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OSRAM metal halide lamps comply with the limit
values of 2 mW/klm or even go below the limit
considerably.
Exceptions are the HQI® lamps without outer bulb
with the power of 1000 W and 2000 W. Here special
safety precautions have to be met by the luminaire.
Standardization of UV variables per “klm” or “lm” offers the advantage of being able to make direct comparison of the relative radiation shares of various lamp
types and wattage classes with regard to the same
application illuminances.
As a comparison:
• Tubular Fluorescent T8 & T5 have an ACGIH UV
value of approx. 0.2 mW/klm (with possible minor
fluctuations depending on wattage class and light
colour).
• Compact lamps have a lower 0.03 mW/klm
Fading occurs not only due to UV but also with shortwave visible light, depending on the spectral object
sensitivity (effect function) of the irradiated object.
There is plenty of information on this subject in the
Division 6 report of the CIE (CIE technical collection)
entitled “On the Deterioration of Exhibited Objects by
Optical Radiation”. Although this deals with objects in
museums, the results are also applicable for example
to shop window lighting. A stronger fading effect could
be achieved by stronger focusing of the light or by a
higher luminous flux in the lamp.
A numerical definition of colour change generated by
irradiation must be expressed in the form of colourimetric differences ΔE*ab. In this way, it is possible to
express exactly every fading, blackening and yellowing
or basically every colour change. Effective radiation
resulting in a colour change of exactly ΔE*ab = 1, is
called threshold effective radiant exposure. This value
is important, as experience shows that colour changes
in this magnitude can be perceived by the average
observer on comparing unexposed areas of a sample
with exposed parts.
Other limit values can be used (ΔE*ab = 2, 3, 4, etc.)
if the correspondingly larger colour differences are
acceptable.
8.4.2 Protective measures to reduce fading
Every protective measure must refer to a reduction in
effective radiant exposure Hdm. Effective radiant
exposure Hdm is the product of the radiation time
tdm and effective irradiance Edm.
Fig. 55: Evaluation function for the sensitivity of human
tissue to UV radiation as per ICNIRP
8.4.1 Fading effect
The colour change in light-sensitive materials resulting
from irradiation with light sources depends on
• irradiance or illuminance,
• spectral distribution of the radiation from the
light source,
• spectral object sensitivity (effect function) and
• irradiation time.
If daylight contributes to the lighting e.g. via skylights
or shop windows, this has to be considered as part of
the irradiation as well. Daylight contains considerable
amounts of light in the UV range und short-wave visible light.
In new objects, colour change is strongest during the
initial period of light exposure. Old wall carpets for example which have been exposed to light for centuries,
show hardly any remaining sensitivity to radiation.
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Reduction can consist of:
• avoiding the critical wavelengths by using
corresponding filters according to the spectral
sensitivity of the irradiated object
• reducing the irradiance
• reducing the exposure time
• enlarging the distance to the luminaire
Remarks on filtering the critical wavelengths:
Relative spectral sensitivity for most samples is not
only very high in the ultraviolet range of irradiation,
and also still fairly high in the visible range for many
exhibits. This would mean that the short-wave visible
range also has to be filtered. To what extent this is feasible depends on the colour rendering characteristics
and the changed colour temperature of the remaining
visible radiation.