Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Pinhole Camera Imaging Science Fundamentals

Chester 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” since the human eye is sensitive to that particular wavelength range. Wavelength

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Simple imaging system: Medical Xray u X-rays either penetrate the object being imaged and are then detected by film, or they are absorbed. Result is a shadowgram. X Ray source Object Film Image Object Film Image Visible light source ? u What if we replaced the x-ray source with a visible light source?

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Typical imaging chain for pinhole camera Visible light source Object Dark box pinhole Film (capture) processing Image

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Light from a Point Source u A point source is a source in which the light appears to be emanating from one point in space. u A point source casts energy in all directions, shown above as rays. u A star can be considered a point source. (Why?) *

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Point Source Casting Rays u Take a look at 7 of the rays cast from one position (the tip) of the object. u With no collection element, all of these rays fall on the “image plane.” There is no way to determine the source of the rays. Object

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Three Points on the Object u This becomes a problem when three points on the object are considered. u The energy can be collected at the image plane, but all information about the object is lost.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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).

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Problems with Ideal Pinhole Camera u An infinitely small aperture allows an infinitely small amount of light (zero) to pass through. u Diffraction (which is ignored in geometric optics) blurs the image when the pinhole is very small.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science 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.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Capture u The image can now be captured using a detection system, such as photographic film. u Film must be processed to yield a permanent, visible image.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Image Capture - Photographic Film u Film consists of a photosensitive compound known as Silver Halide (AgX), which reacts with light. u The area exposed to light turns dark after processing. u The total amount of light hitting a given area of film (known as exposure ) determines how dark that area turns.

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Limitations of Pinhole Camera u Finite pinhole size and diffraction degrade final image. u Light collection is poor - aperture must be small - so image acquisition is slow.