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REU Lecture

REU Lecture. Optics and Optical Design Erik Richard erik.richard@lasp.colorado.edu 303.735.6629. Outline. Brief Review: Nature of Light (Electromagnetic Radiation) Propagation of E&M waves Interaction with matter Wave-particle duality • Brief Review: Optics Concepts Refraction

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REU Lecture

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  1. REU Lecture Optics and Optical Design Erik Richard erik.richard@lasp.colorado.edu 303.735.6629

  2. Outline • Brief Review: Nature of Light (Electromagnetic Radiation) • Propagation of E&M waves • Interaction with matter • Wave-particle duality • • Brief Review: Optics Concepts • Refraction • Reflection • - Diffraction grating characteristics • Imaging characteristics of lenses and mirrors • Detectors • Instrument Design and Function • Drawings • Block Diagram • Mechanisms

  3. 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.

  4. ElectromagneticSpectrum • 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:

  5. 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

  6. Nature of Light: Photon Examples Atoms and Molecules Photoelectric Effect 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 The nature of the interaction depends on photon wavelength (energy).

  7. 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.

  8. Atmospheric absorption of solar radiation N2, O, O2 Solar FUV and MUV radiation is the primary source of energy for earth’s upper atmosphere. ~99% solar radiation penetrates to the troposphere Altitude (km) stratosphere O3 troposphere Altitude “contour” for attenuation by a factor of 1/e I(km) = 37% x Io

  9. Atmospheric Absorption in the WavelengthRange from 1 to 15 m

  10. 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

  11. Blackbody Radiation Curves

  12. Black body radiation • Planck distributions 2 key points 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

  13. Solar Spectral Irradiance SORCE Instruments measure total solar irradiance and solar spectral irradiance in the 1 -2000 nm wavelength range.

  14. 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.

  15. 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.

  16. Atmospheric Observation Modes Direct Solar Radiation

  17. Functional Classes of Sensors

  18. Element of optical sensors characteristics Sensor Spectral bandwidth () Resolution () Out of band rejection Polarization sensitivity Scattered light Detection accuracy Signal to noise Dynamic range Quantization level Flat fielding Linearity of sensitivity Noise equivalent power Field of view Instan. Field of view Spectral band registration Alignments MTF’s Optical distortion Spectral Characteristics Radiometric Characteristics Geometric Characteristics

  19. Reflection and refraction

  20. 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: Examples:

  21. 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)

  22. Brewster’s Angle Examples:

  23. Fresnel Reflection Equations Polarization dependent Reflection fraction vs. incident angle Augustin-Jean Fresnel 1788-1827 Normal incidence Examples: Air-to-water : R=2.0% Air-to-glass : R=4.2%

  24. Fresnel Reflection Air-to-salt salt-to-air Salt: AgCl (near-IR)

  25. 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.

  26. Prism refraction

  27. Second issue: Optical dispersion

  28. 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  < 150 ppm) • In-flight re-calibration • Prism transmission calibration • Duty cycling 2 independent spectrometers

  29. SIM Prism in Littrow Al coated Back surface n’

  30. SIM Optical Image Quality

  31. SIM Measures the Full Solar Spectrum

  32. Optical displacements “Careful!” For small angles:

  33. Focal length (thin lens)

  34. Chromatic Aberration

  35. Chromatic Aberration

  36. Chromatic Aberration

  37. Focal ratio (f/#)

  38. Focal ratio con’t

  39. Optical Transmission

  40. Reflection or Refraction?

  41. Reflection

  42. 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

  43. 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

  44. 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°? Wavelength (nm)

  45. Plane waves, incident on the grating, are diffracted into zero and first order Rotating the grating causes the diffraction angle to change 650 nm 400 nm Zero order a  Reflection Grating Geometry Gratings work best in collimated light and auxiliary optical elements are required to make a complete instrument

  46. 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. Tilted mirrors:1. Produce collimated light when p=f (q=infinity).2. Focus collimated light to a spot with q=f (p=infinity).

  47. Typical Plane Grating Monochromator Design Grating spectrometer using two concave mirrors to collimate and focus the spectrum Entrance Slit 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. Exit Slit Detector

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