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Background luminance
6/60
6/30
6/12
6/9
6/6
6/5
6/4
125 cd/m2
III-11-6 Manual of Civil Aviation Medicine
Luminous flux
11.3.7.1 This is the visible power or light energy per unit of time. The unit is the lumen (lm) which is defined
as the flux emitted within a unit solid angle by an idealized uniform point source of 1 candela of luminous
intensity. In physics, power is measured in watts and there is a correlation between lumens and watts. The
human eye is most sensitive to light of 555 nm, and 1 watt of power at this wavelength (and at this specific
wavelength only) is equivalent to a luminous flux of 675 lumens.
11.3.7.2 Thus 1 lumen is equivalent to approximately 0.0015 watt.
Luminous intensity
11.3.7.3 This is the luminous flux per unit solid angle from a point. Unit solid angle is called a steradian and
is that solid angle which cuts an area of 1 m2 from the surface of a sphere with a radius of 1 m. The light may
be emitted or reflected. The unit of luminous intensity is the candela (cd). One candela is 1 lumen per steradian.
The candela was originally derived from the luminance of a black-body radiator at the temperature of solidifying
platinum. Candle, candela and candlepower are all the same.
Luminance
11.3.7.4 This is the luminous intensity per unit area projected in a given direction, and the unit is the candela
per square metre (cd/m2). This unit is also called the nit.
11.3.7.5 Other luminance units are:
stilb (sb) = 1 candela/cm2
lambert (L) = 1/π candela/cm2
11.3.7.6 It should be noted that the formula used to calculate luminance from illuminance contains the
factor 1/π.
Illuminance
11.3.7.7 This is the luminous flux on unit area of a surface. The unit is the lux (or metre-candle). One lm
uniformly distributed over 1 m2 of surface produces illuminance of 1 lux. For a given luminous flux, the
illuminance decreases as the illuminated area increases.
11.3.8 Most of the units described above are SI units. For countries using the foot-pound system (FPS)
the conversion factors are as follows:
Luminance can be measured in foot-lamberts (fL). 1 fL = 3.426 cd/m2
Illuminance can be measured in foot-candles (fc). 1 fc = 10.75 lux
11.3.9 There are numerous other units used in photometry but describing them all is beyond the scope
of this outline.
11.3.10 To give the above units some practical meaning, the following luminance levels may be helpful
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-7
(Table 11-1):
Table 11-1. Luminance levels for different types of lighting
Environment
Luminance
(cd/m2)
Sun 109
Clear sky at noon 104
Cloudy sky at sunset 10
Clear sky one quarter hour after sunset 1
Night sky, full moon 10–2
Night sky, cloudy, no moon 10–4
Pure scotopic (rod) vision <5 × 10–3
Pure photopic (cone) vision >10
Mesopic vision 5 × 10–3 to 10
11.3.11 The luminous intensity of lamps is measured in units called mean spherical candlepower (MSCP).
The lumen output of a lamp is found by multiplying MSCP by 4π.
11.3.12 If a perfectly reflecting and diffusing surface is illuminated with 1 lux the luminance will be 1/π cd/m2.
White paper reflects about 75 per cent of the incident light so that when it is illuminated with 1 lux its luminance
will be 0.75 × 1/π = 0.24 cd/m2 or 2 400 stilb.
11.3.13 There is no simple relationship between the specified wattage of a given lamp and the illumination
it provides. Factors such as reflectors, angle of incidence of the light rays on the illuminated surface and the
distance from the light source are decisive.
11.3.14 The approximate illuminance given by an ordinary 40-watt desk lamp with a white conical reflector
aimed at 45 degrees to the surface is given in Table 11-2. This table also shows the corresponding luminance
of a white surface (paper or visual acuity chart) viewed at right angles.
Table 11-2. Approximate illuminance from a 40-watt bulb
Bulb-chart distance
Bulb illuminance Chart luminance
50 cm 350 lx 110 cd/m2
75 cm 155 lx 45 cd/m2
100 cm 85 lx 25 cd/m2
III-11-8 Manual of Civil Aviation Medicine
With a 60-watt bulb, the light levels are as in Table 11-3:
Table 11-3. Approximate illuminance from a 60-watt bulb
Bulb-chart distance
Bulb illuminance Chart luminance
50 cm 600 lx 180 cd/m2
75 cm 270 lx 80 cd/m2
100 cm 150 lx 45 cd/m2
11.3.15 Figure 11-2 shows the relationship between illumination of the test chart and visual acuity. The
minimum background luminance recommended for the test charts is 80 cd/m2.
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