eLFH - Light Flashcards
SI unit of luminous intensity
Candela (cd)
Definition of Candela
Luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 x 10^12 Hz and has radiant intensity in that direction of 1/683 watt per steradian
Properties of light travelling
Light travels in straight line until it encounters a surface, where it can then transmit, reflect, refract or diffract
Relationship between light angle of incidence and angle of reflection
They are equal
Refraction definition
When light travels from one medium to another, it can bend / change direction at the interface between the two mediums
Factors which determine the degree of refraction of light
Angle of incidence of incoming light ray
Nature of the mediums
Index of refraction
Obtained by comparing speed of light in a particular substance to the speed of light in a vacuum
E.g. index of refraction for air = 1.0, for water = 1.3
Snell’s law
Total internal reflection definition
If ray of light strikes a medium boundary at an angle larger than the critical angle, it bounces back and does not pass through resulting in total internal reflection
How to determine critical angle
Determined by refractive indices of the two substances
Equipment that rely on total internal reflection
Fibreoptic laryngoscope / bronchoscope
Diffraction definition
Spreading out of light waves as they pass through a gap
Factors which affect extent of diffraction
Wavelength of waves and width of gap
Narrower gap causes wider spread
Construction of fibreoptic laryngoscope
Bundles of fine glass fibres 8 - 10 micrometre diameter
Each coated in 1 micrometre thick cladding glass layer
Cladding glass has lower index of refraction to ensure total internal reflection occurs
Glass fibres arranged into bundles
Number of glass fibres in each bundle in fibreoptic scope
36,000 to 85,000 fibres in viewing bundle depending on scope size
Spectrophotometric analysis
Method of measuring gas concentrations in the anaesthetic gas analyser according to optical density
Involves shining light (radiation) through a sample and determining the quantity of radiation absorbed and therefore gas concentrations
Laws which describe absorption of radiation as it passes through a substance
Beer’s law
Bouguer’s (Lambert’s) law
Beer’s law
Absorption of radiation by a given thickness of solution of a given concentration is the same as that of double the thickness and half the concentration of the solution
Bouguer’s / Lambert’s law
Each layer of a substance of equal thickness absorbs an equal fraction of the radiation that passes through it
Wavelength range of infrared radiation
1 to 40 micrometres
Wavelength at which CO2 maximally absorbs infrared radiation
4.26 micrometre
Gas analyser construction and mechanism
Infrared light emitted
Known path length and wavelength
Changes in infrared light reaching detector must be due to changes in gas concentration
Optical density definition
Unitless measure of the absorbance of a substance
E.g. clear vs foggy evening and light absorption
Factors which determine optical density
Length of light path
Wavelength of light
Substance concentration
2 main types of infrared gas analysers
Side-stream analyser
Mainstream analyser
Side-stream analyser
Sample of gas (typically at 150 ml/m) drawn from breathing circuit and analysed
Moisture trap removes water from sample gas
Sampled gas either returned to circuit or scavenged
Advantages of side-stream analyser
Lightweight at patient end
Multiple gases can be analysed simultaneously
Disadvantages of side-stream analyser
Lag time while it draws off sample
Mainstream analyser
Special connector sits in the breathing circuit at patient end
Analyser shines infrared light across breathing circuit usually across a sapphire window
Advantages of mainstream analyser
No lag time for result
Disadvantages of mainstream analyser
Bulky at patient end
Usually only measure CO2
Issues that can arise with infrared gas analysers
Oxygen can broaden the CO2 absorption spectra
Nitrous oxide can interfere with CO2 absorption and vice versa
Water vapour absorbs infrared light causing falsely high CO2 readings
How do modern gas analysers compensate for nitrous oxide when measuring CO2 from infrared absorption
Most modern gas analysers measure amount of nitrous oxide and automatically compensate for it
How does pulse oximeter measure level of oxygenated Hb
Spectrophotometric technique
Light (red light and infrared light) from two emitting diodes shone through sample every 5 to 10 microseconds and quantity of radiation absorbed is determined by a detector on other side of sample
Wavelength of red light in pulse oximeter
660 nm
Wavelength of infrared light in pulse oximeter
910 nm
Analysis of red and infrared light absorption in pulse oximeter
660 nm red light absorbed less by oxyHb vs deoxyHb (hence why oxyHb is more red as less red light is absorbed)
910 nm infrared light absorbed less by deoxyHb vs oxyHb
Pulsatile component (i.e. arterial blood flow) measured for each wavelength and constant component which is not from arterial blood is subtracted from it
Oxygen saturations are calculated by comparing absorption against measured values from experimental studies
Isobestic points definition
Wavelength at which the absorbance is identical for two chemical substances (e.g. both oxyHb and deoxy Hb)
Isobestic points for oxyhaemoglobin and deoxyhaemoglobin
590 nm and 805 nm
Potential causes for false readings with pulse oximeter
Errors calculating the pulsatile component of light absorption
Machine dysfunction
Increased ratio of non-pulsatile component of light absorption
Causes of errors calculating the pulsatile component of light absorption in pulse oximeter
Hypoperfusion - more difficult for pulse oximeter to determine points of maximum and minimum absorption
Abnormal haemoglobins / IV compounds that interfere with light absorption
Arrythmias - harder for pulse oximeter to predict points of minimum and maximum absorption
Examples of hypoperfusion which cause failure of pulse oximeter
Low pulse pressure
Profound vasoconstriction
Venous pulsation - e.g. torrential tricuspid regurgitation
Examples of abnormal haemoglobins and IV compounds with interfere with light absorption in pulse oximeter
Carboxyhaemoglobin - misleadingly high reading
MetHb (methaemoglobinaemia) - misleadingly low reading of 85%
Methylene blue and Indocyanine green
Causes of machine dysfunction in pulse oximeter
Electrical interference - e.g. surgical diathermy
Causes of increased ratio of non-pulsatile component of light absorption in pulse oximeter
Nail varnish
Dirty fingers
Spurious non-constant background light levels may cause optical interference - e.g. flickering room lights
