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# Lecture #3 - PowerPoint PPT Presentation

Lecture #3. What you see is what you get 1/31/13. Homework. Problems up on web site Due next Tuesday Questions??. What are organisms ’ visual tasks?. Foraging. Finding / choosing mates. Avoiding predators. Knowing when to stop. What happens to light when we see?. Today ’ s topics.

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### Lecture #3

What you see is what you get

1/31/13

• Problems up on web site

• Due next Tuesday

• Questions??

What are organisms’ visual tasks?

Today’s topics

• Reflection

• Absorption / Transmission

• Measuring fR, fA, and fT

• Follow the photon’s path

• Spectral properties of light environments

• Terrestrial

• Aquatic

• Energy of a photon

• Matter will interact with light in one of 4 ways

• Reflected

• Absorbed

• Transmitted = Refracted

• Scattered

• For now we will deal with transparent materials so scattering will be negligible

• Photons are conserved

• Light going in must go somewhere

• Iincident = ITrans + IReflect + Iabsorb = I0

• Express as fraction of I0

• fT + fR + fA = 1

• fT=fraction transmitted

• fR=fraction reflected

• fA=fraction absorbed

Iabsorb

I0

Itrans

Ireflect

θ1 = θ2 = 0

1. Reflection at interface

• Light will reflect at interface between materials with different indices of refraction

• For light perpendicular to

surface

n=1.0

Water n=1.33

• fR, fraction reflected

θ1 = θ2

n=1.0

Water n=1.33

2. Absorption air / water

• Light will interact with molecules in material

• It can excite molecules. If it matches electron resonance, then it will be absorbed

• If not, it will be transmitted

• We see what is not absorbed

In the following, we assume… air / water

• Reflection is pretty small

• Then fT + fR + fA = 1 and fR ≈ 0 so that

• fT + fA = 1 What does that mean???

• Beer’s law

εdepends on what substance is

C is concentration

l is the pathlength

I0

I, light transmitted through

l

Calculating transmission - solution air / water

• Beer’s law

ε depends on what substance is

C is concentration

l is the pathlength

I0

I0

I

I

Low concentration High concentration

Less absorbed More absorbed

More transmitted Less transmitted

Calculating transmission - solution air / water

• Beer’s law

ε depends on what substance is

C is concentration

l is the pathlength

I0

I0

I

I

Short pathlength Longer pathlength

Less absorbed More absorbed

More transmitted Less transmitted

• Beer’s law

α is attenuation coefficient

I0

I

l

• ε length-1 concentration-1 = L-1 molecules-1L3

= L2/molecule

l length

C concentration = molecule / L3

• L-1

• l L

3. Measuring transmission / absorption air / water

Measure I0 - just beam

flashlight

Fiber optic

Spectrometer

Measuring transmission /absorption air / water

Measure I with object in beam

flashlight

Fiber optic

Transmission = I / I0

fT + fR + fA = 1

For small fR

fA = 1-fT

Spectrometer

For reflective objects air / water

Specular reflection

Specular reflection

Scattered

Reflected light vs scattered light

Measuring reflection / scattering air / water

Fiber optic

Light source

Spectrometer

How can we measure I0?

Measuring reflection / scattering air / water

Fiber optic

Light source

Spectrometer

Measure I0 of light

Use white target that reflects all wavelengths

Measuring reflection / scattering air / water

Fiber optic

Light source

Spectrometer

Measure I reflected from object

fRorS = I / I0 fRorS + fA + fT = 1 where reflection and scattering depend on angle

For small fT fRorS = 1 - fA

Examples of absorption and reflection air / water

• The return of the spectrometer

Why does absorption matter? air / water

• Retinal pigments absorb certain wavelengths

• Biological materials

• Photosynthesis uses light to power life

• Wavelengths scattered depend on absorption

• Colors of animals, food

• Define our environment

4. The photon air / water’s path - How do we see?

Sensitivity

• Light from a source, I

• Reflected by object, R

• Detected by eye, S

• Q = I * R * S

Intensity

Reflectance

Q = quanta of light detected

What light illuminates an object? air / water

• Irradiance

• Light flux on a surface - from all directions

• Photons /s m2

Irradiance

• Irradiance

• Light flux on a surface - from all directions

• Photons /s m2

• Radiance

• Light flux from a particular direction and angle

• Photons /s m2sr

Radiance

Irradiance

Light measurement radiance

• Many light meters measure watts / m2

• Watts are joules / s and so are related to photons / s

• We’ll convert that in a minute

• Some light meters measure lux

• This is like watts / m2 but they take human sensitivity into account

Bright sunlight

20,000 lux

Eyes respond to photons radiance

• Eye doesn’t care about watts

• Chemical reactions in eye detect individual photons

75 W light bulb 5 mW laser

How many photons in a Watt radiance

• Watt is a measure of power = energy / time

• 1 watt = 1 J/s

• Convert watts to photons

• E = hf = h c / λ

• h is Planck’s constant = 6.6256 x 10-34 Js

• For 400 nm light:

• E = (6.6256 x 10-34 Js) (2.998 x 108m/s)

• 400 x 10-9 m

• E = 4.96 x 10-19J per photon

Red laser

More photons per W at longer wavelength

Red laser radiance

• Laser power is 3 mW at 650 nm

• # photons/s = Power

• energy per photon

• = 0.003 W

• 3.0x10-19J/photon

• = 9.8 x 1015 photons / s

5. Natural light sources radiance

• Lots of variation in natural light

• Light at high noon

• Light at dawn, dusk

• Light at midnight

• Light in forest

• Light at ocean surface

• Light 100 m depth

• Illuminant shapes what we can see

Solar spectrum radiance

Irradiance in watts / m2

Since there are more photons per watt at longer wavelengths, the curve shape changes when presented as photons / m2 sec

Loew and

McFarland 1990

Why are they similar?

Why are they different?

Loew and

McFarland 1990

Light from sun versus moon radiance

Moon

Sun

Earth

Sun angle changes with time of day

This changes pathlength through atmosphere

Dawn / dusk radiance

Lose mid to long wavelengths at dawn and dusk

Loew and McFarland 1990

Fleishman et al. 1997 radiance

Here are light spectra (irradiance) for forest habitats

Terrestrial habitats

Shade

Fleishman et al. 1997 radiance

Terrestrial habitats

Shade

Sun

Fleishman et al. 1997 chlorophylls

Terrestrial habitats

Shade

Sun

Fleishman et al. 1997 chlorophylls

Terrestrial habitats

Shade

Sun

Affects of the terrestrial environment chlorophylls

• Lighting and contrast with background determines how easily you can be seen

• Cryptic (camouflage) - blend in

• Conspicuous - stand out

• Lighting and contrast with background determines how easily your food can be detected

Light under water chlorophylls

• Water attenuates certain wavelengths more than others

• αλ – attenuation coefficient varies with wavelength

Why does α vary with wavelength? chlorophylls

• Water reflection depends on wavelength

• Water refraction depends on wavelength

• Water absorption depends on wavelength

• None of the above

Attenuation coefficient of pure water chlorophylls

• Which wavelength light is transmitted best?

• 350 nm

• 450 nm

• 550 nm

• 650 nm

α

• We take the light spectrum at the waters surface and

• Multiply it by the fraction of light that is transmitted

Solar illumination at different depths chlorophylls

m

Incident

sunlight

Light penetration chlorophylls

“Blue” oceanic waters

Levine

Sci Am

1982

400 450 500 550 600 650 700 nm

Light penetration chlorophylls

“Blue” oceanic waters

Levine

Sci Am

1982

400 450 500 550 600 650 700 nm

Light penetration chlorophylls

“Blue” oceanic waters

Levine

Sci Am

1982

400 450 500 550 600 650 700 nm

Light at dawn / dusk in air or under water chlorophylls

Loew and McFarland 1990

Note photons/s not Watts

Limit of human sensitivity

Light penetration chlorophylls

“Blue” oceanic waters

Color of transmitted light

Color of water

Levine

Sci Am

1982

400 450 500 550 600 650 700 nm

Different waters attenuate differently chlorophylls

1+2 open ocean

3 ocean with chlorophyll

4 coastal waters with chlorophyll and dissolved organics

chlorophyllsFresh” water

“Green” river water

Swampy “red” waters

4

3

2

1

Aquatic environment chlorophylls

• Depth

• Habitat (coral reef vs ocean)

• Camouflage - blending in

• Light levels (especially in deep ocean)

• Kind of water that you’re in

• How light is transmitted / attenuated

FishBase: Fish at depth viewer chlorophylls

Amphiprion ocellaris

Amphiprion at depth chlorophylls

10 m

50 m

0 m

25 m