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Pinhole Camera Imaging October 23, 2008 Authors: Miguel Alonso, The Institute of Optics Joe Malach, Corning Tropel Andy Murnan, Genawave Paul Murphy, QED Technologies Revised By: Jonathan Petruccelli, The Institute of Optics Christopher Todd, The Institute of Optics Write-up Write-up ............................................................................................................................................ Overview .................................................................................................................................... 1 Background ................................................................................................................................ 4 Imaging Demonstrations................................................................................................................... Pinhole camera ........................................................................................................................... 8 Flashlight illumination as a sum of images.............................................................................. 13 Lens camera.............................................................................................................................. 15 Forming an image with a concave mirror ................................................................................ 21 Forming an image with a soda bottle…………………………………………………………23 Single lens imaging Demonstration ......................................................................................... 27 Optical system demonstration…………………………………………………………………… Anatomy of a camera ............................................................................................................... 36 Overview The basic idea An optical image is a replication of an object using light, where the replication retains the proportions of the original object. Imaging is the process of forming an image. Dictionary definition Im age \ ‘im-ij\ n : An optically formed duplicate, counterpart, or other representative reproduction of an object, especially an optical reproduction formed by a lens or mirror. Source: The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2000 by Houghton Mifflin Company. 1 To form an image, light emitted by or scattered from an object must be selectively allowed to pass through a pinhole or collected and focused by a lens(es) or mirror(s). An obstacle with a pinhole forms an image by blocking all light rays except for one (or a very thin bundle of them) from each point of an object. Therefore, each point on a screen placed behind the pinhole receives only one ray, coming from a corresponding point of the object, as shown in Figure 1. Most light rays are blocked, but a pinhole allows one light ray to pass from each point on the light bulb An image of the light bulb is Light bulb emits light formed on a screen in all directions Figure 1: Pinhole forms image A lens, on the other hand, forms an image by collecting a bundle of light rays from every point on the object and focusing them to corresponding points on the screen. This is shown in Figure 2, where the gray area represents a bundle of rays collected and focused by the lens from one point on the light bulb to the corresponding point on the screen. An image of the light bulb is Light b ulb emits light Lens collects rays from each point of formed on a screen in all directions the light bulb and images each point onto a screen. Figure 2: Lens forms image Definitions Ray : Line along which the energy carried by light travels. Focusing : The action of making a bundle of rays from a point on the object converge to a corresponding point in the image. 2 Lens : a piece of glass, plastic, or other transparent material with smooth sides (at least one of which is curved) that is used to change the direction of rays. Lenses that are thicker at the center than at the edges have the capacity to focus light rays, and therefore to form real images. These are called converging or positive lenses. Lenses that are thicker at the edges cannot form real images on their own, but can be used in combination with converging lenses to make cameras of better quality. They are called diverging or negative lenses. Lenses work by refracting light at each of their surfaces. See Figure 3. Refraction follows Snell’s law. θ 2 θ3 θ4 θ1 n1 n2 n3 R1 R2 Figure 3: Lens showing a single ray refracting at both surfaces. The ray, coming from a medium with index of refraction of n 1, enters a lens whose material has index n 2. The ray hits the lens’s first surface at an angle of θ incidence of 1 with respect to the surface’s normal. This surface, with radius R 1, refracts the ray at an angle θ θ 2 . The ray then travels to the second surface, hitting it at an angle 3 . Finally, the ray leaves the lens at an θ angle 4 from the surface normal and enters a medium with index of n 3. See refraction demonstration for more information. Real image : an image that can be projected onto a screen and is accessible. See Figure 4 Top. Virtual image : an inaccessible image that appears to come from a particular point behind a lens or mirror, but in fact comes from an object at another location. See Figure 4 Bottom. Object Real Image Object Virtual Image Figure 4: Top: Lens forms a real image of the object. Bottom: Lens forms a virtual image of the object. 3 Background History ~500 BC The earliest mention of the pinhole camera was by the Chinese philosopher Mo-Ti. He formally recorded the creation of an inverted image (an image that is rotated by 180 degrees) formed by light rays passing through a pinhole into a darkened room. ~350 BC Aristotle (Greece, 384-322 BC) understood the optical principle of the camera obscura. He viewed the crescent shape of a partially eclipsed sun projected on the ground through the holes in a sieve, and the gaps between leaves of a plane tree. ~300 BC Euclid (Alexandria). In his Optica he noted that light travels in straight lines and described the law of reflection. 965-1020 Ibn-al-Haitham (Basra, Iraq) used spherical and parabolic mirrors and was aware of spherical aberration (a lens or mirror defect in imaging). He also investigated the magnification produced by lenses and atmospheric refraction. He gave a full account of the principle of the pinhole camera including experiments with five lanterns outside a room with a small hole. 1490 Leonardo Da Vinci (Italy) gave two clear descriptions of the pinhole camera in his notebooks. Many of the first camera obscuras were large rooms like that illustrated by the Dutch scientist Reinerus Gemma-Frisius in 1544 for use in observing a solar eclipse. 1558 Giovanni Battista Della Porta (Italy), in his book Magiae Naturalis , recommended the use of a camera with a converging lens as a drawing aid for artists. 1604 Johannes Kepler (Germany). In his book Ad Vitellionem Paralipomena , Kepler explained vision as a consequence of the formation of an image on the retina by the lens in the eye and correctly described the causes of long-sightedness and short- sightedness. 1608 Hans Lippershey (Netherlands) constructed a telescope with a converging objective lens and a diverging eye lens. 1609 Galileo Galilei (Italy) constructed his own version of Lippershey's telescope and started to use it for astronomical observations. 1611 Johannes Kepler (Germany). In his Dioptrice , Kepler presented an explanation of the principles involved in the convergent/divergent lens microscopes and telescopes. 4 1647 B Cavalieri (Italy) derived a relationship between the radii of curvature of the surfaces of a thin lens and its focal length. 1668 Isaac Newton (England). As a solution to the problem of chromatic aberration (image defects related to color) exhibited by refracting telescopes, Newton constructed the first reflecting telescope using mirrors as he believed that chromatic aberration could never be eliminated by lenses. 1733 Chester More Hall constructed an achromatic compound lens using components made from glasses with different refractive indices. 1873 Ernst Abbe (Germany) presented a detailed theory of image formation in the microscope. 1953 Frits Zernike (Netherlands) won the Nobel Prize in Physics for his invention of the phase contrast optical microscope. 1990 The Hubble space telescope was positioned in a low Earth orbit on 25th April. Everyday examples of images At the movie theater, film from a movie reel is imaged onto the large screen for all to see. A camera, film or digital, is a typical example of an instrument that forms images of objects. The image is recorded on film or an electronic detector. You see the result of this image in a photograph or the digital image on a display. The most familiar imaging instrument is the eye. When you look at a car on the street, an image of the car, as well as the rest of the scene, forms on the back of your eye. Your eye is a biological lens and a detector, and is constantly forming images. Applications / engineering Standard applications of imaging are eyeglasses, magnifying glasses, camera lenses, telescopes, and microscopes. All of these applications are also taken to the extreme. Instead of eyeglasses, the cornea of the eye can have custom correction with LASIK surgery using a laser. Professional camera lenses are nearly fully automated and have excellent resolution. The Hubble telescope circles the earth in space to avoid problems caused by the atmospheric absorption of certain wavelengths of light and turbulence. There are optical microscopes that can see objects
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