SUMMARY / RELATED TOPICS

Absolute dating

Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy. Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events. In archaeology, absolute dating is based on the physical and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates. Techniques include tree rings in timbers, radiocarbon dating of wood or bones, trapped-charge dating methods such as thermoluminescence dating of glazed ceramics. Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.

In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from young to systems such as uranium–lead dating that allow acquisition of absolute ages for some of the oldest rocks on Earth. Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications due to the types of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments. One of the most used and well-known absolute dating techniques is carbon-14 dating, used to date organic remains.

This is a radiometric technique. Cosmic radiation entering the earth’s atmosphere produces carbon-14, plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as predators eat other animals. With death, the uptake of carbon-14 stops, it takes 5,730 years for half the carbon-14 to change to nitrogen. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact; the short half-life of carbon-14, 5,730 years, makes dating reliable only up to about 50,000 years. The technique cannot pinpoint the date of an archeological site better than historic records, but is effective for precise dates when calibrated with other dating techniques such as tree-ring dating. An additional problem with carbon-14 dates from archeological sites is known as the "old wood" problem.

It is possible in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating; the development of accelerator mass spectrometry dating, which allows a date to be obtained from a small sample, has been useful in this regard. Other radiometric dating techniques are available for earlier periods. One of the most used is potassium–argon dating. Potassium-40 is a radioactive isotope of potassium that decays into argon-40; the half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated.

Argon, a noble gas, is not incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K–Ar dating was used to calibrate the geomagnetic polarity time scale. Thermoluminescence testing dates items to the last time they were heated; this technique is based on the principle. This process frees electrons within minerals. Heating an item to 500 degrees Celsius or higher releases the trapped electrons, producing light; this light can be measured to determine the last time. Radiation levels do not remain constant over time. Fluctuating levels can skew results – for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.

Because of these and other factors, Thermoluminescence is at the most about 15% accurate. It cannot be used to date a site on its own. However, it can be used to confirm the antiquity of an item. Optically stimulated luminescence dating constrains the time at which sediment was last exposed to light. During sediment transport, exposure to s

Display motion blur

Display motion blur called HDTV blur and LCD motion blur, refers to several visual artifacts that are found on modern consumer high-definition television sets and flat panel displays for computers. Many motion blur factors have existed for a long time in video; the emergence of digital video, HDTV display technologies, introduced many additional factors that now contribute to motion blur. The following factors are the primary or secondary causes of perceived motion blur in video. In many cases, multiple factors can occur at the same time within the entire chain, from the original media or broadcast, all the way to the receiver end. Pixel response time on LCD displays Lower camera shutter speeds common in Hollywood production films, common in miniaturized camera sensors that require more light. Blur from eye tracking fast-moving objects on sample-and-hold LCD, plasma, or microdisplay. Resolution resampling. Deinterlacing by the display, telecine processing by studios; these processes can introduce motion-speed irregularities.

Compression artifacts, present in digital video streams, can contribute additional blur during fast motion. Motion blur has been a more severe problem for LCD displays, due to their sample-and-hold nature. In situations when pixel response time is short, motion blur remains a problem because their pixels remain lit, unlike CRT phosphors that flash briefly. Reducing the time an LCD pixel is lit can be accomplished via turning off the backlight for part of a refresh; this reduces motion blur due to eye tracking by decreasing the time. In addition, strobed backlights can be combined together with motion interpolation to reduce eye-tracking-based motion blur. Different manufacturers use many names for their strobed backlight technologies for reducing motion blur on sample-and-hold LCD displays. Generic names include black frame scanning backlight. Philips created Aptura known as ClearLCD, to strobe the backlight in order to reduce the sample time and thus the retinal blurring due to sample-and-hold.

Samsung uses strobed backlighting as part of their "Clear Motion Rate" technology. This was called "LED Motion Plus" in some previous Samsung displays. BenQ developed SPD more known as "black frame insertion", claim that their images are as stable and clear as CRTs; this is conceptually similar to a strobing backlight. Sharp Corporation use a "scanning backlight" which flashes the backlight in a sequence from the top to the bottom of the screen, during every frame. NVidia has licensed; this is used to reduce crosstalk during 3D Vision, which utilize shutter glasses. A'hack' method or utility tool is needed to take advantage of LightBoost backlights for blur reduction benefits. BenQ developed their own native "BenQ Blur Reduction" technology, integrated into several of their gaming monitors; this offers a strobe backlight which can be turned on and off by the user. There is no control over the strobe timing or strobe length for the user, although third party utilities have been produced for this purpose.

Newer firmware for the BenQ Blur Reduction monitors allow direct user control over the strobe pulse and strobe length directly from the Service Menu. More customization is available by using a higher Vertical Total, which tricks the Mstar scaler into working with a larger blanking interval, as if the vertical screen size were longer; this pushes the strobe crosstalk farther down the bottom of the display, improving strobe image quality, but with some drawbacks. Eizo have introduced their'Turbo 240' option used so far on their Eizo Foris FG2421 gaming display; this allows the user to control the strobe backlight on/off to reduced perceived motion blur LG introduced a similar'Motion 240' option on their 24GM77 gaming monitor ULMB is a technique provided alongside NVIDIA's G-sync technology, linked to the G-sync monitor module. It is an alternative option to using G-sync, offering the user instead an "Ultra Low Motion Blur" mode; this has been provided on various monitors featuring G-sync. For newer games with a higher demand for graphical power, G-Sync is preferable over ULMB.

Some displays use motion interpolation to run at a higher refresh rate, such as 100 Hz or 120 Hz to reduce motion blur. Motion interpolation generates artificial in-between frames that are inserted between the real frames; the advantage is reduced motion blur on sample-and-hold displays such as LCD. There can be side-effects, including the soap opera effect if interpolation is enabled while watching movies. Motion interpolation adds input lag, which makes it undesirable for interactive activity such as computers and video games. 240 Hz interpolation have become available, along with displays that claim an equivalence to 480 Hz or 960 Hz. Some manufacturers use a different terminology such as Samsung's "Clear Motion Rate 960" instead of "Hz"; this avoids incorrect usage of the "Hz" terminology, due to multiple motion blur

Quenching (fluorescence)

Quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisional quenching; as a consequence, quenching is heavily dependent on pressure and temperature. Molecular oxygen, iodide ions and acrylamide are common chemical quenchers; the chloride ion is a well known quencher for quinine fluorescence. Quenching poses a problem such as laser-induced fluorescence. Quenching is made use of in optode sensors. Quenching is the basis for Förster resonance energy transfer assays. Quenching and dequenching upon interaction with a specific molecular biological target is the basis for activatable optical contrast agents for molecular imaging. There are a few distinct mechanisms by which energy can be transferred non-radiatively between two dyes, a donor and an acceptor. Förster resonance energy transfer is a dynamic quenching mechanism because energy transfer occurs while the donor is in the excited state.

FRET is based on classical dipole-dipole interactions between the transition dipoles of the donor and acceptor and is dependent on the donor-acceptor distance, R, falling off at a rate of 1/R6. FRET depends on the donor-acceptor spectral overlap and the relative orientation of the donor and acceptor transition dipole moments. FRET can occur over distances up to 100 Å. Dexter is another dynamic quenching mechanism. Dexter electron transfer is a short-range phenomenon that falls off exponentially with distance and depends on spatial overlap of donor and quencher molecular orbitals. In most donor-fluorophore–quencher-acceptor situations, the Förster mechanism is more important than the Dexter mechanism. With both Förster and Dexter energy transfer, the shapes of the absorption and fluorescence spectra of the dyes are unchanged. Dexter electron transfer can be significant between the dye and the solvent when hydrogen bonds are formed between them. Exciplex formation is a third dynamic quenching mechanism.

The remaining energy transfer mechanism is static quenching. Static quenching can be a dominant mechanism for some reporter-quencher probes. Unlike dynamic quenching, static quenching occurs when the molecules form a complex in the ground state, i.e. before excitation occurs. The complex has its own unique properties, such as being nonfluorescent and having a unique absorption spectrum. Dye aggregation is due to hydrophobic effects—the dye molecules stack together to minimize contact with water. Planar aromatic dyes that are matched for association through hydrophobic forces can enhance static quenching. High temperatures and addition of surfactants tend to disrupt ground state complex formation. An important quenching process in atmospheric physics can be seen in the altitudinal variation of auroral emissions. At high altitudes, the red 630.0 nm emission of atomic oxygen dominates, whereas at altitudes in the E-layer the green 557.7 nm emission is more intense. Both disappear at altitudes below 100 km.

This variation occurs due to the unusually long lifetimes of the excited states of atomic oxygen, with 0.7 seconds for the 557.7 nm and two minutes for the 630.0 nm emission. The mean collision-free paths decrease at lower altitudes due to increasing particle densities, which results in the de-excitation of the oxygen atoms due to the higher probability of collisions, preventing the emission of the red and green oxygen lines. Dark quencher, for use in molecular biology. Förster resonance energy transfer, a phenomenon on which some quenching techniques rely