Detection of light

For many applications in spectroscopy the sensitive detection of light and the accurate measurement of its intensity are of crucial importance for the successful performance of an experiment. The selection of the proper detector for optimum sensitivity and accuracy for the detection of radiation must take into account the following characteristic properties, which may differ for the various detector types  

The time or frequency response of the detector, characterized by its time constant r. Many detectors show a frequency response that can be described by the model of a capacitor, which is charged through a resistor R and discharged through R2 (Fig. 4.72a). When a very short light pulse falls onto the detector, its output pulse is smeared out. If the output is a current i(t) that is proportional to the incident radiation power P(0 (as, for example, in photomultipliers), the output capacitance C is charged by this current and shows a voltage rise and fall, determined by 

If the current pulse i(t) lasts for the time T, the voltage V(t) at the capacitor increases up to r = T and for R2C T reaches the peak voltage T 

The time constant r of the detector causes the output signal to rise slower than the incident input pulse. It can be determined by modulating the continuous input radiation at the frequency /. The output signal of such a device is characterized by (see Exercise 4.12) 

The achievable signal-to-noise ratio which is, in principle, limited by 

The specific detectivity Z) [cms / W ] gives the obtainable signalto-noise ratio Es/Ki of a detector with the sensitive area A = 1 cm and the detector bandwidth Af = 1 Hz, at an incident radiation power of P = 1W. Because the noise equivalent input power is NEP = P En/ K. the specific detectivity of a detector with the area 1 cm and a bandwidth of 1 Hz is D = 1 /NEP. 

The maximum intensity range in which the detector response is linear. It means that the output signal Eg is proportional to the incident radiation power P. This point is particularly important for applications where a wide range of intensities is covered. Examples are output-power measurements of pulsed lasers, Raman spectroscopy, and spectroscopic investigations of line broadening, when the intensities in the line wings may be many orders of magnitude smaller than at the center. 

During recent years the development of image intensifiers, image converters, CCD cameras, and vidicon detectors has made impressive progress. At first pushed by military demands, these devices are now coming into use 

1) The spectral relative response R(x) of the detector, which determines the wavelength range where the detector can be used. The knowledge of R(x) is essential for the comparison of relative intensities I(x ) and I(X2) at different wavelengths. 

2) The absolute sensitivity S(x) = V /P, which is defined as the ratio of output signal to incident radiation power P. If the output is a voltage, as in photovoltaic devices or in thermocouples, the sensitivity is expressed in units of volts per watt. In case of photocurrent devices, such as photomultipliers, S(x) is given in amperes per watt. With the detector area A the sensitivity S can be expressed in terms of the irradiance I, 

3) The attainable signal-to-noise ratio V /V which is in principle limited by the noise of the incident radiation, but may in practice be further reduced by inherent noise of the detector. The detector noise is often expressed by the noise equivalent input power (NEP), which means an incident radiation power which generates the same noise level as the detector itself yielding a signal to noise ratio S/N = 1. In infrared physics a figure of merit for the infrared detector is the detectivity. 

The detectivity D gives the obtainable signal to noise ratio V /V, multiplied by the square root of detector area A and, detector bandwidth Af and divided the incident radiation power P. 

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This chapter explains how optical and infrared detectors work, from basic principles to the state-of-the-art. The role of optical and infrared detectors in an observatory is presented, and the state-of-the-art is related to an ideal detector. An overview of the detector physics is presented, showing that the detection of light is a 5 step process. Each step in this process is explained in detail in the subsequent sections. The chapter concludes with references for further information. 

This kind of analysis is most effectively conducted in a dedicated FEGSTEM instrument, which has an increased beam current while maintaining a small probe size. Such instruments also permit the detection of light elements such as B at boundaries. It has become possible to map the distribution of grain-boundary... 

A cryogenist does not usually need a general purpose mass spectrometer, but the cryogenist cannot work without an LD which is made up of a small vacuum system (rotary pump or diaphragm pump in series with a turbo pump) and a mass spectrometer for the detection of light gases (H2,3He and 4He). 

Clearly high g-factor resonance also benefits the detection of light-absorbing analytes in the fluid region. 

According to Vainstein the relative detectability of light atoms in presence of heavy ones in a 2-dimensional projection can be expressed, in the electron diffraction case, as ... 

Figure 12.15 Effectx)r mechanism for a pheromone. The mechanism is similar to that shown in Figure 12.6 except cyclic AMP leads to opening of a Na ion channel which will result in depolarisation and initiation of an action potential. It is of interest that the physiology/biochemistry is opposite to that involved in the detection of light, in which the signalling system results in hyperpolarisation of a nerve ratfer that hypopolarisation (Chapter 15 see Figure 15.10).

Figure 15.9 (a) The ds- and trans-retinal interconversions in the detection of light Within the photoreceptor cell, light is detected by the conversion of c/s-retinal to trans-retinal, components of the light-sensitive pigment rhodopsin. This apparently small chemical change is sufficient for trans-retinal to dissociate from rhodopsin. (b) The cis/trans q/cte. To continue the process, trans-retinal must be converted back to c/s-retinal. 

Figure 15.11 The biochemical reactions that result in the conversion of trans-retinal to ds-retinal, to continue the detection of light To continue the process, trans-retinal must be converted back to c/s-retinal. This is achieved in three reactions a dehydrogenase converts trans-retinal to trans-retinol an isomerase converts the trans-retinol to c/s-retinol and another dehydrogenase converts c/s-retinol to c/s-retinal. To ensure the process proceeds in a clockwise direction (i.e. the process does not reverse) the two dehydrogenases are separated. The trans-retinal dehydrogenase is present in the photoreceptor cell where it catalyses the conversion of trans-retinal to trans-retinol which is released into the interstitial space, from where it is taken up by an epithelial cell. Here it is isomerised to c/s-retinol and the same dehydrogenase catalyses its conversion back to c/s-retinal. This is released by the epithelial cell into the interstitial space from where it is taken up by the photoreceptor cell. This c/s-retinal then associates with the protein opsin to produce the light-sensitive rhodopsin to initiate another cycle. The division of labour between the two cells may be necessary to provide different NADH/NAD concentration ratios in the two cells. A high ratio is necessary in the photoreceptor cell to favour reduction of retinal and a low ration in the epithelial cell for the oxidation reaction (Appendix 9.7).

Optical mammography Brain imaging Sequential and/or parallel detection of light at selected points reconstruction of 2D, 3D images Reflected or Breast tumours transmitted light, fluorescence Brain perfusion, stroke... 

The detection of light from an extrasolar planet was reported by A. C. Cameron, K. Horne, A. Penny, and D. James, Probable Detection of Starlight Reflected from the Giant Planet Orbiting t Bootis , Nature, 402 (1999), 751. 

All Mars rovers to date have carried alpha-particle X-ray spectrometer (APXS) instruments for chemical analyses of rocks and soils (see Fig. 13.16). The source consists of radioactive curium, which decays with a short half-life to produce a-particles, which then irradiate the sample. Secondary X-rays characteristic of specific elements are then released and measured by a silicon drift detector. The Mars Pathfinder APXS also measured the backscattered a-particles, for detection of light elements, but the Mars Exploration Rovers measured only the X-rays. 

The detection of light, smells, and tastes (vision, olfaction, and gustation, respectively) in animals is accomplished by specialized sensory neurons that use signal-transduction mechanisms fundamentally similar to those that detect hormones, neurotransmitters, and growth... 

In order to increase the sensitivity toward the detection of light elements, a technique known as electron energy-loss spectroscopy (EELS) may be utilized. This method may be carried out within a (S)TEM, and consists of monitoring the loss in energy (due to inelastic scattering) of the beam electrons as they pass through the sample. Since it is more difficult to focus X-rays relative to electrons with appropriate lenses,... 

Fig. 27. Top left image of the device. Inset luminescence image of microwires. Top right I-V curves of microwires produced by slow evaporation (red line) and dip-and-pull approach (black line). Bottom left schematic diagram of the device used for the detection of light. Bottom right photocurrent of the device (excitation wavelength 450 nm), which can be switched on/off rapidly by illumination under voltage bias of 0.5 V. Reproduced with the permission of the Royal Society of Chemistry 244).

Amongst the main parameters of interest in rapid compression studies are the temperature and pressure that are reached at the end of compression. Pressure measurements are made by fast response pressure transducers (>10kHz), and ignition delay times are measured from the pressure-time profiles. The measurements of pressure may be supplemented by the detection of light output through windows, and by chemical analysis at intermediate stages of reaction by rapid expansion and quenching methods [22, 99-101]. 

Rieke, George. Detection of Light From the Ultraviolet to the Submillimeter. New York Cambridge University Press, 2002. 

Detection of Light-Catalyzed Changes in Vitamin Structure... 

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