tool. In order to complete an alignment process it is necessary to quantify the wavefront quality,
and it is desirable to use only a single tool. Methods for quantifying wavefront quality with an autostigmatic
microscope, a tool primarily used for alignment, are described.
It is often desirable to measure an optical component whose aperture exceeds the capacity of the measurement device.
However, stitching of sub-aperture measurement data into a single measurement of an optical component is a
challenging problem since mechanical motions of the test component relative to the reference surface of an
interferometer can not be made with interferometric accuracy. Even more challenging than the need to compensate for
rigid body motion between the sub-aperture measurements is the need to account for imperfections in the reference
surface itself. In this paper we show, both in simulation and experimentally, how the use of a time-delayed source
(TDS) simplifies the stitching of transmitted wavefront measurements from domes and windows. This is accomplished
by making it possible to obtain phase-shifted interferometric measurements using only the light reflected by two
surfaces from a dome or window without the use of a reference surface.
Measurement of the transmitted wavefront of domes and windows is a long-standing problem. One may use a large
return sphere and measure the interference cavity without the dome present and again with the dome present. The
difference between the two measurements is a double-pass measurement of the transmitted wavefront of the dome. Even
so, the long coherence length of the source results in many extraneous fringe patterns. Windows may be tested by using
a collimated source and return flat. A time-delayed source (TDS) having a short-coherence length is used to obtain a
single interference pattern due only to interference of light reflected by the two surfaces of a dome or window. Standard
phase shifting algorithms may be used with the TDS to measure the optical thickness of a dome or window without
errors due to multiple reflections. Since most of the interferometer is common-path, environmental sensitivity is reduced
and alignment is straightforward compared to typical interferometers. Finally, since there is no reference surface,
stitching of sub-aperture measurements is simplified.
We give an example of a Point Source Microscope (PSM) and describe its uses as an aid in the alignment of optical systems including the referencing of optical to mechanical datums. The PSM is a small package (about 100x150x30 mm), including a point source of light, beam splitter, microscope objective and digital CCD camera to detect the reflected light spot. A software package in conjunction with a computer video display locates the return image in three degrees of freedom relative to an electronic spatial reference point. The PSM also includes a Koehler illumination source so it may be used as a portable microscope for ordinary imaging and the microscope can be zoomed under computer control. For added convenience, the laser diode point source can be made quite bright to facilitate initial alignment under typical laboratory lighting conditions. The PSM is particularly useful in aligning optical systems that do not have circular symmetry or are distributed in space such as off-axis systems. The PSM is also useful for referencing the centers of curvatures of optical surfaces to mechanical datums of the structure in which the optics are mounted. By removing the microscope objective the PSM can be used as an electronic autocollimator because of the infinite conjugate optical design.
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