1. The Electromagnetic Spectrum and Blackbody Radiation
Sources of light: gases, liquids, and solids
Boltzmann's Law
Blackbody radiation
The electromagnetic spectrum
Long-wavelength sources
and applications
Visible light and the eye
Short-wavelength sources and applications
Prof. Rick Trebino
Georgia Tech

2. TeraHertz light (a region of microwaves)
TeraHertz light is light with a frequency of ~1 THz, that is, with a wavelength of ~300 mm.
THz light is heavily absorbed by water, but clothes are transparent in this wavelength range.
CENSORED
Fortunately, I couldn’t get permission to show you the movies I have of people with THz-invisible clothes.

3. IR is useful for measuring the temperature of objects.
Hotter and hence brighter in the IR
Old Faithful
Images borrowed from Linda Hermans-Killam,
Such studies help to confirm that Old Faithful is in fact faithful and whether human existence is interfering with it.

4. IR Lie-detection
I don’t really buy this, but I thought you’d enjoy it…
He’s really sweating now…

5. The military uses mid-IR to see various objects.
Mid-IR from heat is easily distinguished from the ambient far infrared, which peaks
near 10 mm and is relatively weak in this range.
Also IR penetrates fog and smoke better than visible light. 4 10
l (mm)

6. The infrared space observatory
Stars that are just forming are cooler than mature stars and so emit light mainly in the IR.

7. Using mid-IR laser light to shoot down missiles
Wavelength = 3.6 to 4.2 mm
Thanks to Michael Gura (Optics I 2003) for this reference!
Tactical High Energy Laser (THEL) Program   The fixed-site version Advanced Concept Technology Demonstration (ACTD) Tactical High Energy Laser (THEL) THEL, was developed by TRW Inc. under a $89 million contract. During several tests in the USA, the system has shot down 25 Katyusha rockets, but has not been deployed. The system has not progressed much since the end of the demonstration program, since the lack of mobility and the fixed base limitations of the system made in insufficient to counter long range rockets currently employed by Hezbulla at the Israeli northern border with Lebanon. While Katyusha rockets had a range of 20 kilometers, and could hit only a few urban targets, the long range rockets have a range of 70 kilometers and can hit strategic facilities and large urban areas in the Haifa bay. A laser-based defense against such weapons must rely on more systems, which could be rapidly mobilized to protect a much larger area. Similar threats could face US contingencies in other parts of the world. This requirement is driving the need for an air-mobile version of the beam weapon. A study completed in 2001 concluded that the rocket interceptor has "lots of promise" and further development should be pursued, primarily in enabling system's mobility. Mobility considerations for the future mobile systems include system mobility (road and off road capabilities) and air transportability, including the type of transport aircraft it should fit on (C-130, C-17 or C-5). Conclusions of these studies will define the necessary size- reduction technologies required for the future version. Further studies of the system include the use of such laser beam weapons to provide "hard kill" defenses against artillery projectiles, UAVs and cruise missiles. The Tactical High Energy Laser uses a high-energy, deuterium fluoride chemical laser to protect against attack by short range unguided (ballistic flying) rockets. In a typical engagement scenario, a rocket is launched toward the defended area. Upon detection by the THEL fire control radar (image on right), the radar establishes trajectory information about the incoming rocket, then "hands off" the target to the pointer-tracker subsystem, which includes the beam director (top of page above). The PTS tracks the target optically, then begins a "fine tracking" process for THEL's beam director, which then places THEL's high-energy laser on target. The energy of the laser causes intense heating of the target, which causes its warhead to explode. The debris from the target falls quickly to the ground, far short of the defended area.
Above: Sequence of a rocket intercept demonstration by e THEL laser, September In these photos, THEL/ACTD laser spot focus on the warhead (top) of the 5 inch diameter rocket, and detonate it (center), thus effectively "neutralizing" the rocket. The gases emitted by the explosion create excessive drag which tears the fragmentation casing into several parts which continue on their ballistic trajectory. (bottom of image series) Below: THEL Radar and fire control system
The Tactical High Energy Laser uses a high-energy, deuterium fluoride chemical laser to shoot down short range unguided (ballistic flying) rockets.

8. Atmospheric penetration depth (from space) vs. wavelength

9. Laser welding
Near-IR wavelengths are commonly used.

10. Visible light Wavelengths and frequencies of visible light
Hecht and

11. Solar wind particles spiral around the earth’s magnetic field lines and collide with atmos-pheric molecules, electronically exciting them.
Auroras
Auroras are due to fluorescence from molecules excited by these charged particles.
Different colors are from different atoms and molecules.
O: 558, 630, 636 nm
N2+: 391, 428 nm
H: 486, 656 nm
Photo: This image of the auroral oval was taken by the Ultraviolet Imager (UVI) onboard the NASA satellite "Polar" on April 4, 1997 at 0519 UT.
Wavelengths due to:

12. Dye lasers cover the entire visible spectrum.

13. Fluorescent lights
“Incandescent” lights (normal light bulbs) lack the emission lines.

14. The human retina Rods Cones
Images and text courtesy of the NDT Resource Center
The retina is a mosaic of two basic types of photoreceptors, rods, and cones.
Cones are highly concentrated in a region near the center of the retina called the fovea. The maximum concentration of cones is roughly 180,000 per mm2 there and the density decreases rapidly outside of the fovea to less than 5,000 per mm2. Note the blind spot caused by the optic nerve, which is void of any photoreceptors.

15. The eye’s response to light and color
The eye’s cones have three receptors, one for red, another for green, and a third for blue.

16. The eye is poor at distinguishing spectra.
Because the eye perceives intermediate colors, such as orange and yellow, by comparing relative responses of two or more different receptors, the eye cannot distinguish between many spectra.
The various yellow spectra below appear the same (yellow), and the combination of red and green also looks yellow!

17. How film and digital cameras work

18. Most digital cameras interleave different-color filters
All other digital camera sensors only measure the brightness of each pixel. As shown in this diagram, a "color filter array" is positioned on top of the sensor to capture the red, green, and blue components of light falling onto it. As a result, each pixel measures only one primary color, while the other two colors are "estimated" based on the surrounding pixels via software. These approximations reduce image sharpness, which is not the case with Foveon sensors. However, as the number of pixels in current sensors increases, the sharpness reduction becomes less visible. Also, the technology is in a more mature stage and many refinements have been made to increase image quality.
Similar to an array of buckets collecting rain water, digital camera sensors consist of an array of "pixels" collecting photons, the minute energy packets of which light consists. The number of photons collected in each pixel is converted into an electrical charge by the photodiode. This charge is then converted into a voltage, amplified, and converted to a digital value via the analog to digital converter, so that the camera can process the values into the final digital image. In CCD (Charge-Coupled Device) sensors, the pixel measurements are processed sequentially by circuitry surrounding the sensor, while in APS (Active Pixel Sensors) the pixel measurements are processed simultaneously by circuitry within the sensor pixels and on the sensor itself. Capturing images with CCD and APS sensors is similar to image generation on CRT and LCD monitors respectively. The most common type of APS is the CMOS (Complementary Metal Oxide Semiconductor) sensor. CMOS sensors were initially used in low-end cameras but recent improvements have made them more and more popular in high-end cameras such as the Canon EOS D60 and 10D. Moreover, CMOS sensors are faster, smaller, and cheaper because they are more integrated (which makes them also more power-efficient), and are manufactured in existing computer chip plants. The earlier mentioned Foveon sensors are also based on CMOS technology. Nikon's new JFET LBCAST sensor is an APS using JFET (Junction Field Effect Transistor) instead of CMOS transistors.
So four times the number of pixels are required. Digital TVs can emit any color from each pixel, so an 8-Mpixel camera only requires a 2-Mpixel TV (which is the full-HDTV standard).

19. Color wheels Hue = wavelength Saturation = spectral width
Hue = wavelength
Saturation = spectral width
Value = brightness (intensity)

20. The Ultraviolet
The UV is usually broken up into three regions, UVA ( nm), UVB ( nm), and UVC ( nm).
UVC is almost completely absorbed by the atmosphere.
You can get skin cancer even from UVA.

21. Flowers in the UV Arnica angustifolia Vahl
Since bees see in the UV (they have a receptor peaking at 345 nm), flowers often have UV patterns that are invisible in the visible.
Arnica angustifolia Vahl
UV picture used several filters: (Filters: U BG-38, SB-140)
Visible
UV (false color)

22. The sun in the UV
Image taken through a 171-nm filter by NASA’s SOHO satellite.
Image from

23. The very short-wavelength regions
Vacuum-ultraviolet (VUV)
180 nm > l > 50 nm
Absorbed by <<1 mm of air
Ionizing to many materials
Soft x-rays
5 nm > l > 0.5 nm
Strongly interacts with core
electrons in materials
Extreme-ultraviolet (XUV or EUV)
50 nm > l > 5 nm
Ionizing radiation to all materials

24. Synchrotron Radiation
Formerly considered a nuisance to accelerators, it’s now often the desired product!
Synchrotron radiation in all directions around the circle
Synchrotron radiation only in eight preferred directions

25. EUV Astronomy
The solar corona is very hot (30,000,000 degrees K) and so emits light in the EUV region.
EUV astronomy requires satellites because the earth’s atmosphere is highly absorbing at these wavelengths.

26. The sun also emits x-rays.
The sun seen in the x-ray region. They’re mostly from the corona, which is 30,000,000°K.

27. Matter falling into a black hole emits x-rays.
Nearby star
Black hole
A black hole accelerates particles to very high speeds.

28. Supernovas emit x-rays, even afterward.
A supernova remnant in a nearby galaxy (the Small Magellanic Cloud).
The false colors show what this supernova remnant looks like in the x-ray (blue), visible (green) and radio (red) regions.

29. X-rays are occasionally seen in auroras.
On April 7th 1997, a massive solar storm ejected a cloud of energetic particles toward planet Earth.
The “plasma cloud” grazed the Earth, and its high energy particles created a massive geomagnetic storm.

30. Atomic structure and x-rays
The Essential Physics of Medical Imaging 2nd Edition, pg. 21 by Jerrold T. Bushberg, J. Anthony Seibert, Edwin M. Leidholdt, Jr, and John M. Boone 2002 by Lippincott Williams & Wilkins in PA ISBN:
Ionization energy ~ 100 – 1000 e.v.
Ionization energy ~ .01 – 1 e.v.

31. Fast electrons impacting a metal generate x-rays.
High voltage accelerates electrons to high velocity, which then impact a metal.
Images from Medical Imaging Physics, Fourth Edition by William R. Hende and E. Russell Ritenour 2002 by Wiley-Liss, New York ISBN: Pages 71, 75
Electrons displace electrons in the metal, which then emit x-rays.
The faster the electrons, the higher the x-ray frequency.

32. X-rays penetrate tissue and do not scatter much.
Roentgen’s x-ray image of his wife’s hand (and wedding ring)

33. X-rays for photo-lithography
You can only focus light to a spot size of the light wavelength. So x-rays are necessary for integrated-circuit applications with structure a small fraction of a micron.
1 keV photons from a synchrotron:
2 micron lines over a base of 0.5 micron lines.

34. High-Harmonic Generation and x-rays
gas jet
x-rays
Femtosecond laser pulse at frequency w
Focusing a femtosecond laser pulse into a gas jet yields photons of frequency nw — generally in the XUV and x-ray regions!
Harmonic orders n > 300, photon energy > 500 eV (a few Angstroms!) have been observed.
Atom
electron
x-ray
The strong field smashes the electron into the nucleus—a highly non-harmonic motion!

35. Gamma rays result from matter-antimatter annihilation.
An electron and positron self-annihilate, creating two gamma rays whose energy is equal to the electron mass energy, mec2. e- e+
hn = 511 kev
More massive particles create even more energetic gamma rays. Gamma rays are also created in nuclear decay, nuclear reactions and explosions, pulsars, black holes, and supernova explosions.

36. Gamma-ray bursts emit massive amounts of gamma rays.
A new one appears almost every day, and it persists for ~1 second to ~1 minute.
They’re probably supernovas.
Image borrowed from
The gamma-ray sky
In 10 seconds, they can emit more energy than our sun will in its entire lifetime. Fortunately, there don’t seem to be any in our galaxy.

37. The universe in different spectral regions…
Gamma Ray
Gamma-ray, visible, and microwave images used with permission from Gabriela González, Department of Physics and Astronomy, Louisiana State University
Gamma-ray view: includes all wavelengths less than about 1.2E-5nm (photon energies greater than 1E8eV)
X-ray and IR images from Optics I student Yujiro Ito, 2004.
X-ray view: from ROSAT shows wavelengths of 0.8nm (blue), 1.7nm (green), and 5.0nm (red)
Infrared view: from IRAS shows emission at 0.1nm, 0.06nm, and 0.012nm
X-Ray
Visible

38. The universe in more spectral regions…IR
Images used with permission from Gabriela González, Department of Physics and Astronomy, Louisiana State University
Microwave