4.1 Spectrographs and monochromators Flashcards
Which is more sensitive interf or spectormeter
Intereferometer by far
Spectrograph
The first instrument for measuring wavelengths
optical instruments that form images S2(lambda) of the entrance slit S1
ththe images of laterally (spatially) separated for different wavelengths of incident radaiation
lateral dispersion due to spectral dispersion in prism or diffraction on plane or concave reflection gratings
Explain the 4f setup of a prism spectrometer
-light source L illuminates colletor lens L0
-L0 focus light into entrance slit s1
-s1 is placed in focal plane of (collimator lens) L1 so that light passing through L1 is collimated
-collimated light from L1 passes through a prism P where it diffracted by angle theta(lambda) dep on the wavlength
-diffracted and spatially sep light passes through L2 which focuses it and forms an image s2(lambda) of the entrance slit S1
the position x(lambda) of this image in the focal plane of L2 is a function of wavelength
what is linear disp of spectrograph
It depends on:
1) spectral dispersion dn/dY of the prism materal
2) focal length of L2
Explain the fourier imaging of 4f system
f1 to lens L1 , f1 to centre (prism here)
f2 to lens L2, f2 to image
midpt is the FT, end is FT(FT) = starting image
General setup of reflection diffraction grating in a spectrometer
1) 2 lenses L1 and L2 in prism spec replected with spherical mirrors M1M2
2) which image entrance slit S1 either onto the exit slit S2 or
3) via a third mirror M to a CCD array
Difference between spectrograph and monochromator
Spectraphs
1)CCD
2) simult measyre large Y region
3) mdoerate time resolution
Monochromators/photoelectric detection
1) high time resolution
2) only small Y interval dY measureable at a time
generally both are called spectrometers
what is ccd how it function to make spectrogra[h
charge coupled device (diode array)
1)placed in focal plane of L2 (prsim) or M2(grating spec)
2) whole spectral range laterally x(Y) is covered by the lateral extent of the idode array (all dY detectected simul.)
3)spectral range limited by spectral sensitivity of CCD materials usually 200-1000nm range
how does monochromator use phtooelectric
select small spectral interval detect successively not simul.
1) exixt slit S2 slecets interval dx2 in the focal plane of output light and only lets small dY thoguh to the photoelectric detector
2) either the slit is moved but IRL the grating is roated through angles to run though the whole dY region
3)
what determiens the signal recieved by a monochromator
Power is measured so that is energy = energy per second
so signal is proportional to product of
1) area of the exit slit with height h with (hdx2)
2) spectral intensity obver that range integral I(Y) dY
How to optimise spec speed
Image light source onto entrance slit st the acceptance angle Omega is fully used
Radiation outside the acceptance angle
1) cannot be detected
2) may increase background by scatttering from lens holders and with spectrometer walls
FSR
Free spectral range is the wavlength interval dY of the incident radiation for which a one valued relation exists between Y and the position x(Y) of the entrance slit image
the range at which you can uniquely determine Y
What is FSR diff between prism and grating spectrometers and interferometer
Prism - FSR covers whole region of normal dipersion of the prism material
grating - dY is determined by diffraction order m and decreases with increasing m
interferometer - very small FSP dY even though they ahvea high spectral resolution R, they require a preselector which lets you measure wavlength within the FSR of the intrument
