Chapter 7

Components of Optical Instruments

8888

Components of optical instruments

1. Absorption3. Emission and chemiluminescence2. Fluorescence and phosphorescence

Source Wavelength Selector Sample Detector Readout

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

Wavelength 100 180 380 850 2000 18000 40000

Region VAC UV Visible Near IR IR Far IR

Sources

Continous

Line

Ar lamp

Xe lamp

H2 or D2 lamp

Tungtstan lamp

Nernst glower ZnO2+Y2O3

Nichrome wire

Glowbar SiC

Hollow cathode lamp

Lasers

Sources for Spectroscopic Instruments

Light Amplification by Stimulated Emission of Radiation

LASER

• Characteristics of a laser:– Spatially narrow and intense– Highly monochromatic– Coherence

Power supply

Pumping source

Radiation

Partially transmitting mirror

MirrorLaser radiation

Nonparallel radiation

Active lasing medium

Schematic of a Laser Source

Ey’’’

Ey’’

Ey’

Ey

Ex

Metastable Excited state

1- Pumping

Excitation by electrical, radiant or chemical energy

1- Pumping

2- Spontaneous emission

1- Pumping

2- Spontaneous emission

3- Stimulated emission

1- Pumping

2- Spontaneous emission

3- Stimulated emission

4- Absorption

Processes in Laser Action

Light attenuation by absorption

Noninverted population

Inverted population

Light amplification by stimulated emission

Three level system Four level system

E0 E0

E1

Ey Ey

Ex

E1

Wavelength Selectors

• Filters– Interference (UV-VIS)

– Absorption (VIS)

– Cut-off

• Monochromators – Gratings

– Prisms

3000 lines/mm Grating 50 lines/mm

Wavelength selectors

Continous

Dis-continous

Fluorite prism

Fused silica or quartz prism

Glass prism

NaCl prism

Interference wedge

Interference filter

KBr prism

Glass filter

Region VAC UV Visible Near IR IR Far IR

Wavelength 100 180 380 850 2000 18000 40000

Wavelength Selectors for Spectroscopic Instruments

Metal filmDielectric layer

Glass plate

Glass plate

Metal film

Interference Filters

tDielectric layer

Metal film

Metal film

Condition for reinforcement n’ =2t/cos

If < 10o n’ =2t

= ’

tn

Interference Filters

5090 5110 6215 6225 6940 6960

Effective bandwidth= 10

Effective bandwidth= 15 A

Effective bandwidth= 15 A

Effective bandwidth

1/2Peak

height

Wavelength

Per

cen

t T

ran

smit

tan

ce

100

80

60

40

20

0

Transmission Characteristic of Interference Filters

Interference Filter

Effective bandwidth=10nm

Absorption Filter

Effective bandwidth= 50nm

40 450 500 550

100

60

40

20

0

80

Per

cen

t T

ran

smit

tan

ce

Effective Bandwidth of Filters

400 500 600 700

100

50

0

Green filter

Orange cut-off filter

Combinationof two filters

Per

cen

t T

ran

smit

tan

ce

Wavelength nm

Coupling of Filters

Monochromators

For many spectroscopic methods its necessary to be able to vary the wavelengthOf radiation that this process called scanning a spectrum. Monochromators were designed for spectral scanning.

Components of Monochromators1- Entrance slit2- Collimating lens or mirrors3- Dispersing element (prism or grating)4- Focusing element (lens)5- Exit slit

Collimating lens

Focusing lensPrism

Exit slit

Entrance slit

Focal plane

Bunsen Prism Monochromator

Reflection grating

Entrance slit Exit slit

Concave mirrors

Focal plane

Czerny Turner Grating Monochromator

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 spacedGrooves.

n=1

Sourc

eDetector

d

d*sin rd*sin i

d* sin i + d sin r =

Monochromatic beams at incident angle i

Diffracted beams at refelected angle r

r

i

Echellette Grating

d*sin r

d*sin i

Detector

d*sin rd*sin i

d

d* sin i + d *sin r = 1.5 *

Monochromatic beams at incident angle i

Diffracted beams at refelected angle r

r

i

Sourc

e

Plate

Photoresist

UV lamp

Mask

Developing solution

Etching solution

Photolithography

i

r

r = i = 63o26’n= 2dsini

Conventional Echelle Focal length 0.5m 0.5m Groove density 1200/mm 79/mm Diffraction angle 10o22’ 63o26’ Order n (at 300 nm) 1 75 Resolution (at 300 nm) 62400 763000 Recirocal linear disperdion 16 A/mm 1.5 A/mm Light gathering power f/9.8 f/8.8

Echelle Monochromator

Performance Characteristics of Grating Monochromators

Quality of monchromators depend on:1- The purity of its radiant output. Stray Radiation2- The ability to resolve adjacent wavelength R = 3- The light gathering power f = F/d4- 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.

Detector

Wavelength

Pow

er Exit slit

Illumination of an Exit Slit

Monochromator

2

Wavelength

Effective bandwidth

Slit width= Slit width= 3(

Pow

er

Slit width= (Slit width=

Ab

sorb

ance

0.100

0.700

Wavelength, nm275220

(a)

0.5 nm bandwidth

Ab

sorb

ance

Wavelength, nm275220

0.100

0.600

(b)

1.0 nm bandwidth

Ab

sorb

ance

Wavelength, nm275220

0.100

0.600

(c)

2.0 nm bandwidth

Effect of Spectral Bandwidth

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

Wavelength 100 180 380 850 2000 18000 40000

Region VUV UV Visible Near IR IR Far IR

Materials for cells, windows, lenses and prisms

LiF

NaCl

KBr

ZnSe

Fused silica or quartz

Corex glass

Silica glass

TlBr or TlI

Construction Materials for Spectroscopic Instruments

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.

Res

pons

e

Power

1. High sensitivity

2. High signal to noise ratioNoise

Signal

Lower S/N

Higher S/N

3. Constant response over considerable range of wavelength

4. Fast response time

5. Zero output signal in the absence of illumination S = kP

S = kP+kd

Res

pons

e

Power6. The electrical signal would be directly proportional to radiant power

PowerR

espo

nse

Res

pons

e

WavelengthRes

pons

e

Time

Ideal Radiation Transducers

Types of Radiation Transducers

• Photon transducers– UV, Vis, near IR

• Heat transducers– IR, far IR

200 600 1000 1400 1800 2200 Wavelength nm

109

1013

1012

1011

1010

1014

1015

Photomultiplier tube

CdS photoconductivity cell

GaAs photovoltaic cell

silicon photodiode PbS photoconductivity cell

CdSe photoconductivity cell

Se/SeO photovoltaic cellThermocouple

Golay

Sp

ectr

al r

esp

onse

.

Response of Detectors

Photon Transducers

• Photovoltaic cells

• Phototube

• Photomultiplier tubes

• Photoconductivity transducers

• Silicon photodiodes

• Charge-coupled device

Plastic case

Glass Thin layer of silver

Selenium

Iron

+ -

Photovoltaic Cell

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

90 Vdc

Wire anodeCathode

Vacuum Photo Tube

Sen

siti

vity

, mA

/w80

20

40

60

400 600 800 1000200

Wavelength, nm

K/Cs/Sb

Ga/As

Ag/O/Cs

Response of some Photoemissive Surfaces

Dynode Potential(V) Number of electrons

1 90 1

2 180 10

3 270 100

4 360 103

5 450 104

6 540 105

7 630 106

8 720 107

9 810 108

Anode 900V Gain =108

12

3

4

5

6

8

7

9

Anode

PhotoemissiveCathode

GrillQuartz envelope

+_

900V dc

Anode

PhotoemissiveCathodeDynodes 1-9

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

p region n region

pn junction

Metal contactLead wire

Silicon Diode

Forward bias

ee

Silicon Diode under Forward Bias

-+

Depletion layer

Reverse bias

Silicon diode under Revese Bias

- +

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

+15V

-15V

To amplifier

Spectrophotometer slit

Thermocouple junction

Reference junction

+_

Detectors

Photo- electric

Thermal

Photographic plates

Photomultiplier tube

Photo tubes

Photo cells

Photo diodes

Photo conductors

Golay pneumatic cell

Pyroelectric cell

Charge coupled devices

Wavelength 100 180 380 850 2000 18000 40000

Region VAC UV Visible Near IR IR Far IR

Detectors for Spectroscopic Instruments

Thermocouples or bolometers

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.