2010 REU Lecture
Optics and Instrument Design
An overview
Erik Richard
erik.richard@lasp.colorado.edu
303.735.6629
Optics – REU Lecture 2010
Richard 1
Outline
•Brief Review: Nature of Light (Electromagnetic Radiation)
–Propagation of E&M waves
–Interaction with matter
–Wave-particle duality
–Atomic, Molecular absorptions and emssions
–Blackbody Radiation – Planck
–Sensors
• Brief Review: Optics Concepts
- Refraction, Reflection, Diffraction, Polarization
- Prisms vs. gratings
–Imaging characteristics of lenses and mirrors
–Aberrations –”You can’t always get what you want”
–Detectors
•Instrument Design, Function and Calibration-Dave Crotzer
Optics – REU Lecture 2010
Richard 2
Nature of Light
(Electromagnetic Radiation)
Classical Definition: Energy Propagating in the form of waves
– Many physical processes give rise to E&M radiation including
accelerating charged particles and emission by atoms and
molecules.
Optics – REU Lecture 2010
Richard 3
Electromagnetic Spectrum
• Velocity, frequency and wavelength are related:
c=l*n where:
• c=3x108 m/sec is the velocity in vacuum
 l and n are the wavelength and frequency respectively
• Electromagnetic radiation is typically classified by
wavelength:
Optics – REU Lecture 2010
Richard 4
Nature of Light: Wave-Particle Duality
• Light behaves like a wave
– While propagating in free space (e.g. radio waves)
– On a macroscopic scale (e.g. while heating a thermometer)
– Demonstrates interference and diffraction effects
• Light behaves as a stream of particles (called photons)
– When it interacts with matter on a microscopic scale
– Is emitted or absorbed by atoms and molecules
• Photons:
– Travel at speed of light
– Possess energy: E=hn=hc/l
• Where h=Planck’s constant h=6.63e-34 Joule hz-1
• A visible light photon (l =400 nm) has n=7.5 x 1014 hz and E=4.97 x 10-19 J
Optics – REU Lecture 2010
Richard 5
Nature of Light: Photon Examples
Atoms and Molecules
Photoelectric Effect
The nature of the interaction depends
on photon wavelength (energy).
Electron kinetic energy: K.E.=hn-W.
W is the work function (depth of the
‘potential well’) for electrons in the
surface. 1ev=1.6x10-19J
Optics – REU Lecture 2010
Richard 6
A closer look at the Sun’s spectrum
Note log-scale for irradiance
The hotter and higher layers produce complex EUV (10-120 nm) emissions
dominated by multiply ionized atoms with irradiances in excess of the
photospheric Planck distribution.
Optics – REU Lecture 2010
Richard 7
Atmospheric absorption
of solar radiation
N2, O, O2
~99% solar radiation
penetrates to the
troposphere
Altitude (km)
Solar FUV and MUV radiation is the primary
source of energy for earth’s upper atmosphere.
stratosphere
O3
troposphere
Altitude “contour” for attenuation by
a factor of 1/e
I(km) = 37% x Io
Optics – REU Lecture 2010
Richard 8
Atmospheric Absorption in the Wavelength
Range from 1 to 15 m
Optics – REU Lecture 2010
Richard 9
Black Body Radiation
• An object radiates unique spectral radiant
flux depending on the temperature and
emissivity of the object. This radiation is
called thermal radiation because it mainly
depends on temperature. Thermal radiation
can be expressed in terms of black body
theory.
Black body radiation is defined as thermal radiation of a black
body, and can be given by Planck's law as a function of
temperature T and wavelength
Optics – REU Lecture 2010
Richard 10
Blackbody Radiation Curves
2hc 2  1 
u(,T)  5  hc


  kT 
e  1

Optics – REU Lecture 2010
Richard 11
Black body radiation
• Planck distributions
Hot objects emit A LOT more
radiation than cool objects
I (W/m2) = x T4
The hotter the object, the
shorter the peak wavelength
T x max = constant
Optics – REU Lecture 2010
Richard 12
Solar Spectral Irradiance
SORCE Instruments measure total solar irradiance and solar
spectral irradiance in the 1 -2000 nm wavelength range.
Optics – REU Lecture 2010
Richard 13
Solar Cycle Irradiance Variations
The FUV irradiance varies by ~ 10-100% but the MUV
irradiance varies by ~ 1-10% during an 11 year solar cycle.
Optics – REU Lecture 2010
Richard 14
Solar variability across the spectrum
• Solar irradiance modulated by presence of magnetic structures on the
surface of the Sun……Solar Rotation (short) Solar Cycle (longer)
• The character of the variability is a strong function of wavelength.
Greatest absolute variability
occurs in mid visible
Greatest relative variability
occurs in the ultraviolet.
Optics – REU Lecture 2010
Richard 15
Atmospheric Observation Modes
Direct Solar Radiation
(Solar or Stellar)
Optics – REU Lecture 2010
Richard 16
Functional Classes of Sensors
Optics – REU Lecture 2010
Richard 17
Elements of optical sensors characteristics
Sensor
What is there?
Spectral Characteristics
Spectral bandwidth ()
Resolution ()
Out of band rejection
Polarization sensitivity
Scattered light
Optics – REU Lecture 2010
How much is there?
Radiometric Characteristics
Detection accuracy
Signal to noise
Dynamic range
Quantization level
Flat fielding
Linearity of sensitivity
Noise equivalent power
Where is it located?
(or Pattern Recognition)
Geometric Characteristics
Field of view
Instan. Field of view
Spectral band registration
Alignments
MTF’s
Optical distortion
Richard 18
The challenge
So… there is light (sometimes lots, sometimes not so much)
How do we collect it?
How do we separate it?
How do we detect it?
How do we record it?
Finally,
How do we know we’re correct?
Optics – REU Lecture 2010
Richard 19
Snell’s Law of Refraction - Derivation
Optics – REU Lecture 2010
Richard 20
Reflection and refraction
refractive index 
speed of light in vacuum
speed of light in medium
Glass : n  1.52
Water : n  1.33
Air : n  1.000292
As measured with respect to the surface normal :
angle of incidence  angle of reflection
Snell ' s law :
n sin   n 'sin  '
Optics – REU Lecture 2010
Richard 21
Critical angle for refraction
An interesting thing happens when light is going from a material with higher index to
lower index, e.g. water-to-air or glass-to-air…there is an angle at which the light will
not pass into the other material and will be reflected at the surface.
Using Snell’s law:
n 'sin  '  n sin 
n
n
o
sin  c  sin 90 
n'
n'
Examples:
Water  to  air
 1 
 48.6
 1.33 
 c  sin 1 
Optics – REU Lecture 2010
Glass  to  air
 1 
 41.1
 1.52 
 c  sin 1 
Richard 22
Total internal reflection
At angles > critical angle, light
undergoes total internal reflection
It is common in laser experiments
to use “roof-top” prisms at 90°
reflectors.
(Note:surfaces are typically
antireflection coated)
Optics – REU Lecture 2010
Richard 23
Polarization
p-polarized waves are linearly polarized waves parallel to the plane of incidence
s-polarized waves are linearly polarized waves perpendicular to the plane of incidence
From German
“parallel” & “senkrecht”
Optics – REU Lecture 2010
Richard 24
Brewster’s Angle
     90o
n sin   n 'sin    n 'sin(90o   )  n 'cos
 n '
  B  arctan  
 n
Examples:
Water  to  air
 1.33 
 53.1
 1 
 B  tan 1 
Optics – REU Lecture 2010
Glass  to  air
 1.52 
 56.6
 1 
 B  tan 1 
Richard 25
Fresnel Reflection Equations
Polarization dependent Reflection fraction vs. incident angle
 sin(    ) 
 n cos  n cos  
Rs ( )  



sin(



)
n
cos


n
cos







2
2
 tan(    ) 
 n cos   n cos 
Rp ( )  



tan(



)
n
cos


n
cos







2
2
Augustin-Jean Fresnel
1788-1827
Normal incidence
 n  n 
R

 n  n 
Optics – REU Lecture 2010
2
Examples:
Air-to-water : R=2.0%
Air-to-glass : R=4.2%
Richard 26
Fresnel Reflection
Air-to-salt
salt-to-air
Salt: AgCl (near-IR)
Optics – REU Lecture 2010
Richard 27
Familiar Examples of Brewster and TIR
Brewster’s: HeNe laser cell
Round trip gain must exceed round trip reflection
losses to achieve laser output
Want to MINIMIZE reflection here
TIR: Diamond cutting
Want to MAXIMIZE reflection here
Brilliant diamond cut must maximize light return through the top.
Optics – REU Lecture 2010
Richard 28
Rainbow
Optics – REU Lecture 2010
Richard 29
Prism refraction
sin 1 n sin  2
 
sin 1 n sin  2
  1   2  


1
1

 2

2

n
Optics – REU Lecture 2010
n
Richard 30
Optics – REU Lecture 2010
Richard 31
Optical dispersion issues
Optics – REU Lecture 2010
Richard 32
Spectral Irradiance Monitor SIM
•
•
•
•
•
•
Measure 2 absolute solar irradiance
spectra per day
Wide spectral coverage
– 200-2400 nm
High measurement accuracy
– Goal of 0.1% (1)
High measurement precision
– SNR 500 @ 300 nm
– SNR  20000 @ 800 nm
High wavelength precision
– 1.3 m knowledge in the focal
plane
– (or ll < 150 ppm)
In-flight re-calibration
– Prism transmission calibration
– Duty cycling 2 independent
spectrometers
Optics – REU Lecture 2010
Richard 33
SIM Prism in Littrow
Al coated
Back surface
n’
 sin 
2  sin 1 
 n'
Optics – REU Lecture 2010

1  sin(   ) 

sin



n'
Richard 34
SIM Optical Image Quality
Optics – REU Lecture 2010
Richard 35
Optics – REU Lecture 2010
Richard 36
SIM Measures the Full Solar Spectrum
Optics – REU Lecture 2010
Richard 37
Optical displacements “Careful!”
For small angles:
Optics – REU Lecture 2010
n 1
d t
n
Richard 38
Focal length (thin lens)
Optics – REU Lecture 2010
Richard 39
Chromatic Aberration
Optics – REU Lecture 2010
Richard 40
Chromatic Aberration
Optics – REU Lecture 2010
Richard 41
Chromatic Aberration
Optics – REU Lecture 2010
Richard 42
Focal ratio (f/#)
Optics – REU Lecture 2010
Richard 43
Focal ratio con’t
Optics – REU Lecture 2010
Richard 44
Optics – REU Lecture 2010
Richard 45
Optical Transmission
Optics – REU Lecture 2010
Richard 46
Reflection or Refraction?
Optics – REU Lecture 2010
Richard 47
Reflection
Optics – REU Lecture 2010
Richard 48
Auxiliary Optical Elements for Gratings
Lenses are often used as elements to collimate and reimage light
in a diffraction grating spectrometer.
Imaging geometry for a concave mirror.
Optics – REU Lecture 2010
Tilted mirrors:
1. Produce collimated light when p=f
(q=infinity).
2. Focus collimated light to a spot with
q=f (p=infinity).
Richard 49
Diffraction grating fundamentals
Beam 2 travels a greater
distance than beam 1 by
(CD - AB)
For constructive interference
m= (CD-AB)
m is an integer called the
diffraction order
CD = dsin & AB = -dsin
m= d(sin + sin)
Note: sign convention is “minus” when diffracted beam is on opposite side of grating
normal than incidence beam; “plus” when on same side
Optics – REU Lecture 2010
Richard 50
Diffraction grating fundamentals
Diffraction gratings use the interference pattern from a large number of equally spaced
parallel grooves to disperse light by wavelength.
Light with wavelength  that is incident on a grating with angle a is diffracted into a
discrete number of angles m that obey the grating equation: m. = d.(sin()+sin(m)).
In the special case that m=0, a grating acts like a plane mirror and =-
Blue (400 nm) and red (650 nm) light are
dispersed into orders m=0,±1, and ±2
Optics – REU Lecture 2010
Richard 51
Grating example
Illuminate a grating with a blaze density of 1450 /mm With collimated
white light and a incidence angle of 48°, What are the ’s appearing at
diffraction angles of +20°, +10°, 0° and -10°?
1mm
6 nm
d
x 10
 689.7 nm
1450
mm
689.7nm
748.4

nm
sin 48  sin 20 
n
n
Wavelength (nm)
Optics – REU Lecture 2010

n=1
n=2
n=3
20
748
374
249
10
632
316
211
0
513
256
171
-10
393
196
131
Richard 52
Reflection Grating Geometry
Gratings work best in collimated light and auxiliary optical elements are
required to make a complete instrument
Plane waves, incident on the grating,
are diffracted into zero and first order
Rotating the grating causes the
diffraction angle to change
650 nm
  d (sin( )  sin(  ))
400 nm
Zero order


Optics – REU Lecture 2010
Richard 53
Typical Plane Grating Monochromator Design
Grating spectrometer using two concave mirrors to
collimate and focus the spectrum
Only light that leaves the grating at the
correct angle will pass through the exit
slit. Tuning the grating through a small
angle counter clockwise will block the
red light and allow the blue light to reach
the detector.
Entrance Slit
Exit Slit
Detector
Optics – REU Lecture 2010
Richard 54
Diffraction
Fraunhofer single slit diffraction involves the
spreading out of waves past openings which are on
the same order of the wavelength of the wave
Optics – REU Lecture 2010
Richard 55
Resolving Power
Na spectral lines
Na D-lines
Instrument & Detector
Optics – REU Lecture 2010
D1=589.6 nm
D2=589.0 nm
Richard 56
Free spectral range
For a given set of incidence and diffraction angles, the grating equation is
satisfied for a different wavelength for each integral diffraction order m. Thus
light of several wavelengths (each in a different order) will be diffracted along
the same direction: light of wavelength λ in order m is diffracted along the same
direction as light of wavelength λ/2 in order 2m, etc.
The range of wavelengths in a given spectral order for which superposition
of light from adjacent orders does not occur is called the free spectral range
Fλ.
m 1
1   
1
m
Optics – REU Lecture 2010
Richard 57
Resolving Power
The resolving power R of a grating is a measure of its ability to
separate adjacent spectral lines of average wavelength λ. It is usually
expressed as the dimensionless quantity

R
 mN

Here ∆λ is the limit of resolution, the difference in wavelength between
two lines of equal intensity that can be distinguished (that is, the
peaks of two wavelengths λ1 and λ2 for which the separation |λ1 - λ2|
< ∆λ will be ambiguous).
Optics – REU Lecture 2010
Richard 58
SOLSTICE: Channel Assembly
‘A’ Channel During Preliminary Alignment Test
Optics – REU Lecture 2010
Richard 59
SOLSTICE: Channel Assembly
Optics – REU Lecture 2010
Richard 60
Solstice Instrument
The SOLar-STellar Irradiance Comparison Experiment consists of two identical
channels mounted to the SORCE Instrument Module on orthogonal axes. They each
measure solar and stellar spectral irradiances in the 115 - 320 nm wavelength range.
SOLSTICE Channels on the IM
SOLSTICE B
Single SOLSTICE Channel
SOLSTICE A
Optics – REU Lecture 2010
- Dimensions: 88 x 40 x 19 cm
- Mass: 18 kg
- Electrical Interface: GCI Box
Richard 61
SOLSTICE Grating Spectrometer
• SOLSTICE cleanly resolves
the Mg II h & k lines
Optics – REU Lecture 2010
Richard 62
Optical Aberrations
Optics – REU Lecture 2010
Richard 63
Optical Aberrations
Optics – REU Lecture 2010
Richard 64
Optical Aberrations
Optics – REU Lecture 2010
Richard 65
Optics – REU Lecture 2010
Richard 66
Optical Aberrations
Optics – REU Lecture 2010
Richard 67
Spherical Aberration
Optics – REU Lecture 2010
Richard 68
Coma
Optics – REU Lecture 2010
Richard 69
Astigmatism
Optics – REU Lecture 2010
Richard 70
Optical Aberrations
Optics – REU Lecture 2010
Richard 71
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Richard 72
Optical Aberrations
Optics – REU Lecture 2010
Richard 73
Focus!!!
Optics – REU Lecture 2010
Richard 74
Optics – REU Lecture 2010
Richard 75
Unwanted & Scattered Light
Optics – REU Lecture 2010
Richard 76
Cassegrain Baffling Example
Optics – REU Lecture 2010
Richard 77
The End Game
Optics – REU Lecture 2010
Richard 78
Optical Detection
Optics – REU Lecture 2010
Richard 79
“What’s the Frequency--Albert?”
Optics – REU Lecture 2010
Richard 80
Photomultiplier Tube Detectors
Single photon detection (pulse counting) with an PMT
Output
pulse
Ground
-1200 V
•A photon enters the window and ejects an electron from the photocathode (photoelectric effect)
•The single photoelectron is accelerated through a 1200 volt potential down series of 10
dynodes (120 volts/dynode) producing a 106 electron pulse.
•The electron pulse is amplified and detected in a pulse-amplifier-discriminator circuit.
•Solstice uses two PMT’s in each channel that are optimized for a specified wavelength range
–CsTe (‘F’) Detector Photocathode) 170-320 nm
–CsI (‘G’) Detector Photocathode) 115-180 nm
Optics – REU Lecture 2010
Richard 81
Optics – REU Lecture 2010
Richard 82
More Nomenclature
Optics – REU Lecture 2010
Richard 83
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Richard 84
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Richard 85