8.2 Colour rendering. Osram HQI-E 150 W/NDL CL
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8.1 Night vision twilight and typical street lighting conditions. This is called mesopic vision which lies between photopic and scotopic vision.
The luminous flux, measured in lumens, is the irradiated output of a light source evaluated by the eye. It is defined by multiplying the physical radiation output with the eye sensitivity curve V(λ). Standard luminous flux measurements only consider the reaction of the eye at high illuminance levels (photopic vision) as is typical for daylight and indoor illumination. Luminous flux measurements measure photopic light as perceived by the central region of the eye.
The change in eye sensitivity comes from the presence of two types of light receivers on the retina: rods and cones. The rods are responsible for vision under low illuminance and are located in the peripheral field of vision. The rods are sensitive to scotopic light while the cones react to photopic light. When the illumination level decreases, the rods are therefore more active, while the cones become inactive.
When the illumination level is very low, for example at night by star light, the vision conditions are said to be scotopic. The reaction of the eye changes under these circumstances. The eye sensitivity curve for low illumination levels (less than 0.1 cd/m²) is the V'(λ) curve, as shown in the figure 43.
The effective, seen “lumen” will differ from the measured photopic luminous flux. When the illumination level falls, the effective “luminous flux”, e.g. of yellow high-pressure sodium lamps, decreases while the effective “luminous flux” of white light with a higher share of green/blue light increases.
Sensitivity for red and yellow light decreases, while there is better perception of blue light. When luminous flux is measured under photopic conditions, this does not correspond to what the eye perceives at low light levels. The reaction of the eye does not change suddenly from high to low illumination levels. The change is gradual when the illumination level decreases to
Figure 47 shows the radiation output of a HCI ® -TC
70 W/NDL and a NAV ® -T 400 W Super 4Y, normalized in the interests of comparability to a luminous flux of
1000 lm. The diagram shows the relative distribution of the radiation in the spectrum.
46
Fig. 47: Physical radiation output in W per 1000 lm and per 5 nm
In Fig. 48, the physical radiation output has been multiplied by the V(λ) curve to ascertain the luminous flux per 5 nm in each case. Integration of the values for all wavelengths between 380 nm and 780 nm results in the specified 1000 lm for both light sources.
The NAV ® lamp radiates more light in the range around
580 nm, which is near the maximum of the V(λ) curve.
This contributes to a high luminous efficacy. On the other hand, there are some gaps in the spectrum, particularly in the blue part of the spectrum, which is responsible for the poorer colour rendering compared to the metal halide lamp.
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