Camera Film-Filter Systems

Lecture 4

Prepared by R. Lathrop 9/99

Updated 9/06

Radiometric Characteristics of aerial photos

Film exposure at a point in a photograph is directly related to the reflectance of the object imaged at that point. Theoretically, film exposure varies linearly with object reflectance, with both being a function of wavelength.

From Lillesand & Kiefer, 1994

Film Exposure

• Exposure, E = s * d2 * t / 4 f2

• whereE = film exposure, J mm-2

s = scene brightness, J mm-

2 sec-1 d = diameter of lens opening, mm t = exposure time, sec

f = focal length of lens, mm

Black & White Film

• B&W Panchromatic: sensitive to ultraviolet - blue -green - red (0.3 to 0.7 m)

• B&W Infrared: sensitive to UV to NIR (0.3 to 0.90 m)

• Ultraviolet blocked by haze filter

B&W Film Exposure process• Silver halide grains in the film emulsion absorb

light energy and are reduced to silver atoms

• During processing, the grains not exposed to light are dissolved and washed away

Emulsion

Base

light

Silver halide grains

Developed film negative with metallic silver

B&W Film Exposure process• After processing, film areas that were

exposed become various shades of gray, depending on the amount of exposure

• Full exposure = black No exposure = white

• In the negative, bright areas appear dark as there is a higher concentration of silver

Processing

Negative-to-Positive Photo Sequence

• To create a positive, the negative is illuminated and photographic print paper is exposed

• In the positive, dark areas on the negative now appear bright and bright areas appear dark, thereby back to the original object tones

Processing

w/ enlarging

Film transmittance

• Transmittance, T, ability of a film to pass light

• Tp = light passing through film at point p total light incident upon the film at pt. p

• Opacity, O, is a measure of “darkness” of a film emulsion

• Op= 1/Tp

40 units transmitted Tp = 40/100 = 0.40

10 units transmitted Tp = 10/100 = 0.10

1 unit transmitted Tp = 1/100 = 0.01Adapted from Lillesand

& Kiefer

Film Density

• Density, Dp = log (Op) = log (1/Tp)

• Image density and visual tone vary in a nearly linear relationship

D - Log Exposure curve• Plot of relative log exposure on X axis vs. density

on Y axis• Slope of straight-line portion of curve

D = logE

• Slope of straight-line portion of curve important determinant of film contrast; steeper the slope, greater the contrast, greater radiometric resolution

D-Log E curvePlot of relative log exposure on X axis vs. density on Y axis

0 1 2 3

Dmin

Dmax

D =

logE

logE

D

Straight line portion

Density

Relative log exposure Adapted from Lillesand & Kiefer

Comparing D-Log E curves

0 1 2 3

Dmin

Dmax

Density

Relative log exposure Adapted from Lillesand & Kiefer

Film 1

Film 2

Density resolution

Radiometric resolution

D - Log Exposure curve

• Slope of straight-line portion of curve, , important determinant of film contrast

• Steeper the slope, greater radiometric resolution (i.e. smallest detectable change in exposure), leads to imagery with greater contrast

• Film 1 has steeper and higher radiometric resolution, greater contrast

Comparing D-Log E curves

0 1 2 3

Dmin

Dmax

Density

Relative log exposure Adapted from Lillesand & Kiefer

Film 1

Film 2

Density resolution

Radiometric resolution

D - Log Exposure curve

• Film speed is the sensitivity of a film to light

• Fast film accommodates low exposure levels (i.e. it lies farther to the left on the log E axis)

• Film 1 is a faster film but has more limited exposure range as compared to Film 2

• Tradeoff between film speed and coarser spatial resolution (larger film grains to capture low light)

• Film 2 provides a finer grain film

Comparing D-Log E curves

0 1 2 3

Dmin

Dmax

Density

Relative log exposure Adapted from Lillesand & Kiefer

Film 1

Film 2

Density resolution

Radiometric resolution

Exposure Falloff

• Falloff: less light, lower exposure towards edge of image frame

White Light is a combination of all the visible wavelengths

400 500 600 700

wavelength (nm)

Human Color Vision3 types of cones: roughly sensitive with peaks in the

blue (445nm), green (535nm) and orange-red (575nm)

Primary Colors

Red Green

Blue

Color Additive Process

R G

B

M

Y

CW

Black background

Subtractive Primary Colors

Yellow (R+G)

absence of blue

Cyan (G+B)

absence of red

Magenta (R+B)

absence of green

Y C

M

R

G

BB

Color Subtractive Process

White background

Color film• 3 dye layers sandwiched to a base

• Yellow dye layer controls blue light passing through image

• Magenta dye layer controls green light

• Cyan dye layer controls red light

Blue sensitive layer

Blue blocking filter

G (& B) sensitive layer

R (& B) sensitive layer

Base & Backing

Color film

Film Image colors

G R

Original colors

Y M C

B G R

Dye Layers

B

Color Infrared (CIR) film• Blue absorbing filter • Near infrared sensitive dye layer - cyan dye which

controls red light on image• Green sensitive layer - yellow dye which controls

blue light on image• Red sensitive dye layer - magenta dye which controls

green light on image

Blue blocking filter

NIR (& B) sensitive layer

G (& B) sensitive layer

R (& B) sensitive layer

Base & Backing

Color Infrared filmOriginal colors

Film Image colors

NIR

Filter

G R

Y M C

B G R

Dye Layers

B

An example-plant leaves• Chlorophyll absorbs large % of red and

blue for photosynthesis- and strongly reflects in green (.55um) mm

• Peak reflectance in leaves in near infrared (.7-1.2mm) up to 60% of infrared energy per leaf is scattered up or down due to cell wall size, shape, leaf condition (age, stress, disease), etc.

• Reflectance in Mid IR (2-4mm) influenced by water content-water absorbs IR energy, so live leaves reduce mid IR return

Color Infrared filmOriginal colors

Red to magenta

NIR

Filter

G R

Y M C

B G R

B

What color is it?

• What color would a green astroturf field (with low NIR reflectance) look like on Color IR film?

• What color would a field of flowering yellow mustard plant look like on Color IR film?

• What color would a magenta (purple) cow (with low NIR reflectance) look like on color IR film?

What color would a green astroturf field (with low NIR reflectance) look like on Color IR film?

Original colors

Film Image colors

NIR

Filter

G R

Y M C

B G R

B

Blue

RU Stadium and Practice Field

Natural grassSynthetic turf

Photo from 1995

‘False color’ imageryOn a computer screen, can re-arrange the individual color layers or bands of a digital image can be switched around

For a CIR, normal rendition

NIR R, R G, G B

The image to the right is scrambled. Match the original wavebands to the display colors.

? R, ? G, ? B

What color would a field of flowering yellow

mustard plant look like on Color IR film?

Original colors

Film Image colors

NIR

Filter

G R

Y M C

B G R

Dye Layers

B

White to pink

What color would a magenta cow look like on

Color IR film?

Original colors

Film Image colors

NIR

Filter

G R

Y M C

B G R

Dye Layers

B

Green

Filters

• Short wavelength blocking filter (high pass) selectively absorbs energy below a certain wavelength

• Haze filters to absorb UV (<0.4 um)

• Yellow filter to absorb blue (<0.5 um)

• Bandpass filters block energy above and below a certain range of wavelengths

Computer Image Display

Computer Display Monitor has 3 color planes: R, G, B

that can display DN’s or BV’s with values between 0-255

3 layers of data can be viewed simultaneously:

1 layer in Red plane

1 layer in Green plane

1 layer in Blue plane

Image Display: RGB color compositing

Red band DN

Blue band DN

Red band DN = 0

Blue band DN = 200

Green band DN

Green band DN = 90

Blue-green pixel (0, 90, 200 RGB)

Landsat MSS bands 4 and 5

GREEN RED

Landsat MSS bands 6 and 7

INFRARED 2INFRARED 1

Note: water absorbs IR energy-no return=black

• combining bands creates a false color composite

Rutgers

Manhattan

PhiladelphiaPine barrensChesapeake BayDelaware River

MSS “false”color composite

RGB Additive Color Processcolor R G B

white 255 255 255

black 0 0 0

red 255 0 0

yellow 255 255 0

cyan 0 255 255

magenta 255 0 255

orange = ___ ___ ___

Additive color process

• Snow reflects highly in the visible and near-IR wavelengths but absorbs (low DN) in the mid-IR and thermal-IR wavelengths.

• For a 'false color' composite that consists of NIR=red, MIR=green, and RED=blue, snow is what color? __________________

Additive color process

• Stressed vegetation with chlorophyll degradation reflects more highly in the red wavelengths than healthy green vegetation.

• For a ‘true color' composite that consists of Red=red, Green=green, and Blue=blue, stressed vegetation is what color? __________________

• For a 'false color' composite that consists of NIR=red, Red=green, and Green=blue, stressed vegetation is what color? __________________

RGB Color Systems

IHS Color System• IHS: Intensity-Hue-Saturation