Photon meter

Three common classes of instruments are used for measuring the amount of incident light (1) photometers or light meters, which measure the available illuminating power, a quantity related to the wavelength sensitivity of the human eye (2) quantum or photon meters, which measure the number of photons and (3) radiometers, which measure the total energy of the radiation (Fig. 4-4). 

In a synchrotron, electrons are accelerated to near relativistic velocities and constrained magnetically into circular paths. When a charged particle is accelerated, it emits radiation, and when the near-relativistic electrons are forced into curved paths they emit photons over a continuous spectrum. The general shape of the spectrum is shown in Fig. 2.4. For a synchrotron with an energy of several gigaelectronvolts and a radius of some tens of meters, the energy of the emitted photons near the maximum is of the order of 1 keV (i.e., ideal for XPS). As can be seen from the universal curve, plenty of usable intensity exists down into the UV region. With suitable mono-... 

A carbon dioxide laser produces radiation of wavelength 106 micrometers (1 micrometer = 10-6 meter). If the laser produces about one joule of energy per pulse, how many photons are produced per pulse ... 

The lead compounds PbS, PbSe, PbTe are narrow-gap semiconductors that have been widely investigated for infrared detectors, diode lasers, and thermo-photovoltaic energy converters. Their photoconductive effect has been utilized in photoelectric cells, e.g., PbS in photographic exposure meters. Integrated photonic devices have been fabricated by their heteroepitaxial growth on Si or III-V semiconductors. 

Figure 7.7 The relationship between photon energy in kcal/mole, wavelength in meter and frequency in Hz is shown schematically. The commonly used acronyms for each major frequency range is indicated at the bottom. (See Colour Plate VI)...

What is the wavelength (in meters) of photons with the following energies In what region of the electromagnetic spectrum does each appear ... 

Photonic activation of redox enzymes results in the light-induced bioelectrocataly-tic activation of an enzyme cascade. This permits application of photoswitchable enzymes as amplifiers of weak photonic signals. Alternatively, sensitive actino-meters, measuring low irradiation doses, might be envisaged. 

The NIST system transmitted a stream of individual photons to create an encryption key (used to encrypt messages ) at the rate of 1 Million bits per second. A very impressive speed but this transmission was between two NIST buildings that are 730 meters apart. One of the issues is how to built repeaters which can allow long quantum channels. A 67 kilometers quantum transmission achieved in 2001 at the University of Geneva may be the longest possible distance with current technology. 

Nanocrystals and nanowires are utilized in a new generation of solar collectors (a nanometer is one billionth of a meter). In conventional solar cells, at the P-N junction one photon splits one electron from its "hole companion" as it travels to the electron-capturing electrode. If solar collectors are made of semiconducting nanocrystals that disperse the light, according to TU Delft s professor Laurens Siebbeles, an avalanche effect results and one photon can release two or three electrons, because this effect maximizes photon absorption while minimizing electron-hole recombination. This effect of the photon-scattering nanoparticles substantially increases cell efficiency. 

Photodetector — Device used to detect photons. After a long period having only thermal photodetectors, quantum photodetectors based on photocurrent were developed and are used quite widely in applications such as photographic meters, flame detectors and lighting control. In the late 1950s the p-i-n photodiode, simply referred to as photodiode, was developed and now is one of the most common photodiodes. There are several types of photodetectors, the most adequate depending on the specific application, like photoconductors, p-i-n photodiodes, Schottky-barrier photodiodes, charge-coupled... 

The energy (in Joules) of a photon with wavelength A (in meters) is given by ... 

By definition, photometers do not respond to radiation in the infrared or the ultraviolet (Fig. 4-4a). They are light meters in the sense that they mimic human vision that is, they respond to photons in the visible region, similar to the light meter on a camera. A candle is a unit of luminous intensity, originally based on a standard candle or lamp. The current international unit is called a candela (sometimes still referred to as a candle ), which was previously defined as the total light intensity of 1.67 mm2 of a blackbody radiator (one that radiates maximally) at the melting temperature of pure platinum (2042 K). In 1979 the candela was redefined as the luminous intensity of a monochromatic source with a frequency of 5.40 x 1014 cycles s-1 (A, of 555 nm) emitting 0.01840 Js-1 or 0.01840 W (1.464 mW steradian-1, where W is the abbreviation for watt and steradian... 

C. A certain optical filter, which passes all wavelengths below 600 nm and absorbs all those above 600 nm, is placed over a radiometric device. If the meter indicates 1 W m-2, what is the maximum photon flux density ... 

A UV filter radiometer provides a convenient measure of the total UV output of a photon source, if properly designed for a specific source. They are broadband meters designed to measure incident radiation in the entire selected, UV region. An optical filter is used to limit the spectral responsivity to a certain band (e.g., UVB, near-UVA, or far-UVA). Ideally, the spectral responsivity is constant within the band and zero outside, but this is not the case in practice. Also, the characteristics of the filters may change with time and the device should be calibrated at least once per year. 

A more practical but less accurate plot of the action spectrum is what I would term the pseudo-action spectrum. The primary difference between normal and pseudo-action spectra derives from how they are obtained and plotted. Normal-action spectra are plots of the effect of a constant number of photons per wavelength versus wavelength, whereas pseudo-action spectra are obtained using a constant energy input, watts/meter, per nanometer. 

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