OPTICAL PRINCIPLES APLIED TO HUMAN VISUAL SYSTEM

Dr. José Young

Basics• LIGHT:

– it is an electromagnetic wave. Composed of an oscilant electric and magnetic field mutually perpendicular to each other. Visible light: 400-700 nm

• Photon: – a particle representing a quantum of light or other electromagnetic

radiation. A photon carries energy proportional to the radiation frequency but has zero rest mass

• Photometry: – science of the measurement of light, in terms of its perceived brightness

to the human eye• Lumen:

– the SI unit of luminous flux, equal to the amount of light emitted per second in a unit solid angle of one steradian from a uniform source of one candela.

The electromagnetic spectrum

UV Radiation• More energy than visible light• Convert some molecules into

vitamins (Vit D)• Phototherapy:

– Highly recommended for neonatal jaundice. BR

• risk for skin cancer• UV-A = long wave (400-315nm)• UV-B = medium wave (315-280)• UV-C= short wave (280-100; germicide)

Phototherapy for Psoriasis with UVB light, or UVA light combined with psoralens (PUVA, furocoumarin*)

*chemical substances that sensitize the skin to the effects of the sun, thus leading to irregular pigmen-tation and increasing the risk of sunburn and phototoxicity.

PHOTOISOMERIZATIONPhototherapy: BR photoisomerization

Short wave UV germicide use

Skin Cancer• It’s the most common Ca in the US • The most frequent types: basal cells and squamous cells • Median erithema dose (MED) is the amount of radiation needed to

develop red skin

¿How UV rays affect human skin?

• Erythema is one of the most obvious efects of UV-B rays on human skin. – superficial reddening of the skin as a result of

injury or irritation causing dilatation of the blood capillaries

• UV can harm genetic material causing Ca– (timidine dimers)

• For those white skin people, UV-B long time exposure can cause skin Ca

• Everytime the ozone layer 1% the cancer risk 2%

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Melanoma

A

B

C

D

E

X Ray• An electromagnetic wave of high energy and very short wavelength, which

is able to pass through many materials opaque to light• They can be produced by deceleration of cathode-produced electrons

(generally a tungsten filament) by hitting a metal – An aluminun cathode inside a tube with gas accelerates particles

towards tha anode where the target is located• Interaction with biological material: they are absorbed, transmitted and

they generate ions• They can pass through opaque objects and print photographic film• Widely used in Medicine

– To study bones and joints– To help in diagnosis of soft tissue

OPTICS PRINCIPLESLIGHT IS ABSORBED, REFLECTED AND REFRACTED

Optics Principles

The reflected angle r has the same value as the incident i with respect to a perpendicular surface• Light rays can be...

• Incident; Reflected; Refracted

• Refraction• Refraction index n: is defined as

the ratio of the sine of the angle of incidence to the sine of the angle of refraction

• Snell´s law: n1 sen i = n2 sen r• Optic fiber: It sends information

coded in a beam of light down a glass or plastic pipe

• Endoscope• Cystoscope

Fibers in bundles are clad by a material that has a lower index of refraction than the core to ensure total internal reflection, even when fibers are in contact with one another. This shows a single fiber with its cladding.

Energy is absorbed

• Absorbed energy generates heat• In medicine, infrared rays are used to warm-up tissues and create images

– Thermogram– With modern technology, a single image may contain several

thousands of temperature points, recorded in a fraction of a second• Thermal imaging can be applied in medicine either as a diagnostic test or

as outcome measure for clinical trials– presence of inflammation: Inflammatory arthritis– blood flow is increased or decreased due to a clinical abnormality

from Infrared thermal imaging in medicine. E F J Ring and K Ammer 2012 Physio

Fluorescence • the property of absorbing light or other

electromagnetic radiation (X ray, UV ray) of short wavelength and emitting light of longer wavelength

• Fluoroscopy• is a type of medical imaging that

shows a continuous X-ray image on a monitor, much like an X-ray movie. During a fluoroscopy procedure, an X-ray beam is passed through the body. The image is transmitted to a monitor so the movement of a body part or of an instrument or contrast agent (“X-ray dye”) through the body can be seen in detail.

Fluorescence

• Fluorescent microscope• is an optical microscope that uses fluorescence and phosphorescence instead

of, or in addition to, reflection and absorption to study properties of organic or inorganic substances.

Lenses

DIVERGING LENSES -+

CONVERGING LENSES

Light deviation by lenses. A. Converging lens (+) concentrates light rays . B. Diverging lens (-) separates light rays .

Basic stuff...

• FOCUS: the point on the axis of a lens or mirror to which parallel rays of light converge or from which they appear to diverge after refraction or reflection

• Focal distance : distance between optical center of the lens.

• The focal length of a lens is determined when the lens is focused at infinity

• A lens system with a focal length of 1.8 cm (0.018 m) is a 56-diopter lens. A lens system with a focal length of 1.68 cm is a 60-diopter lens. A healthy eye is able to bring both distant objects and nearby objects into focus without the need for corrective lenses

• The ability of the eye to adjust its focal length is known as accommodation

Diopter D=1/FThe lens strength in diopters is defined 

as the inverse of the focal length in meters

This unit is expressed with positive (converging) or negative (diverging) the

refraction power.A lens with focal lenght +1 m, will have 1 diopter potency; a lens with +2 diopters is a convergent

lens with focal lenght 0,5m

Image formation and Basic Anatomy

Image formation: real & virtual

REAL Image1. Inverted (if located beyond f)2. Smaller3. Behind the convex lens

closer than object

VIRTUAL Image1. Upright2. Same size as object3. Left-right reversal4. As far behind the mirror as

the object is in front of the mirror.

Does convex lens make virtual image?

Reduced eye model

Useful to calculate image size

AP eye diameter 2,3 cmN= 0,6 cm N = nodal point. Where the light rays are not deviated

Object ?Distance ?

Q’F’QF =

F’NFN

N= nodal point

Q’F’ =F’NFN (QF)

FN

F’N

AP

Tree size 1,8 mObject distance = 5mEye dimeter 2,3 cmN= 0,6 cm

N

Q’F’ =AP-NFP+N (QF)

Q’F’ =2,3cm-0,6cm500cm+0,6cm

(180cm)

Q’F’ = 1,7 500,6

(180cm) = 0,611 cm

WORKING OUT IMAGE SIZE

ImageObject =

image-N Object+N

Common visual acuity deffects

A common form of visual impairment in which part of an image is blurred due to an irregularity the cornea. With ASTIGMATISM, light rays entering the eye are not uniformly focused on the retina. The result is blurred vision at all distances. 

ASTIGMATISM

Age-related farsightedness

Corrections

Human eye anatomy cont...

• Three layers transparent structure

1. Corneal epithelium2. Sstroma3. Endothelium• 44 dioptrías• Astigmatism

Cornea

LENS

• Lens refraction power depends on its proteins hence its higher refraction power as compared to neighboring structures.

• MIP26 protein (“major intrinsic protein”) of the lens Works as ionic channel, allowing proteins to work as an electrical sincitium with gap junctions– MIP26 extracts water from lens and keep transparency– Using patch clamp technique 9-11 K+ channels has been

identified, as well as non-selective cations channels – This lens allows focusing objects and concentrates light

44 D

During phacoemulsification — the most common type of cataract surgery — the rapidly vibrating tip of the ultrasound probe breaks up the cataract, which your surgeon then suctions out (top). After removing the cataract, your surgeon inserts the lens implant into the empty capsule where the natural lens used to be (bottom).

Focusing mechanism Accomodation mechanism: changes in lens AP diameter when cilliary muscle contracts and suspensory ligament changes tension

Retina: Eyeball inner-most layer where the receptors are located.

Cones (5-8mm)• Photopic vision • Color• Higher threshold• Photosensitive

pigments• Rodopsin

Rods (2-5mm)

• Scotopic vision• Lower threshold

sensitive to 5 photons• Vm= -30mV (Na+ channel

partially/open)

Light energy converted to action potential :1. Rodopsin reaction 2. Transducin (G Prot)3. PDE: cGMP

Light transduction

Ca2+

darkness

Ca2+

Receptor

Ganglionar cell

VISUAL SYSTEM TESTING

CAMPIMETRYdetermining the visual field

VisualAcuity

• sharpness of vision, measured by the ability to discern letters or numbers at a given distance according to a fixed standard

• Usefulness» Neurological status of the visual system » Macular function» Need of correction lens» Following-up of surgical procedures

Clinical evaluation

• Snellen’s chart – Every line of letters

(optotypes) has a fraction pointing out subject’s visual acuity as the line he/she can read.

• Normal: 20/20• 20/200 see less = 10% of

vision, myopia

Astigmatism test

Verify if any line looks blurryhttp://www.opeluce.tecnoplace.com/test/testagudeza.php#

• Human eye is able to see color. This property is based upon the photosensitive pigments in the retina, especially in cones

• Opsins are the photosensitive pigments present in cones and rods as well. RHODOPSIN is the most studied. The pigment undergoes chemical changes triggered by light.

• Cones are sensitive to BLUE,GREEN and RED • Eritrhopsin – 560-570 nm red light

– Cloropsin – 530-535 nm green light – Cianopsin – 430-445 nm blue light

COLOR PERCEPTION

RHODOPSIN 505 nm

Humans have Trichromatic vision

due to the three types of cones

activeRhodopsin

ISHIHARA TEST