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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

Lecture #3

What you see is what you get

1/31/13


Homework

Homework

  • Problems up on web site

  • Due next Tuesday

  • Questions??


What are organisms visual tasks

What are organisms’ visual tasks?


Foraging

Foraging


Finding choosing mates

Finding / choosing mates


Avoiding predators

Avoiding predators


Knowing when to stop

Knowing when to stop


What happens to light when we see

What happens to light when we see?


Today s topics

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


Light interactions

Light interactions

  • 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


Light interactions1

Light interactions

  • 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


1 reflection at interface

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


Reflection at biological interfaces is usually pretty small air water

Reflection at biological interfaces is usually pretty small: air / water

  • fR, fraction reflected

θ1 = θ2

n=1.0

Water n=1.33


2 absorption

2. Absorption

  • 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

In the following, we assume…

  • Reflection is pretty small

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

    • fT + fA = 1 What does that mean???


Calculating transmission solution of concentration c

Calculating transmission – solution of concentration, C

  • Beer’s law

εdepends on what substance is

C is concentration

l is the pathlength

I0

I, light transmitted through

l


Calculating transmission solution

Calculating transmission - solution

  • 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 solution1

Calculating transmission - solution

  • 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


Calculating transmission pure substance like water

Calculating transmission - pure substance, like water

  • Beer’s law

α is attenuation coefficient

I0

I

l


Units all cancel so take exponential of a unitless number

Units all cancel so take exponential of a unitless number

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

    = L2/molecule

    l length

    Cconcentration = molecule / L3

  • L-1

  • l L


Transmission absorption depend on wavelength

Transmission / absorption depend on wavelength


3 measuring transmission absorption

3. Measuring transmission / absorption

Measure I0 - just beam

flashlight

Fiber optic

Spectrometer


Measuring transmission absorption

Measuring transmission /absorption

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

For reflective objects

Specular reflection


For opaque objects light scatters in all directions

For opaque objects light scatters in all directions

Specular reflection

Scattered

Reflected light vs scattered light


Scattering reflection depend on wavelength

Scattering / reflection depend on wavelength

  • n depends on λ


Measuring reflection scattering

Measuring reflection / scattering

Fiber optic

Light source

Spectrometer

How can we measure I0?


Measuring reflection scattering1

Measuring reflection / scattering

Fiber optic

Light source

Spectrometer

Measure I0 of light

Use white target that reflects all wavelengths


Measuring reflection scattering2

Measuring reflection / scattering

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

Examples of absorption and reflection

  • The return of the spectrometer


Why does absorption matter

Why does absorption matter?

  • 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 s path how do we see

4. The photon’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

What light illuminates an object?

  • Irradiance

    • Light flux on a surface - from all directions

    • Photons /s m2

Irradiance


Depending on detector set up we might measure irradiance or radiance

Depending on detector set up, we might measure irradiance or radiance

  • 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

Light measurement

  • 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


Lux meter measures irradiance all angles

Lux meter (measures irradiance – all angles)

Bright sunlight

20,000 lux


Eyes respond to photons

Eyes respond to photons

  • Eye doesn’t care about watts

  • Chemical reactions in eye detect individual photons


Energy of light source is given in watts

Energy of light source is given in watts

75 W light bulb 5 mW laser


How many photons in a watt

How many photons in a Watt

  • Watt is a measure of power = energy / time

    • 1 watt = 1 J/s

  • Convert watts to photons


Energy of a photon thank planck

Energy of a photon – thank Planck

  • 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


Energy of photon determines photons watt

Energy of photon determines #photons/watt

Red laser

More photons per W at longer wavelength


Red laser

Red laser

  • 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

5. Natural light sources

  • 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


Light environment sky conditions

Light environment : sky conditions


Light environment sky conditions1

Light environment : sky conditions


Light environment sky conditions2

Light environment : sky conditions


Environment lighting conditions

Environment : Lighting conditions


Light environment viewing angle

Light environment : Viewing angle


Light environment time of day

Light environment : time of day


Light environment time of day1

Light environment : Time of day


Solar spectrum

Solar spectrum

Irradiance in watts / m2


Light spectrum in terms of photon flux

Light spectrum in terms of photon flux

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

Loew and

McFarland 1990


Compare spectra of sunlight and moonlight

Compare spectra of sunlight and moonlight

Why are they similar?

Why are they different?

Loew and

McFarland 1990


Light from sun versus moon

Light from sun versus moon

Moon

Sun

Earth


How does solar spectrum vary for high noon vs dawn dusk

How does solar spectrum vary for high noon vs dawn / dusk

Sun angle changes with time of day

This changes pathlength through atmosphere


Dawn dusk

Dawn / dusk

Lose mid to long wavelengths at dawn and dusk

Loew and McFarland 1990


Terrestrial habitats

Fleishman et al. 1997

Here are light spectra (irradiance) for forest habitats

Terrestrial habitats

Shade


Terrestrial habitats1

Fleishman et al. 1997

Terrestrial habitats

Shade

Sun


Why is light in the forest different absorption of light by chlorophylls

Why is light in the forest different?Absorption of light by chlorophylls


Light reflecting off vegetation

Light reflecting off vegetation


Terrestrial habitats2

Fleishman et al. 1997

Terrestrial habitats

Shade

Sun


Terrestrial habitats3

Fleishman et al. 1997

Terrestrial habitats

Shade

Sun


Affects of the terrestrial environment

Affects of the terrestrial environment

  • 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

Light under water

  • Water attenuates certain wavelengths more than others

  • αλ – attenuation coefficient varies with wavelength


Why does vary with wavelength

Why does α vary with wavelength?

  • Water reflection depends on wavelength

  • Water refraction depends on wavelength

  • Water absorption depends on wavelength

  • None of the above


Attenuation coefficient of pure water

Attenuation coefficient of pure water

  • Which wavelength light is transmitted best?

  • 350 nm

  • 450 nm

  • 550 nm

  • 650 nm

α


Light transmission of clear water

Light transmission of clear water

m


How can we calculate the light spectrum underwater

How can we calculate the light spectrum underwater?

  • 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

Solar illumination at different depths

m

Incident

sunlight


Light penetration

Light penetration

“Blue” oceanic waters

Levine

Sci Am

1982

400 450 500 550 600 650 700 nm


Light penetration1

Light penetration

“Blue” oceanic waters

Levine

Sci Am

1982

400 450 500 550 600 650 700 nm


Light penetration2

Light penetration

“Blue” oceanic waters

Levine

Sci Am

1982

400 450 500 550 600 650 700 nm


Light at dawn dusk in air or under water

Light at dawn / dusk in air or under water

Loew and McFarland 1990

Note photons/s not Watts


Decrease in light intensity with depth

Decrease in light intensity with depth


Decrease in light intensity with depth log scale

Decrease in light intensity with depth - log scale

Limit of human sensitivity


Land nilsson table 2 1

Land & Nilsson Table 2.1


Light penetration3

Light penetration

“Blue” oceanic waters

Color of transmitted light

Color of water

Levine

Sci Am

1982

400 450 500 550 600 650 700 nm


Lecture 3 4813462

Rivers and lakes can vary in water clarity


Different waters attenuate differently

Different waters attenuate differently

1+2 open ocean

3 ocean with chlorophyll

4 coastal waters with chlorophyll and dissolved organics


Fresh water

“Fresh” water

“Green” river water

Swampy “red” waters


Which curve describes light attenuation in green river

Which curve describes light attenuation in Green River?

4

3

2

1


Aquatic environment

Aquatic environment

  • 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

FishBase: Fish at depth viewer

Amphiprion ocellaris


Amphiprion at depth

Amphiprion at depth

10 m

50 m

0 m

25 m


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