Longitudinal chromatic aberrationexamples

Practically, it looks like your photo is of a tilted scale. The primary aberration is likely defocus. Again, from a practical point of view, it would be hard to tease out what is longitudinal chromatic aberration and what is spherochromatism. For one thing, it would help a lot to have images taken at different f/#s. (Since the spherical aberration varies with used f/#). I'm not saying it couldn't be done, but most people would measure the MTF at different colors and use that information. All of the aberrations get wrapped into the MTF, so it is an all-encompassing measure.

The researchers emphasized in a press release that the acoustic control of laser light in gases can probably also be transferred to optical elements such as lenses and waveguides.

Longitudinal chromatic aberrationcauses

In basic science classes, students learn that the changes in refractive index at the interfaces between materials provide the foundation for many ways to control light. Air’s refractive index is so close to 1 that it doesn’t get much attention by itself.

Transversechromatic aberration

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In lens design textbooks, a distinction is often made between longitudinal chromatic aberration and spherochromatism. (See for instance Kingslake's lens design book.) What is simple way to understand the difference? As far as I can tell, both involve the focal plane varying in distance from the lens as the wavelength of the light changes.

Note that transverse aberrations (the size of the spot in the plane perpendicular to the optical axis) behave differently than longitudinal aberrations. A topic for a different time.

Sphericalaberration

Longitudinal chromatic aberrationformula

Spherochromatism is simply the change in longitudinal chromatic by lens zone. Both aberrations are closely related. If you have a lens with longitudinal, there will be some change in color focus by lens zone (spherochromatism). The use of conventional optical glass in long lenses was not able to control either longitudinal nor the spherochromatism to a great degree. Stopping down the lens to avoid the poorly corrected marginal rays was necessary to attain an acceptable image. With the advent of low dispersion glass, both problems were nearly eliminated. I view spherochromatism as a subset of longitudinal chromatic. Fringing in the picture is from longitudinal chromatic. To say that an optic has spherochromatism, one must compare color focus of various lens zones (Paraxial, zonal and marginal). You would need 3 pictures; one for each lens zone to determine if the optic had noticeable spherochromatism.This cannot be determined in a single picture. The green fringing (mid spectrum) indicates that the chromatic is not lateral chromatic.

The DESY–Helmholtz team directed a beam of ultrashort near-infrared laser pulses to make seven passes through an ultrasound field volume 7 cm across. The multiple passes lengthened the beam’s path through the acoustic field. The first experiment used a mode-locked femtosecond laser with an average output power in the tens of milliwatts, but the second experiment involved a burst-mode laser with an average power of 3.5 kW and peak powers up to 20 GW, while still preserving excellent beam quality.

In particular, how do we identify the following image as primarily an example of spherochromatism and not longitudinal CA?

Yet, at the tiniest of incident angles, even a refractive index change on the order of 10−5 at a gaseous boundary layer can have large effects. For example, air layers at varying temperatures can produce mirages.

Lateralchromatic aberration

Now, researchers based in Germany have harnessed this effect by creating an optical Bragg grating with high-pressure ultrasound waves (Nat. Photon., doi:10.1038/s41566-023-01304-y). The experiments, conducted in ambient air, demonstrated a deflection efficiency of greater than 50 percent while avoiding the pitfalls of solid media, such as narrow spectral ranges, light-induced damage and nonlinear effects.

Schrödel, the study’s first author, credited team members from Technische Universität Darmstadt, Germany, for building the ultrasound transducer, which operated at 490 kHz—far above the upper limit of human hearing. “A transducer of this size, that transmits ultrasound waves at high frequency into air at our truly ludicrous acoustic power, is unmatched,” he adds.

Longitudinal chromatic aberrationreddit

According to Schrödel, the DESY-Helmholtz team’s air-based acousto-optic modulator is only the most primitive optical element in future “gas-phase sonophotonics” because it requires only a planar acoustic wave. The researchers emphasized in a press release that the acoustic control of laser light in gases can probably also be transferred to optical elements such as lenses and waveguides. How differently shaped sound waves could affect light beams is one of the group’s next avenues of study.

A laser light beam passes between a loudspeaker-reflector array that creates a grating of air. The laser beam interacts with this grating and is deflected without contact. [Image: Science Communication Lab for DESY]

According to Yannick Schrödel, a doctoral student at DESY and Helmholtz, the deflection effect first became noticeable when the sound was at 130 dB, but the team cranked up the sound to 148 dB for the main experiments. That decibel level is above the threshold of pain for humans and close to the noise levels that NASA uses to test spacecraft before launch—but the researchers did not need to put their equipment inside an acoustic chamber. “Surprisingly, because the ultrasound is so confined in the 7-cm diameter and 2-cm height resonator and is absorbed after a relatively short distance, the experiments were performed in the ambient laboratory air!” Schrödel says.

Scientists already use the interaction between optical and acoustic waves for Q-switching in solid-state lasers and other applications requiring signal modulation, but practical acousto-optic devices involve solid and liquid media. Calculations of Bragg diffraction by the researchers at Deutsches Elektronen-Synchrotron (DESY) and Helmholtz Institute Jena, both in Germany, revealed several requirements for efficient modulation in air: a sufficiently shallow incidence angle of the light beam and optimization of the length of the sound transducer.

“The potential of contactless control of light and its extension to other applications can currently only be imagined,” explains project leader Christoph Heyl of DESY and Helmholtz. “Modern optics is based almost exclusively on the interaction of light with solid matter. Our approach opens up a completely new direction.”