1. Chapter 3. Components of Optical Instruments

2. §1 General designs of optical instruments
Source
container
Wavelength selector
Radiation detector
Signal processor and readout
Absorption
Fluorescence
Phosphorescence
Scattering
Emission
Chemiluminescence
Components of typical spectroscopic instruments
Phenomena for optical spectroscopic methods
Three configurations

3. B. fluorescence, phosphorescence, scattering
A. absorption
(1)
(2)
(3)
(4)
(5) hv hv hv I
Source lamp
Or heated solid
Sample
holder
Wavelength
selector
Photoelectric
transducer
Signal processor
and readout
B. fluorescence, phosphorescence, scattering
(2)
(3)
(4)
(5) hv hv I
Sample
holder
Wavelength
selector
Photoelectric
transducer
Signal processor
and readout hv
(1)
Source lamp
Or heated solid
C. emission, chemiluminescence
(3)
(4)
(5)
(1) and (2) hv hv I
Wavelength
selector
Photoelectric
transducer
Signal processor
and readout
Source and
Sample holder

4. §2 Sources of radiation Enough Energy & Stability
Sources for spectroscopic instruments

5. Continuum Sources

6. Line Sources

7. • monochromatic (narrow bandwidths)(line sources)
Laser Sources
Light Amplification by Stimulated Emission of Radiation
Advantages
• intense
• monochromatic (narrow bandwidths)(line sources)
• pulsed ( s) or continuous wave (cw)
• coherent nature of outputs
• small beam divergence

8. §3 Wavelength Selectors
For spectroscopic analysis, a limited, narrow, continuous group of wavelength is required.
干涉光劈

9. Two types of wavelength selectors Filters monochromators
Fig Output of a typical wavelength selector (p.155)

10. b. Absorption filter - colored glass or dye between two glass plates
Filters
a. Interference filter - two thin sheets of metal sandwiched between glass plates, separated by transparent material (Interference for transmitted wave through 1st layer and reflected from 2nd layer )
100
Band-reject interference filter
Transmittance
b. Absorption filter - colored glass or dye between two glass plates
400
Wavelength (nm)
750
100
Useful in non-scanned situations and when a specific, known band of radiation needs to be monitored.
Band pass interference filter
Transmittance

11. Monochromators:
Five components:
• Entrance slit • Collimating lens or mirror
• Dispersion element (prism or grating) • Focusing lens or mirror • Exit slit

13. Grating (most modern instruments)
Echellette grating:

14. Performance characteristics of monochromators
(1) Spectral purity scattered or stray light in exit beam
Use entrance and exit windows, dust and light-tight housing, coat interior with light absorbing paint
(2) Dispersion ability to separate small wavelength differences
( quantify the spatial separation of wavelengths on the exit focal plane)

15. Angular Dispersion:
Da=dθ/dλ (property of dispersive element)
Linear Dispersion:
D=dy/dλ (property of dispersive device)
Shown in instrument handbook

16. Resolving Power of monochromators

17. Resolving Power for a Grating
So, in order to just resolve these two spectral lines: λ1 = 4501 Å
λ2 = 4499 Å
We need an instrument with a resolving power of:
R = 4500/2 = 2250
The resolving power of a diffraction grating:

19. Effect of slit width on resolution
Fig Illumination of an exit slit by monochromatic radiation 2 at various monochromator settings. Exit and entrance slits are identical. (p.165)
Bandwidth: range of wavelengths exiting the monochromator
Related to dispersion and entrance/exit slit widths
Effective bandwidth (eff ):
eff =
When y is equal to slit width w,  is the effective bandwidth, we know

20. Effect of slit width on spectra (p.166)
No spectral resolution of three wavelengths
Partial resolution of three wavelengths
Complete resolution of two lines, only if the slit width is adjusted so that the effective bandwidth of the monochromator is equal to one half the difference between l's

21. Example:

22. Example 7-2 (p.165):
A grating monochromator with a reciprocal linear dispersion of 1.2 nm/mm is to be used to separate the sodium lines at and In theory, what slit width would be required?
Solution: we know D-1 = 1.2 nm/mm
( )
eff = = 0.3 nm 2
Since , then
w = 0.3/1.2=0.25 mm

23. How Can We Improve Resolution?

24. Sample Containers and Optics:
• Cuvettes
• Lenses
• Prisms, gratings, filters
Made of suitable material (see table 7-2):
Glass nm (vis-near IR)
Silica/quartz nm (UV-near IR)
NaCl ,000 nm (UV-far IR)

26. Radiation Transducers:
Ideally:
• high sensitivity
• low noise
• wide wavelength response
• fast response time
• linear output (S=k·P)
[Many real transducers exhibit dark current (kd) (S=k·P+kd)]

27. Detectors for spectroscopic instruments
Two general types

28. Photon Transducers:
(A) Photovoltaic cells - metal-semiconductor-metal sandwiches that produce voltage when irradiated ( nm)

29. (B) Phototube - electrons produced by irradiation of cathode travel to anode. l response depends on cathode material ( nm)
The basic phototube is packaged in vacuum and presents a photocathode (such as alkaline earth metals, Hg, Au, Ag... ) covered with a particular photoemissive material. A biased anode collects the ejected electrons and an external meter measures the current that flows.
photon
photocathode
anode

30. Secondary Electron Emissive Materials for Dynodes
(C) Photomultiplier tube (PMT) - irradiation of cathode produces electrons, series of anodes (dynodes) increases gain to electrons per photon. Low incident fluxes only!
light
dynodes
anode
photochathode
electrons
voltage divider network
high voltage
Secondary Electron Emissive Materials for Dynodes
BeO (Cs) GaP (Cs) MgO (Cs) Cs3Sb KCl

31. (D) Photodiode
A photodiode is formed by sandwiching an undoped layer of Si between a heavily doped p-layer and a heavily doped n-layer. Photons can be absorbed in the intrinsic layer, producing an electron -hole pair. The bias potential sweeps these carriers to the opposite regions, producing a current in the external circuit.
Photodiodes are more sensitive than phototubes, but less sensitive than PMT’s.
(E) Photodiode Array

32. Thermal detectors - sensitive to IR (l>750 nm)
thermocouples - junction thermometer
bolometers - resistance thermometer
pyroelectric devices - piezoelectric effect
Charge-transfer device (CTD)
Charge-coupled device (CCD)
Charge-injection device (CID)