Optical/UV Flashcards
Order of UV/IR
UV C, B, A
Visible
IR A, B, C
UV wavelengths
UV A 315 - 380/400nm
UV B 280 - 315nm
UV C 100 - 280nm
IR wavelengths
IR A 760/780 - 1400nm
IR B 1.4 - 3 μm
IR C 3μm - 1mm
Light delivery
Deliver light away from where it is generated - optical fires use total internal reflection. Low loss and high bandwidth.
How does a photodiode work
Converts incident light into electrical current. Creates electron hole pair that produces current. Si/Ge used to create photodiodes that detect in visible range.
PMTs
Good for low light levels but fragile and need stable power supply.
Thermopile
Converts thermal to electrical energy - output in response to temp.
Light interaction with tissues
Reflection
Absorption
Scattering
Fluorescence
Laser doppler flowmetry
Measure blood flow in tissue. Skin illuminated by coherent collimated laser beam. Reflected light collected, moving particles (RBCs) doppler shift light and can calculate speed and concentration of RBCs from this.
Laser Doppler Imaging
Use same principle as LDF but build up image. Used in assessment of burn depth.
Pulse oximetry
Wavelength of light affects how it is absorbed. Shine IR and visible through finger and see what is absorbed, difference based on amount of haemoglobin and oxygen. (Not necessarily suggesting not enough oxygen - miss CO poisoning as Hb still bounded to CO)
Tissue Oxygen Saturation
Use reflected or scattered light instead of absorbed. Uses visible light spectroscopy to quantify HbO2 in different depths of skin as white light reflected is split into components and converted to signal
Near IR Imaging
Light near IR on skin - IR camera can image this and display in visible spectrum. Can see deeper veins. Needs to be aligned perfectly and not real time image
Microscopy
Imaging on small scale using lenses and bright visible light Enhances characteristics such as staining. Look at capillaries in nail folds.
Optical Coherence tomography
Two beams of light have travelled same optical distance. Areas of sample reflecting a lot of light create greater interference - tells us about spatial dimensions and locations of structures.
Laser speckle contrast imaging
Closer representation of blood flow, directly measure movement at superficial depth of skin using laser light. Laser light passed through diffuser makes speckle pattern, contrast max if nothing moves - shows what is high and low movement.
Uses of UV
Treatment of:
Psoriosis
Vitiligo
Polymorphic light eruption
Effects of UV on skin, acute and delayed
Acute erythema (within hours): dilation of superficial blood vessels
Delayed tanning (2-7 days later): production and redistribution of melanin, thickening of skin
How to measure UV
Thermopile - not very sensitive but uniform response
Photodiode - more sensitive but non-uniform
Which UV more readily creates erythema
UVB
Radial exposure (or dose) calculation
effective irradience x time
Why does UV treat psoriasis
UV selectively kills cells that cause the condition. Action spectra 300-320nm - UVB, direct action causing apoptosis.
UVA - indirect action which has mutagenic effect
Hazards of UV for skin and eyes
Skin: sunburn, skin cancer, aging
Eyes: acute - snow blindness
Chronic - cataracts
What are the three types of effect of UV
Photo-electrical
Photo-chemical
Thermal
What needs to be in place if an exposure limit may be exceeded?
Risk assessment
Local rules
UV radiation protection supervisor
Access control
PPE
Training
Audit
What are limits for artificial sources of UV and UVA
Artificial 30J/m^2 over 8 hours
UVA 10000J/m^2 over 8 hours
