The dynamo theory of the lunar magnetic variations is developed as a three‐dimensional problem. The finite thickness of the ionospheric‐current layer and the anisotropy of the conductivity are considered. The currents responsible for the lunar magnetic variations are assumed to flow in the same layer of the ionosphere as those responsible for the solar magnetic variations, i.e., from 100 to 120 km above the earth's surface. The pattern of the results is similar to that for the solar variation, especially the magnetic‐field results. The horizontal magnetic field is greatest in the middle of the current layer and at that height it is about 7 times as great as that at the ground. The pressure variation in the lower half of the layer is about 180° out of phase with that at the ground.

The impacts of uncertainty in mirror movements in mechanically scanned interference pattern structured illumination imaging (IPSII) are discussed. It is shown that uncertainty in IPSII mirror movements causes errors in both the phase and amplitude of the Fourier transform of the resulting imaging. Finally, we demonstrate that iterative phase retrieval algorithms can improve the quality of IPSII images by correcting the phase errors caused by mirror movement uncertainties.

We discuss the use of wave plates with arbitrary retardances, in conjunction with a linear polarizer, to split linearly polarized light into two linearly polarized beams with an arbitrary splitting fraction. We show that for non-ideal wave plates, a much broader range of splitting ratios is typically possible when a pair of wave plates, rather than a single wave plate, is used. We discuss the maximum range of splitting fractions possible with one or two wave plates as a function of the wave plate retardances, and how to align the wave plates to achieve the maximum splitting range possible when simply rotating one of the wave plates while keeping the other one fixed. We also briefly discuss an alignment-free polarization rotator constructed from a pair of half-wave plates.

We present a laser wavelength meter based on a commercial color sensor chip. The chip consists of an array of photodiodes with different absorptive color filters. By comparing the relative amplitudes of light on the photodiodes, the wavelength of light can be determined. In addition to absorption in the filters, etalon effects add additional spectral features which improve the precision of the device. Comparing the measurements from the device to a commercial wavelength meter and to an atomic reference, we found that the device has picometer-level precision and picometer-scale drift over a period longer than a month.

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