Light eyes cameras

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Basic Principles of Imaging and Lenses Light Electromagnetic Radiation Photons Light These three are the same… • Light * pure energy • Electromagnetic Waves * energy-carrying waves emitted by vibrating electrons • Photons * particles of light EM Radiation Travels as a Wave c = x 108 m/s EM Radiation Carries Energy • Quantum mechanics tells us that for photons E = hf where E is energy and h is Planck’s constant • But f = c/λ • Putting these equations together, we see that E = hc/λ Electromagnetic Wave Velocity • The speed of light is the same for all seven forms of light • It is 300,000,000 meters per second or 186,000 miles per second The Electromagnetic Spectrum • • • • • • • Radio Waves - communication Microwaves - used to cook Infrared - “heat waves” Visible Light - detected by your eyes Ultraviolet - causes sunburns X-rays - penetrates tissue Gamma Rays - most energetic Sampling and Quantization Interlace vs progressive scan Progressive scan Interlace Color Sensing in Camera (RGB) • 3-chip vs 1-chip: quality vs cost • Why more green? Why colors? http://www.cooldic http://www.cooldi Practical Color Sensing: Bayer Grid • Estimate RGB at ‘G’ cels from neighboring values words/Bayer-filter.wikipedia Image Formation f(x,y) = reflectance(x,y) * illumination(x,y) Reflectance in [0,1], illumination in [0,inf] White Balance White World / Gray World assumptions Problem: Dynamic Range The real world has High dynamic range 1500 25,000 400,000 2,000,000,000 Is Camera a photometer? Image pixel (312, 284) = 42 42 photos? Long Exposure Real world Picture 10-6 High dynamic range 10-6 106 106 to 255 Short Exposure Real world Picture 10-6 High dynamic range 10-6 106 106 to 255 Image Acquisition Pipeline Lens scene radiance Shutter sensor irradiance (W/sr/m ) ∫ sensor exposure ∆t CCD ADC analog voltages Remapping digital values Camera is NOT a photometer! pixel values Varying Exposure What does the eye sees? The eye has a huge dynamic range Do we see a true radiance map? [...]... Lincoln? A Brief History of Images 1558 1568 1837 Silicon Image Detector, 1970 1970 A Brief History of Images 1558 1568 1837 Digital Cameras 1970 1995 A Brief History of Images 1558 1568 1837 Hasselblad HD2-39 1970 1995 2006 Geometric Optics and Image Formation Pinhole Cameras • • • Pinhole camera - box with a small hole in it Image is upside down, but not mirrored left-to-right Question: Why does a... z y' y = f' z Problems with Pinholes • Pinhole size (aperture) must be “very small” to obtain a clear image • However, as pinhole size is made smaller, less light is received by image plane • If pinhole is comparable to wavelength of incoming light, DIFFRACTION effects blur the image! • Sharpest image is obtained when: pinhole diameter d = 2 f 'λ Example: If f’ = 50mm, λ = 600nm (red), d = 0.36mm The... Formation using (Thin) Lenses • Lenses are used to avoid problems with pinholes • Ideal Lens: Same projection as pinhole but gathers more light! o i P P’ f Gaussian Lens Formula: 1 1 1 + = i o f • f is the focal length of the lens – determines the lens’s ability to bend (refract) light • f different from the effective focal length f’ discussed before! Focus and Defocus aperture Blur Circle, aperture diameter... The Multi-Wavelength Sun X-Ray UV Composite Infrared Visible Radio EM Spectrum Relative Sizes The Visible Spectrum Light waves extend in wavelength from about 400 to 700 nanometers A Brief History of Images Camera Obscura, Gemma Frisius, 1558 1544 Camera Obscura "When images of illuminated objects penetrate through... blur circle is less than the resolution of the imaging sensor Problems with Lenses Compound (Thick) Lens Vignetting B L3 L2 L1 principal planes α α A nodal points thickness Chromatic Abberation more light from A than B ! Radial and Tangential Distortion ideal FB FG FR actual ideal actual image plane Lens has different refractive indices for different wavelengths Spherical Aberration Spherical lenses... The best modern lenses may contain aspherical elements Human Eye • The eye has an iris like a camera • Focusing is done by changing shape of lens • Retina contains cones (mostly used) and rods (for low light) • The fovea is small region of high resolution containing mostly cones • Optic nerve: 1 million flexible fibers Slide credit: David Jacobs
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