4 - EM Waves, Lasers Flashcards
Nature of light waves
Spatiotemporal electromagnetic wave
Electic + magnetic fields that oscillate in space and time
-oscillations are normal to the direction of propagation
The nature of light across media
- frequency
- wavelength
- velocity
- equations (2)
F: invariant
W + V: dependent upon media
n = c/υn = speed of light in a vacuum/speed in a medium with IR of n
n = λ0/λn = wavelength in a vacuum/wavelength in medium with IR of n
Give λ in vacuum (nm) for the parts of spectrum
- ultraviolet
- visible
- infrared
UV: <400 nm
Visible: 380-760 nm
IR: >700 nm
UV = short Visible-GBIV = short Visible-ROY = long IR = long
List overall spectrum: highest to lowest frequency
Gamma
X-ray
UV
Visible
IR
Microwaves
Radar
TV
Radio
UV light and the eye
UV-A/near: 315-400 nm
UV-B/middle: 280-315 nm
UV-C/far: 100-280 nm
Absorption: -overall: —UV-C (<300) = cornea (epi, bowmans) —UV-A (300-400) = lens, vitreous (300-350) —visible (400-700) = retina
-tears/cornea/aqueous: <290nm (no UV-C reaches lens)
-UV-A/B will be partially transmitted to lens - esp in children (elderly lens transmits only a small amount of UV-A retina)
—why an aphakic pt is at higher risk for UV damage
Describe population inversion
To initiate a chain rxn/avalanche, we need the MAJORITY OF ATOMS IN EXCITED STATE
Achieved by adding energy to the system PUMPING
-UV, chemical, etc.
Describe metastable states
Long-lived intermediate energy states of atoms
Steps to laser functioning (5)
1) pump -> pop inversion
2) atoms decay to metastable state (Emeta)
3) eventually decay to ground state (E0)
4) a PHOTON IS RELEASED DURING THIS PROCESS
5) the released photon stimulates the release of other photons (stimulated emission)
CSLT
Confocal Scanning Laser Tomography
Scanning laser ophthalmoscope
Used to IMAGE RETINAL SURFACE; precisely image the topography of the OPTIC DISC
- computer arranges into 3-D representation
- to measure thickness and cross-sectional areas of the RNFL
Commercially = Heidelberg Retinal Tomograph (HRT)
SLP
Scanning Laser Polarimetry
Measures RNFL THICKNESS using scanning laser ophthalmoscope + polarimeter*
- GLAUCOMA
- device used to measure the change in polarization of light reflected back from retina, which is directly proportional to thickness
Commercially = GDx VCC
OCT
Optical Coherence Tomography
~ultrasound with light rays (not sound)
- constructs 2-D images of NFL, macula, optic disc
- near IR beam which is reflected backwards - temporal delay is measured
Ophthalmic applications
-argon laser
PRP - condns such as neo and mac edema
ALT - on TM for open-angle glauc
LPI
Retinal tears
Ophthalmic applications
-excimer
LASIK/PRK - ablate corneal tissue
Recall:
- measured IOP will decr after
- flap thickness ranges from 160-200 microns
- need 250 microns residual bed
- ablation depth/diopters is ~12microns for traditional LASIK, ~15um for wavefront LASIK
Ophthalmic applications
-Nd:YAG
PCO
SLT - uses Q-switched
LPI - angle-closure glauc
Laser sclerotomy - trabeculectomy surgery
Cyclodestructive procedures on CB - stop aqueous prod
Ophthalmic applications
-helium neon
Illuminate retina to view fundus
