4. APTA SS-PS-004-99, Rev. 1 Standard for Low-Location Exit Path ...

Annex D (informative)
Passive LLEPM field test and criteria
D.1 Technical considerations
There are three variables that affect the visibility of the LLEPM signs:
a) Illuminance charging levels provided by ambient light
b) Amount of time that the sign is exposed to light
c) Type of activating light source
4.19
APTA SS-PS-004-99, Rev. 1
Edited 4-1-04
An illumination level of 5 foot-candles (fc) has been shown to provide adequate charging
light for LLEPM material to meet the criterion of 7.5 mcd/m 2 in rail passenger cars, 1.5
hours after normal power is removed. Adequate light for HPPL charging to meet the 7.5
mcd/m 2 criterion is generally available at all locations, except directly under the seats or
if there are overhangs or other obstructions that block light from reaching the LLEPM.
Another variable is the available HPPL charging time. To meet the 7.5 mcd/m 2 criterion,
the adequate charging time at 5 fc would be at least 30 minutes from dark storage to
departure. Most HPPL materials reach saturation at about 30 minutes. The standard time
for the test procedure is set at 60 minutes to make certain that all samples have reached
saturation and to make the results less sensitive to small errors in the timing of
measurements. While 5 fc provides adequate charging light with cool-white fluorescent
lighting, warm-white fluorescent or incandescent lights are less effective at charging. If
warm-white fluorescent or incandescent fixtures are used, the minimum actual charging
level may need to be increased 20 to 50 per cent respectively to ensure that the charging
level and time for LLEPM products is adequate.
As a general rule, if the value of the ambient illuminance level (fc) at the location where
the product is installed, multiplied by the number of minutes of exposure to a light source
(natural or artificial), exceeds 150, then the charge is adequate.
In addition, some samples of HPPL material may be more efficient than others at
absorbing an ambient light charge, thus actual charging times may differ. However, It
should also be noted that for the same charging time (but in locations where the
illuminance on the surface of the HPPL signs is higher for the same amount of time; e.g.,
20 fc for 60 minutes), the sign luminance may be only be increased by 25%.
Finally, proper placement must avoid damage from cleaning.
Volume VI - Passenger System