imaging science fundamentalschester f. carlson center for imaging science classical photography and...

Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Classical PhotographyClassical Photographyand Geometric Opticsand Geometric Optics

Imaging Science Fundamentals


Typical Imaging Chain Typical Imaging Chain for Photographyfor Photography

source (sun)

object

processing

imagecollection (lens)exposure (aperture & shutter)detection (photographic film)

camera


LightLight

Gamm

a Ray

s

Ultrav

iolet

Ray

s

X Ray

s

Light

Infra

red (I

R)

Microwav

e

Radio

wav

e

400 nm 750 nm

People detect visible wavelengths as “colors;” the human eye is sensitive to that particular range of wavelengths.

Wavelength


Sources: Sun vs. light bulbSources: 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.


ObjectObject

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.


CollectionCollection

For a camera to be efficient, the pinhole is replaced by a lens.

The lens redirects light rays emanating from the object.


RefractionRefraction

Light slows down in materials. Imagine a line of marching Girl

Scouts . . .

Direction of travel


““Girl Scouts in the Mud”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


Wavefronts at Normal Wavefronts at Normal Angle of IncidenceAngle of Incidence


Index of RefractionIndex of Refraction

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

Medium Index of Refraction Vacuum 1 (exactly) Air 1.0003 Water 1.33 Glass 1.5 Diamond 2.4


““Girl Scouts in Mud” at an AngleGirl Scouts in Mud” at an Angle

The direction of travel changes when the marching line hits the mud at a non-normal angle.


Wavefront at Non-Normal Wavefront at Non-Normal Angle of IncidenceAngle of Incidence


Snell’s LawSnell’s Law

This change in direction is described by Snell’s Law

AIR

GLASS AIR

GLASS

normal


Trigonometry ReviewTrigonometry Review

sin() = y/r (opp/hyp) cos() = x/r (adj/hyp) tan() = y/x (opp/adj)

Adjacent Side (x)

OppositeSide (y)

Hypotenuse (r)

RULES THAT DEFINE SIN, COS, TAN of an ANGLE:


Snell’s LawSnell’s Law

Snell’s Law: n1sin1= n2sin2

1

2

n1

n2

(Or, if 1 and 2 are small, n11= n22)


Refraction for Different MaterialsRefraction for Different Materials

AIR

WATER

GLASS

DIAMOND

light


Flat to Curved SurfaceFlat to Curved Surface

A curved surface can be approximated with small straight segments.


Curved InterfaceCurved Interface

Concave interface diverges rays.

Convex interface converges rays.

Assuming n’ > n

n n’nn’


Graphical Ray TracingGraphical Ray Tracing

A way to analyze optical systems. Light rays always travel from left to

right for analysis purposes.

Axis of symmetry

Image side (+)Source side(-)

Light Rays

Lens


Convex LensConvex Lens

Image focal point, F’, is half the distance to the effective center of curvature of the lens.

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


Convex LensConvex Lens

Image focal length, f’, is the distance from the lens to the image focal point.

Object focal length, f, is the distance from the lens to the object focal point.

F’

f’

F

f


Ray Diagrams Ray Diagrams for a Positive (Convex) Lensfor a Positive (Convex) Lens

(infinity)

Object Location Image Type and Location

Real, at F’

F

Real, at 2F’

Real, at (infinity)

Virtual

2F

< F


Concave LensConcave Lens

Image focal point, F’, is on the object side

Focal length, f’, is negative.

Axis of symmetry

Light Rays

Lens

F’

f’


Ray Diagrams Ray Diagrams for a Negative (Concave) Lensfor a Negative (Concave) Lens

Rays convergingtoward 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)


DispersionDispersion

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.


Chromatic AberrationChromatic Aberration

Dispersion results in a lens having different focal points for different wavelengths - this effect is called chromatic aberration.

Results in a “halo” of colors.

Solution: Use 2 lenses of different shape and material (“achromatic doublet”).

F’Red

.

Object (small dot) Image with chromatic aberration

F’Blue

White light


Spherical AberrationSpherical Aberration

All the rays do not bend toward the focal point, resulting in a blurred spot.

Solution: use lenses with aspherical curvature, or use a compound lens.

F’

.

Object (small dot) Image with spherical aberration


ExposureExposure

Exposure is defined as the total amount of light falling on the film.

Exposure = Illuminance * Time

camera

Aperture Shutter

Lens


IlluminanceIlluminance

Illuminance is the rate of light falling on a given area (i.e. energy per unit time).

Illuminance is controlled by aperture: a larger aperture brings more light to the focus.

Small aperture Large aperture


Aperture and “F number”Aperture and “F number”

F# (F number) is often used in photography to describe the aperture.

F# = focal length of the system/diameter of aperture

d


Exposure TimeExposure Time

Exposure time is controlled by the shutter: when closed, the film is not exposed to light.

Exposure time is simply the time interval between opening and closing the shutter.

Shutter Closed Shutter Open


Types of ShuttersTypes of Shutters

Simplified Camera

Between the Lens (BTL) Or

Leaf Shutter

Focal Plane Shutter


BTL or Leaf ShutterBTL or Leaf Shutter

Made of overlapping “leaves” that slide out of the way when shutter opens.

Located between the imaging lens elements.

CLOSED

OPEN


Focal Plane ShutterFocal Plane Shutter

Metal or fabric with a narrow slit opening which traverses the area to be exposed.

Located just before the detector (film) at the focal plane.


Why control exposure with aperture Why control exposure with aperture and shutter?and shutter?

Flexibility! Fast shutter speed for freezing action

(e.g. sports photography). Slow shutter speed for low light levels

(e.g. sunsets). Small aperture for bright scenes or to

enable longer exposures. Large aperture for low light conditions

(taking candle lit or moon lit pictures).

imaging science fundamentalschester f. carlson center for imaging science classical photography and...

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