Components of Optical Instruments Chapter 7 Components of Optical Instruments

Components of optical instruments 2. Fluorescence and phosphorescence 3. Emission and chemiluminescence 1. Absorption Source Wavelength Selector Sample Detector Readout l 8888 Rgb back 146,184. 148

Source of Radiation A source should be generate a beam of radiation with sufficient power Its out put power should be stable for reasonable period Continuum Sources (D2 lamp, Ar lamp, Xe lamp, Tungsten lamp) Line Sources (Hollow cathode lamp, Hg vapor, Na vapor, Electrodeless discharge lamp) Laser Sources

Sources for Spectroscopic Instruments Wavelength 100 180 380 850 2000 18000 40000 Region VAC UV Visible Near IR IR Far IR Ar lamp Sources Continous Line Xe lamp H2 or D2 lamp Tungtstan lamp Nernst glower ZnO2+Y2O3 Nichrome wire Glowbar SiC Hollow cathode lamp Lasers

Light Amplification by Stimulated Emission of Radiation LASER Characteristics of a laser: Spatially narrow and intense Highly monochromatic Coherence

Schematic of a Laser Source Nonparallel radiation Active lasing medium Laser radiation Mirror Partially transmitting mirror Radiation Pumping source Power supply

Processes in Laser Action 1- Pumping 2- Spontaneous emission 3- Stimulated emission 4- Absorption 1- Pumping 2- Spontaneous emission 3- Stimulated emission 1- Pumping 2- Spontaneous emission 1- Pumping Excitation by electrical, radiant or chemical energy Ey’’’ Ey’’ Ey’ Ey Ex Metastable Excited state

Light attenuation by absorption Noninverted population Light amplification by stimulated emission Inverted population

E1 E1 Ey Ey Ex E0 E0 Three level system Four level system

Wavelength Selectors Filters Monochromators Interference (UV-VIS) Absorption (VIS) Cut-off Monochromators Gratings Prisms

Wavelength Selectors for Spectroscopic Instruments Region VAC UV Visible Near IR IR Far IR Fluorite prism Wavelength selectors Continous Dis-continous Fused silica or quartz prism Glass prism NaCl prism KBr prism 3000 lines/mm Grating 50 lines/mm Interference wedge Interference filter Glass filter

Interference Filters Glass plate Metal film Dielectric layer

Interference Filters Metal film Dielectric layer t Metal film q Metal film Dielectric layer t Metal film Condition for reinforcement nl’ =2t/cos q If q < 10o nl’ =2t = l’h l = 2th/n

Transmission Characteristic of Interference Filters 100 Effective bandwidth= 15 A 80 Effective bandwidth= 15 A 60 Effective bandwidth= 10 A Percent Transmittance Effective bandwidth 40 1/2 Peak height 20 5090 5110 6215 6225 6940 6960 Wavelength

Effective Bandwidth of Filters 100 Interference Filter 80 60 Effective bandwidth=10nm Percent Transmittance 40 Absorption Filter 20 Effective bandwidth= 50nm 40 450 500 550

Percent Transmittance Coupling of Filters 100 Orange cut-off filter Green filter 50 Percent Transmittance Combination of two filters 400 500 600 700 Wavelength nm

Monochromators Components of Monochromators 1- Entrance slit For many spectroscopic methods its necessary to be able to vary the wavelength Of radiation that this process called scanning a spectrum. Monochromators were designed for spectral scanning. Components of Monochromators 1- Entrance slit 2- Collimating lens or mirrors 3- Dispersing element (prism or grating) 4- Focusing element (lens) 5- Exit slit

Bunsen Prism Monochromator Entrance slit Focal plane l1 Exit slit Collimating lens l2 Focusing lens Prism

Czerny Turner Grating Monochromator Concave mirrors Reflection grating l1 l2 Entrance slit Focal plane Exit slit

Gratings 1- Transmission Gratings 2- Reflection Gratings Replica Grating Echellette Grating Concave Grating Halographic Grating

Replica Grating Replica grating are manufactured from a MASTER grating which consists Of a hard, optically flat, polished surface upon which have been ruled with A suitable shaped diamond tool a large number of parallel and closely spaced Grooves.

Echellette Grating Detector Source d* sin i + d sin r = l r d*sin i i Diffracted beams at refelected angle r Monochromatic beams at incident angle i d* sin i + d sin r = l r n=1 d*sin i i d*sin r d

Detector Source d*sin i d*sin r d* sin i + d *sin r = 1.5 * l r Diffracted beams at refelected angle r Source d*sin i d*sin r d* sin i + d *sin r = 1.5 * l Monochromatic beams at incident angle i r d*sin i i d*sin r d

Photolithography UV lamp Mask Photoresist Plate Developing solution Etching solution

Echelle Monochromator i r = i = b = 63o26’ nl = 2dsini r

Performance Characteristics of Grating Monochromators Quality of monchromators depend on: 1- The purity of its radiant output. Stray Radiation 2- The ability to resolve adjacent wavelength R = l/Dl 3- The light gathering power f = F/d 4- The spectral band width Bandwidth is defined as the span of monochromator settings (in units of wavelength) needed to move the image of the entrance slit across the exit slit.

Illumination of an Exit Slit Monochromator Exit slit Power l3 l2 l1 Wavelength Detector

Slit width= Slit width= (l2-l1)/2 Slit width= 3(l2-l1)/4 Effective bandwidth Power l3 l2 l1 Wavelength

Effect of Spectral Bandwidth 0.700 0.600 Absorbance 1.0 nm bandwidth Absorbance 0.5 nm bandwidth 0.100 0.100 220 275 220 Wavelength, nm 275 Wavelength, nm (a) (b) 0.600 Absorbance 2.0 nm bandwidth 0.100 220 275 Wavelength, nm (c)

Sample Containers Cells or cuvettes that hold the samples must be made of materials that is transparent to radiation in the spectral region of interest. Quartz or fused silica : UV region (below 350 nm) and also up to 3000 nm Silicate glasses: The region between 350 and 2000 nm Plastic containers : Visible region Crystalline sodium Chloride : IR region

Construction Materials for Spectroscopic Instruments Wavelength 100 180 380 850 2000 18000 40000 Region VUV UV Visible Near IR IR Far IR LiF Fused silica or quartz Materials for cells, windows, lenses and prisms Corex glass Silica glass NaCl KBr TlBr or TlI ZnSe

Radiation Transducers The detectors for early spectroscopic instruments were the human eye or a photographic plate or film. These detection devices have been largely supplanted by transducer that convert radiant energy into an electrical signal. These transducer are modern detectors.

Ideal Radiation Transducers Noise Signal Lower S/N Higher S/N Response Power 1. High sensitivity Response Wavelength 2. High signal to noise ratio Response Time 3. Constant response over considerable range of wavelength Response Power 4. Fast response time 5. Zero output signal in the absence of illumination S = kP S = kP+kd Power Response 6. The electrical signal would be directly proportional to radiant power

Types of Radiation Transducers Photon transducers UV, Vis, near IR Heat transducers IR, far IR

Response of Detectors 1015 Photomultiplier tube 1014 1013 CdS photoconductivity cell Spectral response. 1012 GaAs photovoltaic cell CdSe photoconductivity cell 1011 PbS photoconductivity cell silicon photodiode 1010 Se/SeO photovoltaic cell Thermocouple Golay 109 200 600 1000 1400 1800 2200 Wavelength nm

Photon Transducers Photovoltaic cells Phototube Photomultiplier tubes Photoconductivity transducers Silicon photodiodes Charge-coupled device

Photovoltaic Cell - Glass Thin layer of silver Selenium Plastic case Iron + -

Characteristics of Photovoltaic Cells Cell current = 10 – 100 mA No external electrical energy required. Usually used for low level signals. Low internal resistance = amplification not convenient Fatigue Low cost

Vacuum Photo Tube Cathode Wire anode 90 Vdc

Response of some Photoemissive Surfaces 80 K/Cs/Sb 60 Sensitivity, mA/w 40 Ga/As 20 Ag/O/Cs 200 400 600 800 1000 Wavelength, nm

Dynode Potential(V) Number of electrons 5 1 90 1 2 180 10 7 3 3 270 100 4 6 4 360 103 2 1 5 450 104 8 9 6 540 105 7 630 106 Quartz envelope Anode Grill 8 720 107 9 810 108 Photoemissive Cathode Anode 900V Gain =108 900V dc Photoemissive Cathode Dynodes 1-9 Anode + _ To readout

Features of Photomplier Tubes High sensitivity in UV, Vis, and NIR Limited by dark current Cooling to -30oC improves response Extremely fast time response Limited to measuring low-level signals

Silicon Diode pn junction Metal contact Lead wire p region n region

Silicon Diode under Forward Bias + - e

Silicon diode under Revese Bias Reverse bias - + Depletion layer

Photoconductivity Transducers The most sensitive transducers for monitoring radiation in the NIR region are semiconductors whose resistance decrease when they absorb radiation within this range. Absorption of radiation by these material promotes some of their bound electrons into an energy state in which they are free to conduct electricity. The resulting change in conductivity can then be measured. Examples: CdS, CdSe, CdTe, PbS (specially sensitive at room temp.) PbSe, InS, InSe.

Thermal Transducers This kind of transducers generally used in IR regions which photons lack the energy to cause photoemission of the electrons. The radiation impinges upon and is absorbed by a small black body, and the resultant temperature is measured. The heat capacity of the absorbing elements must be as small as possible if a detectable temperature change is to be produced. Thermocouples Bolometers: resistance thermometer, Pt, Ni or semiconductor (thermistor) Pyroelectric transducers electric polarization, slight relative shift of positive and negative electric charge in opposite directions within an insulator, or dielectric, induced by an external electric field. Polarization occurs when an electric field distorts the negative cloud of electrons around positive atomic nuclei in a direction opposite the field. This slight separation of charge makes one side of the atom somewhat positive and the opposite side somewhat negative

Spectrophotometer slit +15V Spectrophotometer slit Thermocouple junction + _ To amplifier Reference junction -15V

Detectors for Spectroscopic Instruments Wavelength 100 180 380 850 2000 18000 40000 Region VAC UV Visible Near IR IR Far IR Photographic plates Detectors Photo- electric Thermal Photomultiplier tube Photo tubes Photo cells Photo diodes Charge coupled devices Photo conductors Thermocouples or bolometers Golay pneumatic cell Pyroelectric cell

Signal Processors and Readouts The signal processor is ordinarily an electronic device that amplifies the electronic signal from the transducer. In addition, it may change the signal from dc to ac (or the reverse), change the signal phase, and filter it to remove unwanted components. Also they may perform some mathematical operation on the signal such as differentiation or integration or conversion to logarithm. Readout devices are found in modern instrument. Some of them include digital meters, potentiometer or cathode array tube.