15/20/2015bae2023 physical properties of biological materials 1 spectroscopy

104/18/23 BAE2023 Physical Properties of Biological Materials

1

Spectroscopy

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Electrical and magnetic properties

• Electromagnetic fields are propagated through and reflected by materials– Characterized as:

• Current flow at low frequencies• Magnetism in metals• Optical absorbance / reflectance in light

• Frequency is a major factor in the primary characteristics– Low frequency – “electrical” properties– High frequency – “optical” properties

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Fundamentals of high frequency electromagnetic waves (Light)

• Light = Energy (radiant energy)– Readily converted to heat

• Light shining on a surface heats the surface• Heat = energy

• Light = Electro-magnetic phenomena– Has the characteristics of electromagnetic

waves (eg. radio waves)– Also behaves like particles (e.g.. photons)

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The electromagnetic spectrum

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Relationship between frequency and wavelength

Plus

Minus Minus

Plus

Wavelength = speed of light divided by frequency

(miles between bumps = miles per hour / bumps per hour)

= Wavelength [m]= Frequency [Hz]c = 3x108 m/s in a vacuum

c

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Relationship between frequency and wavelength

Plus

Minus Minus

Plus

Antenna

+ -

KOSU = 3 x 108 / 97.1 x 106

KOSU = 3 m

red = 6.40 x 10- 7 m = 640 nmBohr’s Hydrogen = 5 x 10 - 11 m

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Plants light harvesting structure - model

Jungas et. al. 1999

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Light emission / absorption governed by quantum effectsPlanck - 1900

E nh E is light energy fluxn is an integer (quantum)h is Planck’s constant is frequency

E hp Einstein - 1905

One “photon”

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Frequency bands and photon energy

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Changes in energy states of matter are quantified

Bohr - 1913

h E Ek j

Where Ek, Ej are energy states (electron shell states etc.) and frequency, , is proportional to a change of state

and hence color of light. Bohr explained the emission spectrum of hydrogen.

Hydrogen Emission Spectra (partial representation)

Wavelength

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Measurement of reflected intensity –Typical Multi-Spectral Sensor Construction

Analog toDigitalConverter

Computer

One Spectral Channel

Photo-Diode detector/ Amplifier

Optical Filter

Collimator

Target

Illumination

CPU

Radiometer

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Measurement of reflected intensity - Fiber-Optic Spectrometer

OpticalGlass Fiber

Photo Diode Array

Optical GratingAnalog toDigitalConverter

Computer

CPU

Element selection

One Spectral Channel at a time

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Visual reception of color

• Receptors in our eyes are tuned to particular photon energies (hn)

• Discrimination of color depends on a mix of different receptors

• Visual sensitivity is typically from wavelengths of ~350nm (violet) to ~760nm (red)

Wavelength

400 nm 700 nm500 nm

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Quantification of color• Spectral measurements can be used to quantify

reflected light in energy and spectral content, but not very useful description of what we see.

• Tri-stimulus models – represent color as perceived by humans– Tri-stimulus models

• RGB - most digital work• CYM - print• HSI, HSB, or HSV - artists• CIE L*a*b*• YUV and YIQ - television broadcasts

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CIE XYZ model

• Attempts to describe perceived color with a three coordinate system model

X

Y

Z= luminance

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CIE Lab model

• An improvement of the CIE XYZ color model.

• Three dimensional model where color differences correspond to distances measured colorimetrically

• Hue and saturation (a, b) – a axis extends from green (-a) to red (+a)– b axis from blue (-b) to yellow (+b)

• Luminance (L) increases from the bottom to the top of the three-dimensional model

• Colors are represented by numerical values• Hue can be changed without changing the

image or its luminance.• Can be converted to or from RGB or other

tri-stimulus models

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Photo-Chemistry• Light may be absorbed and precipitate (drive) a chemical

reaction. Example: Photosynthesis in plants

6 6 62 2 6 12 6 2CO H O h C H O O

• The wavelength must be correct to be absorbed by some participant(s) in the reaction

• Some structure must be present to allow the reaction to occur

• Chlorophyll• Plant physical and chemical structure

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Silicon Responsivity

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Primary and secondary absorbers in plants

• Primary– Chlorophyll-a– Chlorophyll-b

• Secondary– Carotenoids– Phycobilins– Anthocyanins

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Chlorophyll absorbance

Chla: blackChlb: redBChla: magentaBChlb: orangeBChlc: cyanBChld: bueBChle: green

Source: Frigaard et al. (1996), FEMS Microbiol. Ecol. 20: 69-77

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Radiation Energy BalanceIncoming radiation interacts with an object and may follow three exit paths:

• Reflection• Absorption• Transmission

+ + = 1.0, , and are thefractions taking each pathKnown as:

fractional absorption coefficient,fractional transmittance, andreflectance respectively

I0

I0 I0

Iout = I0

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Internal Absorbance (Ai)• Lambert's Law - The amount of light absorbed is directly

proportional to the logarithm of the length of the light path or the thickness of the absorbing medium. Thus:

l = length of light path

k = extinction coefficient of medium• Normally in absorbance measurements the measurement is

structured so that reflectance is zero

klI

IA

outi

)1(log 0

klTI

IA

outi

1loglog 0

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Reflectance

– Ratio of incoming to reflected irradiance– Incoming can be measured using a “white”

reflectance target– Reflectance is not a function of incoming

irradiance level or spectral content, but of target characteristics

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0

100

200

300

400

500

600

700

0 250 500 750 1000 1250 1500 1750 2000Wavelength (nm)

Sp

ectr

al Irr

adie

nce

(w

/m^2

nm

)

Extraterrestrial SolarIrradience

Terrestial SolarIrradience

Adapted from Thekaekara, M. P. 1973.Solar Energy Outside the Earth's Atmosphere.Solar Energy, Vol 14, p 109.

Solar Irradiance

NIRUV

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Soil and crop reflectance

0

0.1

0.2

0.3

0.4

0.5

0.6

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

Fra

cti

on

al

Re

fle

cta

nc

e

43 Soils

27 Soybeans

25 Potatoes

9 Sunflower

73 Cotton17 Corn

P. S. ThenkabailR. B. SmithE. De PauwYale Center for Earth Observation

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Soil Reflectances - Oklahoma

0

0.2

0.4

0.6

0.8

1

350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Ref

lect

ance

(Fra

ctio

n)Tipton Stillwater

Perkins Mangum

Lahoma Haskell

Goodwell Ft. Cobb

Chickasha Altus

Agron. Stwr.

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Electromagnetic properties

Review:• Electromagnetic radiation is energy• Interaction with materials is affected by the

properties of the material• Can give indication of physical damage,

mold presence, foreign material, contaminating chemicals or ID of materials

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• ApplicationsNear-infrared: measuring moisture, % oils and

proteinsXrays: internal defectsMicrowaves: heating/cookingMagnetic properties: moisture content and

compositionGamma Rays: sterilization of food products

during processing

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• Electromagnetic radiation (ER) is transmitted in the form of waves

– Wavelength λ (lambda)– Frequency ν (nu)– λ ν = c, speed of light in a vacuum– 3.0 x 108

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• Xrays and gamma rays have shortest wavelengths 10-12 m and highest frequencies 1020 hz

• 60 cycle AC: 60 hz and 5 x 106 m (coast to coast distance for 1 wavelength!!!!)

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• Interactions with visible light, Infrared and UV radiation

– Used for sorting and quality evaluation– Iref = reflected– I1 = energy entering the object– I2 = energy striking the opposite face

after rectilinear transmission– Iout = leaving the opposite face

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– Transmittance: T=Iout/I0

– Absorbance: Ai=-log (I1/I2)

– Reflectance: R=Iref/I0

– Optical Density: log10(I0/Iout)• Amount of energy transmitted through

the material

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– Flourescence: excited by energy at a particular wavelength and then emits energy at a different wavelength (aflatoxin test for aspergillus...fungi)

– Delayed-light emission: radiation is emitted for a time after the exciting radiation is removed (chlorophyll)

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Electromagnetic properties Resistance, Capacitance and Dielectric

Properties

– Biological materials act as a combination of resistors and capacitors

– Varies with moisture content and internal structure

– Used to evaluate quality and composition– Dielectric loss factor is important in heating

(microwave)

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Properties

– Resistance:• measure by placing material between two

metal plates and incorporating into an electric circuit

– Value of R is inversely correlated with moisture content

– Pressure of plates and temperature also affect R

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Properties

– Resistivity: ρr (rho)R = (ρr L)/A ,

Ω-1m-1 or Siemen/m, S/m– Resistance and resistivity are variableSo…we use capacitance instead.In an AC circuit, capacitor causes a phase shift

between voltage and current. (perfect vacuum = 90°)

With biomaterials in place < 90°See Figure 11.5

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Properties

– Dielectric Properties: dielectric constant ε' and dielectric loss factor ε”.

– ε‘ = ability of material to store energy– ε” = ability of mateials to dissipate energy– Loss tangent = ε” / ε‘ – Rate of heat generation per unit volume (Q) at a

location inside material:– Q = 2πf ε0 ε”E2, where – f = frequency, ε0 = free space dc (8.854E-12

F/m), E = electric field

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Properties

– Distance waves will penetrate material before being reduced to 36.8% of original value….power penetration depth (δp)

δp = λ0((1+ (ε”/ ε‘)2)1/2)-1/2) / (2π(2 ε' )1/2

λ 0 = wavelengh in free space

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Properties

Example 4.2 pg 176 of handout

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Properties

Moisture content effects on dielectric properties

Pg 177 handout figure 4.18

Free water : found in capillaries (I)

Bound water: physically adsorbed to the surface of dry materials (II)

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Properties

Example of dielectric properties:

Page 183 handout Table 4.2

Measuring dielectric properties

pg 187 handout figure 4.23

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Problem 1. Estimate the penetration depth of raw beef during cooking in a home microwave oven. Assume that dielectric properties are constant throughout heating.

Problem 2. Determine the angle of signal lag for wheat, corn and rice.

Problem 3. 11.2 in Stroshine book

Problem 4. 11.4 in Stroshine book

15/20/2015bae2023 physical properties of biological materials 1 spectroscopy

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