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Liquid Crystal Cells
Published in Russell A. Chipman, Wai-Sze Tiffany Lam, Garam Young, Polarized Light and Optical Systems, 2018
Russell A. Chipman, Wai-Sze Tiffany Lam, Garam Young
The IPS cell configuration has both electrodes on the same substrate, as shown in Figure 24.26. Therefore, the electric field is predominantly parallel to the glass plates. The molecules are not anchored at the boundaries. The applied voltage rotates the directors along the applied field direction, which is in the plane of the glass plates. The directors always remain perpendicular to the display normal, so the LC molecules change very little with switching.6 The IPS cell acts like a rotating half-waveplate. At low voltage, the directors are aligned with one of the polarizers to give a dark state. The applied voltage twists the directors about the z-axes. At the highest voltage shown, the director is 45° from the polarizers, providing a bright white state. Unlike the TN cell, when a pixel fails, IPS gives a dark dead pixel. The dark and white states could be reversed by orienting the polarizers parallel to the directors in the off state.
Megavoltage cone-beam computed tomography
Published in Ross I. Berbeco, Beam’s Eye View Imaging in Radiation Oncology, 2017
The projection image first undergoes a series of steps designed to improve image quality prior to performing the reconstruction. These steps first include offset, gain, and dead pixel corrections, and transmission filtering, using either an average or a diffusion filter to reduce noise. The diffusion filter uses different levels of averaging depending on the local gradient. A logarithmic conversion and normalization is then applied to the corrected pixel value I(u, v): () N(u,v,ϑ)=ln(I0(u,v,ϑ)I(u,v,ϑ))
X-ray Cone Beam Computed Tomography
Published in Paolo Russo, Handbook of X-ray Imaging, 2017
The above correction is actually performed on a pixel-by-pixel base, followed by dead-pixel correction (using the averaged value from neighboring pixels to replace the dead pixel). Figure 35.2 shows an example of a projection image before (a) and after (b) flat-fielding correction. We will cover this topic in more details in the Section 35.4.
Tutorial: Luminance Maps for Daylighting Studies from High Dynamic Range Photography
Published in LEUKOS, 2021
C. Pierson, C. Cauwerts, M. Bodart, J. Wienold
Because it deteriorates over time, the camera sensor should be checked for stuck, dead, or hot pixels (Jacobs and Wilson 2007). Stuck pixels are damaged pixels that will always appear colored or overexposed, even if no light reaches the sensor. Therefore, by taking a picture with the lens cap on the camera, stuck pixels are detected as the colored or white spots in the image. Dead pixels are permanently damaged pixels that do not receive any power. They will thus always appear black, even when the image is fully overexposed. Hot pixels are not permanently damaged, but show up during long exposures as the sensor heats up. To detect hot pixels, a picture should be taken with the lens cap on using a long exposure. Every non-black and non-stuck pixel in that long-exposed image is a hot pixel. Hot pixels should not be a problem since for daylit scenes, the maximal exposure time should be kept under 2 seconds to prevent the lighting conditions to vary too much. However, the presence of several stuck or dead pixels requires to change camera to avoid distorted measurements.