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Katherine Johnson’s mathematical calculations helped guide some of America’s most important early space missions while she confronted the racial and gender barriers faced by Black women in twentieth-century America.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
 | This article may be too technical for most readers to understand. (June 2020) |
The frequency-doubling illusion is an apparent doubling of spatial frequency when a sinusoidal grating is modulated rapidly in temporal counterphase.[1][2] Recently, it has been proposed that the illusion arises from a spatially nonlinear ganglion cell class.[3] The contrast threshold values needed for perceiving this physiological effect are used in frequency doubling technology perimetry for the detection of even early phases of glaucoma.[4][5][6] A more recent study's results argue against the hypothesis that spatially nonlinear retinal ganglion cells are the physiological substrate of the frequency-doubling illusion. A cortical pathway of temporal phase discrimination may be the principal cause of the illusion, whereas spatial phase information (i.e., grating position) is retained.[7]
Sensitivity to the spatial-frequency-doubling illusion was also positively correlated with reading lag and coherent motion. The results provide good support for a magno deficit in dyslexia that has its origins at a retinal level with impairment in—at least partially—M(y)-cell activity.[8]
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
The frequency-doubling illusion is an apparent doubling of spatial frequency when a sinusoidal grating is modulated rapidly in temporal counterphase. Recently, it has been proposed that the illusion arises from a spatially nonlinear ganglion cell class. The contrast threshold values needed for perceiving this physiological effect are used in frequency doubling technology perimetry for the detection of even early phases of glaucoma. A more recent study's results argue against the hypothesis that spatially nonlinear retinal ganglion cells are the physiological substrate of the frequency-doubling illusion. A cortical pathway of temporal phase discrimination may be the principal cause of the illusion, whereas spatial phase information (i.e., grating position) is retained. Sensitivity to the spatial-frequency-doubling illusion was also positively correlated with reading lag and coherent motion. The results provide good support for a magno deficit in dyslexia that has its origins at a retinal level with impairment in—at least partially—M(y)-cell activity.
A blue laser emits electromagnetic radiation with a wavelength between 400 and 500 nanometers, which the human eye sees in the visible spectrum as blue or violet. Blue lasers can be produced by: direct, inorganic diode semiconductor lasers based on quantum wells of gallium(III) nitride at 380-417nm or indium gallium nitride at 450 nm diode-pumped solid-state infrared lasers with frequency-doubling to 408nm upconversion of direct diode semiconductor lasers via thulium- or praseodymium-doped fibers at 480 nm metal vapor, ionized gas lasers of helium-cadmium at 442 nm and 10–200 mW argon-ion lasers at 458 and 488 nm Lasers emitting wavelengths below 445 nm appear violet, but are nonetheless also called blue lasers. Violet light's 405 nm short wavelength, on the visible spectrum, causes fluorescence in some chemicals, like radiation in the ultraviolet ("black light") spectrum (wavelengths less than 400 nm).
Gallium nitride (GaN) is a binary III/V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronics, high-power and high-frequency devices. For example, GaN is the substrate that makes violet (405 nm) laser diodes possible, without requiring nonlinear optical frequency doubling. Its sensitivity to ionizing radiation is low (like other group III nitrides), making it a suitable material for solar cell arrays for satellites. Military and space applications could also benefit as devices have shown stability in high-radiation environments. Because GaN transistors can operate at much higher temperatures and work at much higher voltages than gallium arsenide (GaAs) transistors, they make ideal power amplifiers at microwave frequencies. In addition, GaN offers promising characteristics for THz devices. Due to high power density and voltage breakdown limits GaN is also emerging as a promising candidate for 5G cellular base station applications. Since the early 2020s, GaN power transistors have come into increasing use in power supplies in electronic equipment, converting AC mains electricity to low-voltage DC, as well as higher-voltage applications such as electric vehicle traction inverters due to their small size, fast switching speed and high efficiency.
Potassium titanyl phosphate (KTP) is an inorganic compound with the formula K+[TiO]2+PO3−4. It is a white solid. KTP is an important nonlinear optical (NLO) material that is commonly used for frequency-doubling diode-pumped solid-state lasers such as Nd:YAG and other neodymium-doped lasers. Related NLO materials include lithium niobate, ammonium dihydrogenphosphate, and potassium dihydrogenphosphate.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Madam C.J. Walker