Radiation safety
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Light emissions
When the stream of electrons is directed at the phosphor-coated surface of
the CRT screen, the electrons in the molecules of the phosphor are shifted
and, in the process, light is emitted. All of the wavelengths of the light
from the phosphor also are found in natural sunlight.
When a phosphor is developed and made available for use, it is given a reference
number. For example, the phosphor used in one Lenovo display terminal is P42.
The distribution of wavelengths of light for a P42 phosphor
is shown in Figure 5.
Energy distribution curve for P42 phosphor (wavelength in nanometers).
It may be seen from that relative-energy distribution that the emissions
represent a wide range of wavelengths.
In the figure, the emission peak of radiant energy at approximately 525 nanometers
has been arbitrarily assigned the value 100, thereby showing energy levels
at other wavelengths proportional to that peak.
It is possible to see how low the emissions from a P42 phosphor are if they
are compared with the relative radiation energies of a common light source
at the same wavelengths.
Energy distribution curve for a daylight fluorescent lamp.
Figure 6 shows the relative radiant energy emitted by a daylight fluorescent
lamp, such as those commonly found in most office environments. As in the case
of P42 phosphor, emissions occur over a wide range of wavelengths. Again, the
energy peak, here at approximately 425 nanometers, is arbitrarily given the
value 100.
Although the energy peaks in Figures 5 and 6 have been assigned the arbitrary
reference level of 100, the two energy peaks are actually very different, hence,
Figures 5 and 6 do not adequately relate the two radiant energy sources to
each other. It is Figure 7 that shows the relative contribution of the two
light sources with respect to the amount of light striking the observer's eye.
Figure 7 was constructed by retaining the fluorescent energy peak (Figure 6)
as the reference and combining with it the plot (Figure 5) of P42, thus showing
the difference between the two light sources. From Figure 7, one may deduce,
for example, that if the P42 display were set at a nominal brightness and all
the character positions were filled, a light meter held about 35 cm in front
of the display would read approximately 2 lux. By contrast, a fluorescent lamp
illuminating the same area would read about 400 lux.
Combined relative energy curves for P42 phosphor and daylight fluorescent
lamp.
It should be obvious, of course, that as the ambient light level is changed,
or as the brightness setting on the display is changed, the relationship
between the two light sources will also change. The important point, however,
is that the comparison shows that P42 phosphor energy levels are an almost
negligible fraction of the energy levels emitted by a common fluorescent lamp.
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