Lecture for Med Students at SENACYT/Clayton University of Panama

1. OPTICAL PRINCIPLES APLIED TO
HUMAN VISUAL SYSTEM
Dr. José Young

2. 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.

5. 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.

6. Phototherapy: BR photoisomerization

7. Short wave UV
germicide use

8. 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

9. ¿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%

10. ‘
Melanoma
A
B
C
D
E

11. 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

13. OPTICS PRINCIPLES
LIGHT IS ABSORBED, REFLECTED AND REFRACTED

14. 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

15. • 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.

17. 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

18. 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.

19. 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.

20. Lenses
DIVERGING LENSES
-+
CONVERGING LENSES

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

22. 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

23. Diopter D=1/F
The 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

24. Image formation
and Basic
Anatomy

25. Image formation: real & virtual
REAL Image
1. Inverted (if located beyond f)
2. Smaller
3. Behind the convex lens
closer than object
VIRTUAL Image
1. Upright
2. Same size as object
3. Left-right reversal
4. As far behind the mirror as
the object is in front of the
mirror.

26. Does convex lens make virtual image?

27. Reduced eye model
Useful to calculate
image size
AP eye diameter 2,3 cm
N= 0,6 cm
N = nodal point. Where the light
rays are not deviated
Object ?
Distance ?

28. Q’F’
QF
=
F’N
FN
N= nodal point
Q’F’ =
F’N
FN
(QF)
FN
F’N
AP
Tree size 1,8 m
Object distance = 5m
Eye dimeter 2,3 cm
N= 0,6 cm
N
Q’F’ =
AP-N
FP+N
(QF)
Q’F’ =
2,3cm-0,6cm
500cm+0,6cm
(180cm)
Q’F’ = 1,7
500,6
(180cm) = 0,611 cm
Image
Object
=
image-N
Object+N

29. 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

30. Age-related farsightedness

31. Corrections

32. Human eye anatomy cont...

33. • Three layers transparent
structure
1. Corneal epithelium
2. Sstroma
3. Endothelium
• 44 dioptrías
• Astigmatism

34. 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

35. 44 D

36. 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).

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

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

41. 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)

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

44. Light transduction
Ca2+
darkness
Ca2+

45. Receptor
Ganglionar cell

48. VISUAL SYSTEM TESTING

49. CAMPIMETRY
determining the
visual field

51. • 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

52. 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

53. Astigmatism test
Verify if any line looks blurry
http://www.opeluce.tecnoplace.co
m/test/testagudeza.php#

54. • 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
RHODOPSIN 505 nm

55. Humans have
Trichromatic vision
due to the three
types of cones

56. active
Rhodopsin

57. ISHIHARA TEST