640
23 Electricity-to-Light Conversion
(a)
(b)
Fig. 23.1 a Relative eye sensitivity curves for photopic (light adapted, solid line) and (dark adapted, dashed line) vision.
b Conversion of lumen to Watt for light- and dark-adapted vision
Table 23.1 Radiometric and photometric quantities and units. The photometric units are lumen (lm), lux (lx = lm/m 2 )
and candela (cd = lm/sr)
Radiometric
Photometric
Quantity
Symbol
Unit
Quantity
Symbol
Unit
Radiant flux
e
W
Luminous flux
v
lm
Radiant intensity I e
W/sr
Luminous
intensity
I v
cd
Irradiance
E e
W/m 2
Illuminance
E v
lx
Radiance
L e
W/m 2 /sr
Luminance
L v
lm/m 2 /sr
v = K m
∞
0
(λ) V (λ) dλ ,
(23.1)
with K m = 683 lm/W. This formula is also the definition of the unit ‘lumen’. In Fig. 23.1b, the
conversion function
2 V (λ) is shown for light and dark adapted vision.
3
Further derived photometric quantities are luminous intensity (luminous flux per solid angle), measured in candela (cd), the illuminance (luminous flux per area), measured in lux (lx), and the luminance
(luminous flux per area and solid angle). The latter is particularly important if the radiation enters an
optical system, e.g. for refocusing. The radiometric and photometric quantities are summarized in
Table 23.1.
2 The V (λ) curve has been experimentally determined by letting several observers adjust (decrease) the perceived brightness of a monochromatic light source at 555 nm to that of light sources of the same absolute radiation power at other
wavelengths with so-called heterochromatic flicker photometry. The ‘relative sensitivity curve for the CIE Standard
Observer’ was determined in 1924. The ‘standard observer’ is neither a real observer nor an average human observer.
The curve has shortcomings, e.g., due to the used spectral band width (20–30 nm) of the light sources and the comparison
of spectral power instead of the photon flux.
3 While photopic vision is due to cones, the scotopic (dark-adapted) vision is due to rods. Rods are more than one thousand
times as sensitive as the cones and can reportedly be triggered by individual photons under optimal conditions. Rods
predominate in the peripheral vision and are not color sensitive.
23 Electricity-to-Light Conversion
(a)
(b)
Fig. 23.1 a Relative eye sensitivity curves for photopic (light adapted, solid line) and (dark adapted, dashed line) vision.
b Conversion of lumen to Watt for light- and dark-adapted vision
Table 23.1 Radiometric and photometric quantities and units. The photometric units are lumen (lm), lux (lx = lm/m 2 )
and candela (cd = lm/sr)
Radiometric
Photometric
Quantity
Symbol
Unit
Quantity
Symbol
Unit
Radiant flux
e
W
Luminous flux
v
lm
Radiant intensity I e
W/sr
Luminous
intensity
I v
cd
Irradiance
E e
W/m 2
Illuminance
E v
lx
Radiance
L e
W/m 2 /sr
Luminance
L v
lm/m 2 /sr
v = K m
∞
0
(λ) V (λ) dλ ,
(23.1)
with K m = 683 lm/W. This formula is also the definition of the unit ‘lumen’. In Fig. 23.1b, the
conversion function
2 V (λ) is shown for light and dark adapted vision.
3
Further derived photometric quantities are luminous intensity (luminous flux per solid angle), measured in candela (cd), the illuminance (luminous flux per area), measured in lux (lx), and the luminance
(luminous flux per area and solid angle). The latter is particularly important if the radiation enters an
optical system, e.g. for refocusing. The radiometric and photometric quantities are summarized in
Table 23.1.
2 The V (λ) curve has been experimentally determined by letting several observers adjust (decrease) the perceived brightness of a monochromatic light source at 555 nm to that of light sources of the same absolute radiation power at other
wavelengths with so-called heterochromatic flicker photometry. The ‘relative sensitivity curve for the CIE Standard
Observer’ was determined in 1924. The ‘standard observer’ is neither a real observer nor an average human observer.
The curve has shortcomings, e.g., due to the used spectral band width (20–30 nm) of the light sources and the comparison
of spectral power instead of the photon flux.
3 While photopic vision is due to cones, the scotopic (dark-adapted) vision is due to rods. Rods are more than one thousand
times as sensitive as the cones and can reportedly be triggered by individual photons under optimal conditions. Rods
predominate in the peripheral vision and are not color sensitive.