Thursday, 14 January 2010 Lecture 4: The spectrum, color theory and absorption and photogrammetry Reading Ch 2.3 http://www.bway.net/~jscruggs/film.html (color film) 1

Color vs. B/W, shape and composition Interpretation of some images Previous lecture: Spatial data - photointerpretation & basic color theory What we covered: Image geometry Color vs. B/W, shape and composition Interpretation of some images Color mixing 2

Color Color is a sensation that can be predicted and controlled Color has 3 dimensions and can be simulated by radiances at three different l’s In natural color those are red, green and blue but In remote sensing any 3 may be combined as a “false-color” image Therefore we need to understand color Color is created by selective absorption, so we need to understand that first 3

For SI units frequently used in Remote Sensing, see back cover of text The electromagnetic spectrum Light is energy - Q =hn in ergs or joules (J) where h = Planck’s constant, 6.63·10-34 J s n = frequency (s-1) = c/l (c = speed of light, 3.00x108 ms-1, l = wavelength (µm,nm,mm,cm,m) For SI units frequently used in Remote Sensing, see back cover of text In remote sensing we commonly measure the flux of photons from a unit surface for a certain amount of time and by a camera or scanner a certain distance away with a lens of a particular diameter This flux is called the radiance L and the units are W m-2 sr-1. Watts W (power) are energy per unit time (J s-1) Sr stands for steradian and is the solid angle subtended by the pixel 4

Review On Solid Angles, class website (Ancillary folder: Steradian Review On Solid Angles, class website (Ancillary folder: Steradian.ppt) On solid angles… On a plane, we can measure the angle q between 2 vectors sharing endpoint P, the center of a circle of radius r. A radian is defined as the angle that subtends an arc on a circle equal to the radius. It is about 57 degrees (360/2p). A circle is divided into 360 degrees, or 2p radians. In a volume, we can measure solid angles as shown to the right, where P is the center of a sphere of radius r and q is the solid angle of a cone that intersects the sphere in a small circle of circumference p*C. A sphere (area = 4pr2) contains 4p steradians, where a steradian (sr) is the unit of solid angle. The cone defined to the right subtends a solid angle of 1 sr. Airborne scanners have angular resolutions that are commonly 0.0025 or 0.001 sr, for example. As a rule of thumb, the pixel size for a 2.5 milliradian instrument is about 1 m for every thousand feet of elevation above terrain.

Section of the eye Let’s start with how humans sense color: Cone-shaped cells within the eye absorb light in 3 wavelength ranges – RGB They send signals to the brain proportional to how much light is absorbed The brain turns these signals into the sensation of color Color has three attributes – hue, saturation, and intensity or lightness color (perception) is related to radiance (physical flux) Section of the eye 5

1 nanometer (nm) = 10-9 m = 10-3 mm DAY Bright light NIGHT Dim light Rods are more sensitive than cones In bright light, the three sets of cones send strong signals to the brain that drown out the signal from the rods. The signals are interpreted as the sensation of color In dim light, the signal from the single set of rods is dominant. It is interpreted as the sensation of black/white (gray) 1 nanometer (nm) = 10-9 m = 10-3 mm 6

Additive Color 7

The spectrum and color Spectral yellow brightness Gray Wavelength, l (mm) Blue Green Red Blue Green Red Blue Green Red Cartoon spectrum – A useful tool 8

+ = Additive Color Blue Green Red Blue Green Red Blue Green Red Blue 9

Additive mixtures – another framework 0, 100, 0% g 50, 50, 0% 33, 33, 33% b r 100, 0, 0% 0, 0, 100% 10

A D I T V E M X N G 11

To work with color, we use three different data “spaces”: *Perceptual data space – how we sense color intuitively (Hue, saturation, intensity) *Radiance data space – how the color stimulus is described by the image data *Transformed DN space – a mathematical description of color that is related to radiance 12

INTENSITY (LIGHTNESS) A simple perceptual color space (HSI) HUE SATURATION INTENSITY (LIGHTNESS) 13

2) RGB radiance space B b G g r R r=R/(R+G+B) g=G/(R+G+B) b=B/(R+G+B) r R 14

3) Transformed data space r=R/(R+G+B) g=G/(R+G+B) b=B/(R+G+B) The CIE system: characterizes colors by a brightness parameter Y plus two color coordinates x and y. The response of the eye is best described in terms of three tristimulus coordinates rgb. Colors that can be matched by combining a set of three primary colors (ie, Red, Green, Blue) are represented on the chromaticity diagram by a triangle joining the coordinates for the three colors. Any H,S pair can be expressed in terms of the CIE color coordinates x and y, but intensity is not represented. g y r b x 15

g Additive mixtures b r 16

Transformation from a Cartesian XYZ radiance space to a spherical color space Longitude = hue (H) Co-latitude = saturation (S) Radius = intensity (I) XYZ may be any three tristimulus fluxes but are treated as RGB Z Y X 17

Transformed Viking Lander HUE SAT RGB images of Mars INT Natural color Intensity INT 18

Color is created by selective Absorption Bouguer If L is the radiance from a source at strength Lo after passage through an absorbing medium such as the atmosphere, then: L = e-kz Lo W m-2 sr-1 (Beer-Lambert-Bouguer Law) Light must either be reflected, absorbed, or transmitted This is the “rat” law of conservation: L= Lr + La + Lt e-kz describes the % of light transmitted through the medium (assuming Lr =0) k is a value characteristic of the absorptivity of the medium z is the length of passage through the medium (which we take to be homogeneous) 19

Absorption by a homogeneous medium is a constant-rate process – for every mm of material the light passes through, a certain fraction is absorbed. If it goes through z mm of medium, the total light remaining is e-kz %, where 1/k is the scale depth – that is, for every 1/k passage through the medium, 1/e = 1/2.718 % = 36.8% of the light remains. Fraction of light transmitted Thickness, mm Graph of absorption as a function of medium thickness 20

Absorption and color k is commonly different from wavelength to wavelength (kl) eg, more light might be absorbed in green than in red or blue When we see light having passed through such a filter, it appears magenta to us (ie, no green). We need to consider remote-sensing fluxes to be functions of wavelength Thus, radiance L (W m-1 sr-1) becomes spectral radiance Ll (W m-1 sr-1 µm-1) 21

A word about filters… Filters Transmittance l, mm Filter functions 22

“Subtractive” Color 23

* = “Subtractive” Color 1% 1% 100% Blue Green Red Blue Green Red Blue Input spectrum Red-transmitting filter 1% 1% 100% Blue Green Red * Blue Green Red Filtered spectrum = Filter Blue Green Red Scene 24

Remember: “subtractive” mixing is physically done by multiplication white light green filter yellow dark R: 1.0 * 0.0 = 0.0; * 0.8 = 0.0 G: 1.0 * 0.9 = 0.9; * 0.8 = 0.7 B : 1.0 * 0.0 = 0.0; * 0.0 = 0.0 Additive #White circle – all white plus red, saturates white in digital image – will be pink in real life #Chartreuse background plus red background – deep yellow or orange, ie some green and some yellow plus lots of red #cyan crane plus red bg: B & R = purple #cyan crane plus grn circle: cyan plus green = more green #grn circle & ochre bg: a little greener #black circle plus green circle =- no change #black bg plus red bg – no change Subtractive (really, multiplicative) #red bg “minus” white circle: red (red * red) #red bg times ochre bg: darker red #red bg times cyan crane: dark purple (liottle color overlap) #red bg times black dot – black (everything is absorbed) #G circle times black dot – black #G circle times ochre bg: dark yellow #G circle times cyan stork – dark green The way to view the subtractive process is light with some color passing through a class filter that only transmits another color 25

Next lecture: Radiative transfer & sources we will trace radiation from its source to camera 26