Optical Spectroscopy with dispersive Spectrometers
Basics - Building Blocks - Systems - Applications

Basics, Chapter 4: Detectors for
Monochromators and Spectrographs.
 

Please note: The content of the BASICS pages is available in printed shape since June 2014:
 

"Fundamentals of dispersive optical Spectroscopy Systems",
SPIE-Monograph, ISBN No.: 9780819498243

The internet pages have been reduced to the headlines, equations, and figures of the different chapters . We hope, that the brief collection and the complete and extended book will be as helpful and useful for you, as the spectra-magic.de/Basics have been.

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4.0.0 Fundamentals
Aim of the “Fundamentals” is to help starters with the definition and verification of the instrumentation for optical spectroscopy. Experienced spectroscopists and expert physicists will know some of the issues better and in deeper detail. But hopefully, every reader may find things worth mentioning.


4.02 The Detectors chapter consists of 5 parts:
1) the signals involved,
2) single element detectors,
3) data acquisition techniques,
4) diode arrays, CCDs, and other techniques,
5) definition of signal functions: response time, bandwidth, and roll-off.

4.1.0 Introduction and Representation of Symbols

4.1.1 Work and Power of Light Signals

Energy Conversion Curves
Graph 81: the presentation of photon energy

4.1.2 Basic Parameters of Detectors

 General Parameters in Detection
Graph 82 displays some general signal parameters of optical detectors.

4.1.3 Detection Limit, Noise, and Signal/Noise-Ratio

4.1.5 Detection Limit, Noise and SNR for relative Measurements

4.2 Single Point Detectors

 PMT Parameters
Graph 83: General and normalized data curves of photo tubes
typical PMT QE curves
Graph 84: typical Quantum Efficiencies of common Cathode materials

4.2.2 Comments on the interpretation of PMT data sheets

4.2.3 A Sample Calculation for PMT, valid for an Integration Time of 1 s

4.2.4 The Photon Counter

4.2.5 UV PMTs and Scintillators

4.3 The Illumination of Detectors, combined with Image Conversion

Coupling Methods
Graph 85: Four of many options to couple detectors and spectrometers

4.4 Channeltron and Micro-Channelplate (MCP)

Channeltron and Channelplate
Graph 86: Channeltron and Micro-Channelplate Intensifier, according to 4.4.1 und 4.4.2

4.4.1 The Channeltron,
4.4.2 Basis of a Micro Channelplate system is the plate itself, in the sketch marked red and with MCP.

4.5 Intensified PMT and Single Photon Counting

4.6 Solid State Detectors

Solid State Efficiency Curves
Graph 87: Some Efficiency Curves of Solid-State Detectors for the Range of 20 µm, simplified.

The General Effect of Cooling


4.6.1 Planck´s Radiation = Blackbody Radiation

Blackbody Radiation
Graph 88: Planck´s Radiation at low Temperatures in linear and logarithmic scale in the Range of 0.5 bis 20 µm.

4.6.2 The background charge´s magnitude of IR detectors strongly depends on the material
.

4.6.3 The Parameter D* connects important specifications of IR detectors in a single equation.

4.6.4 Detectors and the ambient Temperature

 IR Detector setup
Graph 89: Principle of a Detector with cold aperture

Two exemplary examples shall demonstrate the Signal/Noise Ratio at IR Detectors
4.6.5 Estimation under continuous Light Situations


4.6.6 A Consideration of synchronized Measurements

Absorption Setup with / without Lock-In
Graph 90A: Simple IR spectroscopy setup for Absorption with/without Lock-In Amplifier, the Lock-In typical signals are marked violet, description follows with graph 90B.

4.6.6.1 Typical Signals at the Lock-In in optical Applications:

Signals at the Lock-In
Graph 90B: The Lock-In Amplifier´s most important signals:

4.6.6.2 Comparison of Measurements with DC Background, clock-pulsed Background, and Lock-In-Detection

4.6.8 Estimation of the modulated Measurement
at the same Conditions as with continuous Light, described under 4.6.5


4.6.9 Tandem Detectors, also called Sandwiches

 Sandwich Detector
Graph 91: Principle of Sandwich Detectors.

4.6.10 Typical Parameters of Solid State Detectors, and their Interpretation


4.6.11 The Illumination of small Detector Elements


4.6.12 Charge storing Semiconductor Elements, thermal Recombination and Holding Time


4.6.13 PIN and Avalanche Diodes


4.7 Detector coupling by Fibre Optics

Fibre Coupler
Graph 92: Detector coupling by fibre cable and fibre taper

4.8 Area Detectors:  CCD and Array

4.8.1 Mounting of Area Detectors, and the Distribution of Wavelengths

4.8.1.1 What are the popular Kinds of Area Detectors?


4.8.2 Basic Parameters of Arrays and CCDs with and without Cooling

4.8.2.1 Pixel Size, Capacity, Sources of Noise, Dynamic Range, Shift Times, Read-out Time, ADC Conversion Time


4.8.2.2 The applicability of CCD for Spectroscopy, Image Processing, and Photography


4.8.3 Signal Transfer and Read-Out

graph 93: CCD read out scheme
Graph 93: Shifting processes to read-out a CCD

4.8.3.1 Combination of read-out in Imaging Mode and Display in Spectroscopy Fashion

4.8.4 CCD Architecture

 CCD Architecture
Graph 94: CCD Architecture

4.8.5 CCD and Array Efficiency

4.8.5.1 Front illuminated CCD

Front illuminated CCD Efficiency
Graph 95A: Typical Efficiency Curves of different types of Front illuminated CCD

4.8.5.2 Rear Side illuminated CCD

 Back illuminated CCD Efficiency
Graph 95B: Some typical Efficiency Curves of rear-side illuminated CCDs.
 IGA Array Efficiency
Graph 95C: Three representative InGaAs Efficiency Curves.

4.8.5.3 Interferences with Rear side illuminated CCD - Etaloning
As mentioned above, rear-side illuminated CCD show an interference, which is affected by the double pass though refractive index changes: between air or vacuum and the substrate and the substrate and the light sensitive element.

Etaloning
Graph 95D: typical CCD behaviour in the NIR


4.8.6 Time Control – Synchronization, Shutter, and Gating


4.8.6.1 Shutter Control


4.8.6.2 Micro Channelplate Image Intensifiers

4.8.7 Current Formats of Area Detectors

4.8.7.1 Diode Arrays

4.8.7.2 CCD Sensors

4.8.8 Read-out Techniques: Multi Spectra Spectroscopy, Binning and Virtual CCD Partition

4.8.8.1 Multi spectra or Multitrack Spectroscopy and vertical Binning

 Ray Travel in a Multitrack Scpetroemter
Graph 96: Example of the beam travel in a multi spectra experiment.
Multitripe Read-out Pattern
Graph 97: Read-out pattern of a square shaped CCD for Multitrack Spectroscopy


4.8.8.2 Virtual CCD Programming


4.8.9 CCD and Array Systems with Image Intensification


4.8.9.1 CCD with „On-chip Multiplication“ or „Electron Multiplication“ (EMCCD).

4.8.9.2 CCD with additional Micro channelplate image intensifier (MCP-CCD)


4.8.10 Measurements in the ms - µs Time Frame

4.8.10.1 Kinetic Measurements
 CCD modified for Kinetics
Graph 98: CCD modified for Kinetic Measurements

4.8.10.2 Double Pulse Measurements


4.8.11 Extension of the spectral Efficiency into the deep UV


4.9 Other Area Detectors

4.9.1 CID and C-MOS Array

4.9.2 NIR and IR Area Detectors


4.9.2 The Positions sensitive Detectorplate (PSD)

 Position Sensitive Detector
Graph 99: Function of a PSD Detector

4.10. Definitions

4.10.1 Exponential Functions and Signal Damping
e-funtions
Graph 100A: the general behaviour of e-functional Processes,
damping devides - analogue and digital behaviour
Graph 100B:
 Definition of the time constant

4.10.2 Low Pass Filter Functions
Filter Roll-off Functions
Graph 101: The Suppression of Signals outside the desired Bandwidth

4.10.2.2 Additional Note on the dB Interpretation
.

4.10.3 Definition of Bandwidth in electric vs. opto-spectroscopic Systems.
Defintion of Bandwidth
Graph 102: Comparison of the Bandwidth Definition for System in electronic technology versus optical Spectroscopy, with Signal behaviour following a Gaussian normal distribution
 

Hoping, the study of the book Elements of dispersive optical Spectrometers will be of help for you, we remain with special thanks for your interest.

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