Lecture 5: Interaction of light with matter/biological systems; photon transport phenomena

Question 1

What are the interactions of light with matter ?

Answer 1

1. Reflection
2. Refraction
3. Absorption
4. Emission
5. Scattering
6. Interference
   The strength of these interactions depends on the material properties

Question 2

Describe the steps in the path of a signal from reception to reaction ?

Answer 2

1) Reception: the receptor (for example, the eye) receives the stimulus.
2) Transduction: the stimulus is converted to an electrical impulse.
3) Conduction: the electrical impulse is carried to the brain through the nervous system
through an action potential (more on that below!).
4) Analysis and Interpretation: the signal is processed by the brain.
5) Feedback: a reaction is triggered as a result of the stimulus.

Question 3

What molecules are responsible for the absorption of light in biological systems ?

Answer 3

Pyrroles, Porphyrins, Heme, Chlorophyll, Cytochromes, Phycobiliproteins, Carotenoids, Ferredoxins, Flavins, Melanin

Question 4

What are the main scatterers of biological systems ?

Answer 4

Lipids (Adipose tissue absorbs the most light) . Their content depends on the type of tissue.

Question 5

What does scattering depends on and what are the structures that absorb light through Mie and Rayleigh ?

Answer 5

• Type/size of scatterer
• Mitochondria, lysosomes, vesicles (for Mie)
• Collagen fibrils, macromolecules, membranes (for Rayleigh scattering)

Question 6

What’s transmittance of light in biological systems?

Answer 6

Measure of how much light from a light source is able to pass through a material to the other side.
- How much light is NOT being absorbed
- How much light is NOT being scattered

Question 7

What does the penetration of light through tissue depend on and what does it consist of ?

Answer 7

1. Mean free path: The average distance over which a moving particle travels before changing its direction or energy, as a result of collisions with other particles.
   - The greater the mean free path means greater penetration and deeper imaging.
2. Refractive index variations
3. Size of scatterer (Mie or Rayleigh)
4. Wavelength (longer wavelength, greater mean free path)

Question 8

How can we use the interactions with
light and tissue as engineers?

Answer 8

To build devices for medical applications.
Green light is sent through the
tissue (fingertip, wrist)
● Uses a photoplethysmography
(PPG) sensor to measure the
scattered or transmitted light
● This quantifies the beats per
minute or oxygenation, as
passing blood cells will differ in
absorption.

Question 9

Why can’t we use light in the
visible range for deep tissue
penetration? Which wavelengths
of light can be used?

Answer 9

Visible light is only feasible for skin and other directly accessible tissues because
components that heavily absorb and scatter light, such as lipids, are present in
deep layers of animal tissue.
One modality that allows for imaging at larger depths is near-infrared (NIR) light
between 650nm and 950nm. These long wavelengths are ideal for deep
penetration as they are not absorbed by soft tissue and molecules such as
melanin, which allows the light to travel to deep tissue.