Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Classical Photography and Geometric Optics Imaging Science Fundamentals

Chester F. Carlson Center for Imaging Science Typical Imaging Chain for Photography source (sun) object processing image collection (lens) exposure (aperture & shutter) detection (photographic film) camera

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Light Gamma Rays Ultraviolet Rays X Rays Light Infrared (IR) Microwave Radio wave 400 nm 750 nm People detect visible wavelengths as “colors;” the human eye is sensitive to that particular range of wavelengths. Wavelength

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Sources: Sun vs. light bulb Sun emits many different types of radiation, including X-rays, ultraviolet (UV), optical light, infrared (IR), and radio. Most harmful wavelengths (X-ray, UV) are blocked by the atmosphere. A light bulb -- like the Sun -- emits energy over a broad range of wavelengths; most of its energy comes out in the IR, but a lot comes out in the optical.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Object Energy from the source interacts with the object to be imaged. Some energy is absorbed, while some energy is reflected or scattered. The wavelengths that are scattered -- i.e., not absorbed -- are the ones which determine the color of the object.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Collection For a camera to be efficient, the pinhole is replaced by a lens. The lens redirects light rays emanating from the object.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Refraction Light slows down in materials. Imagine a line of marching Girl Scouts... Direction of travel

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science “Girl Scouts in the Mud” As the marching line steps into the mud, they will slow down, depending on how thick the mud is. Mud

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Wavefronts at Normal Angle of Incidence

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Index of Refraction u Index of Refraction (n) is the ratio between the speed of light in vacuum (c) and the speed of light in the medium (v). n = c/v MediumIndex of Refraction Vacuum 1 (exactly) Air Water 1.33 Glass 1.5 Diamond 2.4

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science “Girl Scouts in Mud” at an Angle u The direction of travel changes when the marching line hits the mud at a non-normal angle.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Wavefront at Non-Normal Angle of Incidence

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Snell’s Law u This change in direction is described by Snell’s Law AIR GLASS AIR GLASS normal

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Trigonometry Review  sin(  ) = y/r (opp/hyp)  cos(  ) = x/r (adj/hyp)  tan(  ) = y/x (opp/adj)  Adjacent Side (x) Opposite Side (y) Hypotenuse (r) RULES THAT DEFINE SIN, COS, TAN of an ANGLE:

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Snell’s Law Snell’s Law: n 1 sin  1 = n 2 sin  2 11 22 n1n1 n2n2 (Or, if  1 and  2 are small, n 1  1 = n 2  2 )

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Refraction for Different Materials  AIR WATER GLASS DIAMOND    light

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Flat to Curved Surface A curved surface can be approximated with small straight segments.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Curved Interface u Concave interface diverges rays. u Convex interface converges rays. Assuming n’ > n nn’n

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Graphical Ray Tracing u A way to analyze optical systems. u Light rays always travel from left to right for analysis purposes. Axis of symmetry Image side (+)Source side(-) Light Rays Lens

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Convex Lens u “Positive Lens” u “Burning Glass” u Magnifying lens shorthand In real life Double Convex Plano-Convex Positive Meniscus

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Convex Lens u Image focal point, F’, is half the distance to the effective center of curvature of the lens. u Object focal point, F, is exactly the same distance on the object side of the lens. Axis of symmetry Light Rays Lens F’ F Object side Image side

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Convex Lens u Image focal length, f’, is the distance from the lens to the image focal point. u Object focal length, f, is the distance from the lens to the object focal point. F’ f’ F f

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Ray Diagrams for a Positive (Convex) Lens  (infinity) Object Location Image Type and Location Real, at F’ F Real, at 2F’ Real, at  (infinity) Virtual 2F < F

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Concave Lens u “Negative Lens” u Near-sighted corrective lens shorthand In real life Double concave Plano- concave Negative meniscus

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Concave Lens u Image focal point, F’, is on the object side u Focal length, f’, is negative. Axis of symmetry Light Rays Lens F’ f’

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Ray Diagrams for a Negative (Concave) Lens Rays converging toward F Object Location Image Type and Location Virtual, at  (infinity) Approaching the lens from  (infinity) Virtual, at F’ Virtual, between F’ and the lens  (infinity)

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Dispersion u Dispersion - Index of refraction, n, depends on the frequency (wavelength) of light. Dispersion is responsible for the colors produced by a prism: red light “bends” less within the prism, while blue light “bends” more.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Chromatic Aberration u Dispersion results in a lens having different focal points for different wavelengths - this effect is called chromatic aberration. u Results in a “halo” of colors. u Solution: Use 2 lenses of different shape and material (“achromatic doublet”). F’ Red. Object (small dot)Image with chromatic aberration F’ Blue White light

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Spherical Aberration u All the rays do not bend toward the focal point, resulting in a blurred spot. u Solution: use lenses with aspherical curvature, or use a compound lens. F’. Object (small dot)Image with spherical aberration

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Other Aberrations u Coma u Off axis blur which looks like the “coma” of a comet. u Astigmatism u Different focal lengths for different planes. u Distortion u Images formed out of shape...

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Exposure u Exposure is defined as the total amount of light falling on the film. u Exposure = Illuminance * Time camera Aperture Shutter Lens

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Illuminance u Illuminance is the rate of light falling on a given area (i.e. energy per unit time). u Illuminance is controlled by aperture: a larger aperture brings more light to the focus. Small aperture Large aperture

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Aperture and “F number” u F# (F number) is often used in photography to describe the aperture. u F# = focal length of the system/diameter of aperture d

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Exposure Time u Exposure time is controlled by the shutter: when closed, the film is not exposed to light. u Exposure time is simply the time interval between opening and closing the shutter. Shutter Closed Shutter Open

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Types of Shutters Simplified Camera Between the Lens (BTL) Or Leaf Shutter Focal Plane Shutter

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science BTL or Leaf Shutter u Made of overlapping “leaves” that slide out of the way when shutter opens. u Located between the imaging lens elements. CLOSED OPEN

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science BTL or Leaf Shutter u Advantages u Uniform illumination independent of film size. u Entire film frame illuminated at once. u Disadvantages u Illumination of frame not constant over time. u Limitations on shutter speed.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Focal Plane Shutter u Metal or fabric with a narrow slit opening which traverses the area to be exposed. u Located just before the detector (film) at the focal plane.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Focal Plane Shutter u Advantages u Cost effective (one shutter needed for all lenses - great for interchangeable lens systems) u Can achieve very fast shutter speeds (~1/10000 sec) u Disadvantages u May cause time distortion if the film size is large (since the shutter slit must traverse the film)

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Why control exposure with aperture and shutter? u Flexibility! u Fast shutter speed for freezing action (e.g. sports photography). u Slow shutter speed for low light levels (e.g. sunsets). u Small aperture for bright scenes or to enable longer exposures. u Large aperture for low light conditions (taking candle lit or moon lit pictures).