The document discusses spectral instruments, which analyze the interaction between matter and the electromagnetic spectrum, focusing primarily on photon detectors such as photodiodes and photomultiplier tubes. It provides details on their working principles, performance parameters, and various applications in fields like nuclear medicine and light measurement. The document also covers different light sources used in conjunction with these instruments, including incandescent lamps and discharge tubes.

1. SPECTRAL INSTRUMENTS
PRESENTED BY :
Preeti Choudhary (17/MAP/016)

2. The Electromagnetic spectrum

3. What are Spectral Instruments ?
Spectral Instruments are the devices which are
used for the analysis of the interaction between
matter and any portion of the electromagnetic
spectrum.

4. Photodetector
• A photodetector is a device which absorbs
light and converts the optical energy to
measurable electric current.
• Detectors are classified as Thermal detectors
and Photon detectors, we will focus on photon
detectors only.

5. Photon detectors
• work on the principle of conversion of
photons to electrons
• a device may absorb photons only if the
energy of incident photons is above a certain
minimum threshold.
• Photon detectors, in terms of the technology,
could be based on (a) Vacuum tubes - e.g.
Photomultipliers and (b) Semiconductors - e.g.
Photodiodes.

6. Performance Parameters
• Responsivity : Responsivity of a detector is given
as the ratio of the generated photocurrent (I) to
the amount of optical power (P0 ) incident on the
detector The unit of responsivity is
amperes/watt.
• Responsivity depends on the wavelength and for
an ideal photodetector, varies linearly with
wavelength.

7. • Quantum Efficiency: The number of electrons
produced per incident photon is defined as
the quantum efficiency , which is usually
expressed as a percentage .

8. Photodiode
• A photodiode is a p-n junction diode that can
absorb photons and generate either a
photovoltage or free carriers that can produce
photocurrent.
• A photodiode is designed to be used in
reverse bias.

10. Cross sectional view of silicon photodiode
•The figure shows a p-n
junction diode with a heavily
doped p-side. The donor
concentration on the n side of
the junction is less than the
acceptor concentration on the
p+ side.
• The p- layer is very thin and
is formed on the front surface
of the device on an n-type
silicon.
•The active area is coated
with an antireflection coating
of material (like silicon
nitride) so that most of the
light falling on the device can
be trapped by it.
•Metallized contacts provide
the terminals.
•Photons absorbed in depletion region produces
charge carriers that are immediately swept across
the junction by the natural bias.
•This movement of charge carriers across the
junctions upsets the electrical balance and
produces small photocurrent, which is detected at
electrodes.

11. Photomultiplier tube

12. What is it ?
• A photomultiplier converts light into an
electrical signal, then amplifies that signal to a
useful level by emission of secondary
electrons

13. Components
• a photocathode which converts light flux into
electron flux
• an electron-optical input system which
focuses and accelerates the electron flux
• an electron multiplier consisting of a series of
secondary-emission electrodes (dynodes)
• an anode which collects the electron flux from
the multiplier and supplies the output signal

15. Applications
• Photodiodes are used in the receivers for infrared remote
control devices used to control equipment
from televisions to air conditioners.
• Photodiodes are often used for accurate measurement of
light intensity in science and industry.
• For smaller photon fluxes, the photomultiplier can be
operated in photon-counting or Geiger mode.
• Photomultipliers tubes are used in gamma cameras which
are used in field of nuclear medicine.

16. SOURCES OF LIGHT

17. Sources Of Light
1. Incandescent Solid Lamp 3. Discharge Tubes2. The Bunsen Flame
ii. Cold-cathode Discharge
Tubes
i. Hot-cathode Discharge
Tubes
b. Hydrogen Discharge
Tube
a. Neon Discharge Tube
b. Mercury Vapour Lamps
a. Sodium Vapour Lamps

18. 1. The Incandescent Solid Lamp
 A solid heated to a high temperature
has been used as a source of light.
 It emits a continuous spectrum.
 All the colours are gradually added up
as the temperature rises higher and
higher.
 e.g. Tungsten filament lamp

19. 2. The Bunsen flame
 Any substance introduced in
the non-luminous part of the
flame gives the characteristic
spectrum.
 It may consists of a number of
spectral lines.
 To obtain sodium flame, the
sodium salt is introduced in the
flame using an asbestos ring
dipped in the solution of the
salt.

20. 3. Discharge Tubes
 Discharge tubes are lamps in which an
electric discharge is passed through a
rarefied gas.
 The atoms of the gas get excited to a
higher energy states.
 When these atoms fall back to the
lower energy state, emits radiation.
 The whole space inside the tubes gets
filled with a luminous.
 Two types:
 (i) Hot-cathode discharge tubes
 (ii) Cold-cathode discharge tubes

21. (i) Hot-cathode Discharge Tubes
These discharge tubes have high operating
current in them (~lamp) because of which the
electrodes get heated
e.g. Sodium vapour lamp,
Mercury vapour lamp,
etc.

22. Sodium Vapour lamp
 U-tube is filled with Neon gas
containing small amount of sodium.
 Neon:Sodium=8000:1
 Neon gas pressure is 10 mmHg.
 Operating temperature inside U-tube
~280oC.
 Operating voltage of lamp, 440V.
 The visible light emitted by sodium
lamp is a doublet called D1 (5896Å) and
D2 (5890Å) lines.

23. Sodium Vapour Lamp
Tungsten
electrodes
Metallic sodium pecks
U-shaped tube

25. Spectrum Of Sodium Lamp

26. Mercury Vapour lamp
 Arc tube contains Argon
gas and some amount of
Mercury.
 Argon gas pressure is 10
mmHg.
 Operating voltage: 140V.
 Choke helps in regulating
the current flow through
the lamp.
 Operating temperature
inside arc tube: ~600oC.

27. Mercury Vapour Lamp

28. Mercury Vapour Lamp (simplified
schematic)
Double walled bulb
Electrodes
Starting Electrode
Arc Tube

29. Spectral distribution for Mercury Lamp

30. Cold Cathode Discharge Tube
 These are also called Geissler Tubes.
 Mostly used to study the spectra of
different gases (neon, hydrogen, argon
or helium) at low pressure.
 They are usually of two shaped (fig.).
 Tubes are filled with the gas whose
spectrum to be study.
 Gas is filled at the pressure of 1 or 2
mmHg.
 Operating voltage: >2000V.
Operating current in these tubes is of the order of few milliamperes, which can not heat the
electrodes, that’s why they are known as cold cathode discharge tubes.

31. Cold Cathode Discharge Tube
e.g. (i) Neon discharge tube
(ii) Hydrogen discharge tube
(iii) Nitrogen discharge tubes.
etc.

32. Cold cathode tubes

33. Sodium lamps (45W, 80W, 160W,
200W)

34. Mercury lamp (90W)

35. THANK YOU !