eLFH - Light

Question 1

SI unit of luminous intensity

Answer 1

Candela (cd)

Question 2

Definition of Candela

Answer 2

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

Question 3

Properties of light travelling

Answer 3

Light travels in straight line until it encounters a surface, where it can then transmit, reflect, refract or diffract

Question 4

Relationship between light angle of incidence and angle of reflection

Answer 4

They are equal

Question 5

Refraction definition

Answer 5

When light travels from one medium to another, it can bend / change direction at the interface between the two mediums

Question 6

Factors which determine the degree of refraction of light

Answer 6

Angle of incidence of incoming light ray

Nature of the mediums

Question 7

Index of refraction

Answer 7

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

Question 8

Snell’s law

Answer 8

Question 9

Total internal reflection definition

Answer 9

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

Question 10

How to determine critical angle

Answer 10

Determined by refractive indices of the two substances

Question 11

Equipment that rely on total internal reflection

Answer 11

Fibreoptic laryngoscope / bronchoscope

Question 12

Diffraction definition

Answer 12

Spreading out of light waves as they pass through a gap

Question 13

Factors which affect extent of diffraction

Answer 13

Wavelength of waves and width of gap

Narrower gap causes wider spread

Question 14

Construction of fibreoptic laryngoscope

Answer 14

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

Question 15

Number of glass fibres in each bundle in fibreoptic scope

Answer 15

36,000 to 85,000 fibres in viewing bundle depending on scope size

Question 16

Spectrophotometric analysis

Answer 16

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

Question 17

Laws which describe absorption of radiation as it passes through a substance

Answer 17

Beer’s law

Bouguer’s (Lambert’s) law

Question 18

Beer’s law

Answer 18

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

Question 19

Bouguer’s / Lambert’s law

Answer 19

Each layer of a substance of equal thickness absorbs an equal fraction of the radiation that passes through it

Question 20

Wavelength range of infrared radiation

Answer 20

1 to 40 micrometres

Question 21

Wavelength at which CO2 maximally absorbs infrared radiation

Answer 21

4.26 micrometre

Question 22

Gas analyser construction and mechanism

Answer 22

Infrared light emitted
Known path length and wavelength
Changes in infrared light reaching detector must be due to changes in gas concentration

Question 23

Optical density definition

Answer 23

Unitless measure of the absorbance of a substance

E.g. clear vs foggy evening and light absorption

Question 24

Factors which determine optical density

Answer 24

Length of light path

Wavelength of light

Substance concentration

Question 25

2 main types of infrared gas analysers

Answer 25

Side-stream analyser Mainstream analyser

Question 26

Side-stream analyser

Answer 26

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

Question 27

Advantages of side-stream analyser

Answer 27

Lightweight at patient end Multiple gases can be analysed simultaneously

Question 28

Disadvantages of side-stream analyser

Answer 28

Lag time while it draws off sample

Question 29

Mainstream analyser

Answer 29

Special connector sits in the breathing circuit at patient end Analyser shines infrared light across breathing circuit usually across a sapphire window

Question 30

Advantages of mainstream analyser

Answer 30

No lag time for result

Question 31

Disadvantages of mainstream analyser

Answer 31

Bulky at patient end Usually only measure CO2

Question 32

Issues that can arise with infrared gas analysers

Answer 32

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

Question 33

How do modern gas analysers compensate for nitrous oxide when measuring CO2 from infrared absorption

Answer 33

Most modern gas analysers measure amount of nitrous oxide and automatically compensate for it

Question 34

How does pulse oximeter measure level of oxygenated Hb

Answer 34

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

Question 35

Wavelength of red light in pulse oximeter

Answer 35

660 nm

Question 36

Wavelength of infrared light in pulse oximeter

Answer 36

910 nm

Question 37

Analysis of red and infrared light absorption in pulse oximeter

Answer 37

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

Question 38

Isobestic points definition

Answer 38

Wavelength at which the absorbance is identical for two chemical substances (e.g. both oxyHb and deoxy Hb)

Question 39

Isobestic points for oxyhaemoglobin and deoxyhaemoglobin

Answer 39

590 nm and 805 nm

Question 40

Potential causes for false readings with pulse oximeter

Answer 40

Errors calculating the pulsatile component of light absorption Machine dysfunction Increased ratio of non-pulsatile component of light absorption

Question 41

Causes of errors calculating the pulsatile component of light absorption in pulse oximeter

Answer 41

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

Question 42

Examples of hypoperfusion which cause failure of pulse oximeter

Answer 42

Low pulse pressure Profound vasoconstriction Venous pulsation - e.g. torrential tricuspid regurgitation

Question 43

Examples of abnormal haemoglobins and IV compounds with interfere with light absorption in pulse oximeter

Answer 43

Carboxyhaemoglobin - misleadingly high reading MetHb (methaemoglobinaemia) - misleadingly low reading of 85% Methylene blue and Indocyanine green

Question 44

Causes of machine dysfunction in pulse oximeter

Answer 44

Electrical interference - e.g. surgical diathermy

Question 45

Causes of increased ratio of non-pulsatile component of light absorption in pulse oximeter

Answer 45

Nail varnish Dirty fingers Spurious non-constant background light levels may cause optical interference - e.g. flickering room lights