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



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