FIU CHM 4130 - CHAPTER 7B_Xiao_Components_of_Optical_Instruments_2018 (1) (44 pages)

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CHAPTER 7B_Xiao_Components_of_Optical_Instruments_2018 (1)



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Components of Optical Instruments The generic spectrometer n Wavelength Separators monochromators and slits n Detectors Monochromators Prism monochromator Monochromators Grating monochromator d Spacing between the reflecting surfaces Beam 2 travels a greater distance than beam 1 for constructive interferences to occur CB BD n angle i CAB angle r DAB CB dsini BD dsinr n d sini sinr Echelle grating Higher dispersion and high resolution than a Echellette n 2dsini Performance Characteristics of Grating Monochromators 1 Spectral Purity Scattered radiation Stray radiation Imperfections of monochromator components 2 Dispersion of Grating Monochromators Dispersion is the ability of a monochromator to separate the different wavelengths Angular dispersion is equal to the rate of change of the angle of deviation with respect to the change in wavelength D 1 1 d d cos f d nf 3 Resolving power of a grating monochromator The resolving power R of monochromator the limit of its ability to separate adjacent images that have a slight difference in wavelength R 4 Light Gathering Power f number F a measure of the ability of a monochromator to collect the radiation that emerges from the entrance slit f F d Monochromator slits Slits hole in the wall Control the entrance of light into and out from the monochromator They control quality Entrance slits control the intensity of light entering the monochromator and help control the range of wavelengths of light that strike the grating Less important than exit slits Exit slights help select the range of wavelengths that exit the monochromator and strike the detector More important than entrance slits Can be Fixed just a slot Adjustable in width effective bandwidth and intensity Adjustable in height intensity of light Wider slits greater intensity Poorer resolution Narrower slits lower intensity Better resolution DETECTORS Just photon transducers Radiation transducers Radiant energy Electrical signal Proper3es of an Ideal Transducer 1 High sensi vity The transducer should be capable of detec3ng very small signals 2 Signal to noise ra o S N A high signal to noise ra3o is an important characteris3c of a good transducer 3 Constant response When radia3on of di erent wavelengths but of the same intensity are measured the transducer should give a constant response 4 Fast response A short response 3me is essen3al especially for scanning instruments 5 Zero dark current In absence of illumina3on the detector output should read zero 6 Zero dri If radia3on of constant intensity hits the transducer signal should be constant with 3me 7 Signal S kP P radiant power Types of radiation transducers Respond to Photons the intensity of EMR striking them by changing a voltage or current emitted or required by themselves for UV VIS and near infrared Response to Heat thermal detector e g for IR radiation Do NOT respond selectively to specific wavelengths that is what the wavelength selector is for but work over a range of wavelengths Various types of photo transducers Photographic films not widely in use any more Phototubes used in simpler instruments Photomultiplier tubes used in more complex instruments Multichannel transducers Diode arrays Charged coupled devices CCD s like in many camcorders Different wavelengths require different detectors Most UV VIS instruments have two photomultiplier tubes 1 Photovoltaic cells Operate in the visible region 350 750 nm with maximum sensitivity at about 550 nm Low sensitivity Fatigue its current output decreases with time although the intensity is constant 2 Vacuum phototubes Good for the general detection of radiation intensity in the UV Vis region Reliable Function based on the photoelectric effect A small dark current is always available 3 Photomultiplier tubes Also function based on photoelectric effect Additional signal is gained by multiplying the number of electrons produced by the initial reaction in the detector Each electron produces as series of photo electrons multiplying its signal Thus the name PMT Very sensitive to incoming light Most sensitive light detector in the UV VIS range VERY rugged They last a long time Sensitive to excessive stray light room light powered PMT DEAD PMT Always used with a scanning or moveable wavelength selector grating in a monochromator Modes of Operations Dynodes D1 D7 Photoemissive cathode 104 107 electrons produced for every photo emitted from the cathode Photomultiplier Tube Detector High sensitivity at low light levels Cathode material determines spectral sensitivity Good signal noise Shock sensitive Anode PMT Gain and Offset 8 19 dynodes 9 10 is most common Gain is the amount of the amplifier multiplies the differential input voltage to produce the output voltage 0 1000V Offset is an error either voltage or current at the input that adds to the input signal before gain takes effect 100 100V 1 Raising gain voltage on PMT can amplify weak signal but also amplify noise 2 Raising offset on PMT threshold can cut off background noise but signal is equally affected the image quality won t improve Improvements increase photon number such as average accumulation slow scan speed lower scan format for bigger pixel size and longer pixel time larger pinhole size etc Multichannel Photon Transducers 1 Photodiode Array Semiconductors Silicon and Germanium Group IV elements Formation of holes via thermal agitation excitation Doping n type Si or Ge doped with group V element As Sb to add electrons As Ar 4S23d104p3 p type Doped with group III element In Ga to added holes In Kr 5S24d105p1 Semiconductor Diodes Diode is a nonlinear device that has greater conductance in one direction than in another Adjacent n type and p type regions pn junction the interface between the two regions High resistant e Forward biasing Reverse biasing Diode I V diagram Photodiode Transducer A silicon photodiode transducer consists of a Reversed Biased pn junction formed on a silicon chip A photon promotes an electron from the valence bond filled orbitals to the conduction bond unfilled orbitals creating an electron hole pair The concentration of these electron hole pairs is dependent on the amount of light striking the semiconductor Convert light into either current or voltage Photodiode 1 In a reverse biased p n junction no current flows 2 When an incident photon strikes the junction it is absorbed and an electronhole pair is created 3 The electron and the hole are swept through the junction in opposite directions creating a current


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