Pinhole Camera

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Pinhole Camera

• Imaging Science Fundamentals

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Light
Ultraviolet Rays
Infrared (IR)
Gamma Rays
Radio wave
Microwave
X Rays
Light
750 nm
400 nm
Wavelength

• People detect visible wavelengths as colors
  since the human eye is sensitive to that
  particular wavelength range.

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Simple imaging system Medical Xray
X Ray source
Film
Object
Image

• X-rays either penetrate the object being imaged
  and are then detected by film, or they are
  absorbed. Result is a shadowgram.

• What if we replaced the x-ray source with a
  visible light source?

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Typical imaging chain for pinhole camera
Visible light source
Film (capture)
pinhole
processing
Object
Image
Dark box
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Light from a Point Source

• A point source is a source in which the light
  appears to be emanating from one point in space.
• A point source casts energy in all directions,
  shown above as rays.
• A star can be considered a point source. (Why?)

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Point Source Casting Rays
Object

• Take a look at 7 of the rays cast from one
  position (the tip) of the object.
• With no collection element, all of these rays
  fall on the image plane. There is no way to
  determine the source of the rays.

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Three Points on the Object

• This becomes a problem when three points on the
  object are considered.
• The energy can be collected at the image plane,
  but all information about the object is lost.

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Need for Collection Element

• A crucial element in most imaging systems is the
  component responsible for collecting the energy
  emerging from a given point on the object, such
  that it is brought to a single point in the image.

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Ideal Pinhole Camera
An ideal pinhole allows only a single ray from
each point on the object to pass through. This
preserves the spatial distribution of energy,
creating a sharp image. An ideal pinhole camera
has an infinitely small aperture (opening).
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Problems with Ideal Pinhole Camera

• An infinitely small aperture allows an infinitely
  small amount of light (zero) to pass through.
• Diffraction (which is ignored in geometric
  optics) blurs the image when the pinhole is very
  small.

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Realistic Pinhole Camera
Since the pinhole cannot be infinitely small,
more than one ray actually gets through, blurring
the image formed at the image plane.
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Capture

• The image can now be captured using a detection
  system, such as photographic film.
• Film must be processed to yield a permanent,
  visible image.

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Image Capture - Photographic Film

• Film consists of a photosensitive compound known
  as Silver Halide (AgX), which reacts with light.
• The area exposed to light turns dark after
  processing.
• The total amount of light hitting a given area of
  film (known as exposure) determines how dark that
  area turns.

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Limitations of Pinhole Camera

• Finite pinhole size and diffraction degrade final
  image.
• Light collection is poor - aperture must be small
  - so image acquisition is slow.