Tighter photometric specification needed for the LED elevated taxiway
edge light
LED's have been introduced for use in L861T elevated
taxiway fixtures. There have been complaints that the LED's are too bright
and not the same color as incandescent fixtures. Both observations have
merit since we currently have no maximum candela output specified for
these fixtures and we have incandescent fixtures that have noticeable
variations in the color they project.
The current requirement in FAA AC 150/5345-46 for the
L861T elevated taxiway fixture output signal is that it provides a minimum
2 candela for all radials within vertical angles between 0-6 degrees.
At all other vertical angles the minimum candela output shall be .2 candelas.
The color shall be aviation blue as defined by the AC.
Actual light output values of today's L861T fixtures
The actual light output of existing fixtures in the field varies considerably
due to the different light sources being utilized and there being no main
beam maximum candela value specified in AC 150/5345-46.
Incandescent
Tests conducted utilizing different incandescent sources give the results
shown in the below graph. The results were presented in an IES paper several
years back by Crouse-Hinds. It compares tungsten-halogen lamps with that
of standard incandescent. Both 30 and 45 watt tungsten-halogen lamps are
shown. Only 45 watt incandescent is shown.
The lumen maintenance of the tungsten-halogen is good and the average
lumen output is still increasing after 1000 hours at maximum intensity.
The standard incandescent has the characteristic reduction in light output
that takes over after approximately 400 hours of operation and descends
rather rapidly as we approach 1000 hours of operation. Some airports utilize
this lamp for several thousand hours of operation and we can only guess
as to whether the fixture output is still within the specification requirements
at end of life. Intertek Testing Laboratories advises that depending on
filament placement they sometimes find radials at which the incandescent
lamp falls below the 2 cd minimum due to the interference caused by the
filament supports.
The candela averages at the end of 1000 hours at maximum intensity for
the three lamp sources shown are:
- Tungsten-halogen 45 watt lamp 14.2 candela
- Tungsten-halogen 30 watt lamp 7.4 candela
- Incandescent 45 watt lamp 6.6 candela
We know that if we increase the light output of a fixture by 50% this
brightness change is perceived by the human eye. Knowing this indicates
we already have a mixture of brightness levels existing on the airport
when there is a mixture of lamps. This is probably the exception rather
than the rule since most airports tend to use the same type lamp airport
wide for the same application. When mixing LED's and incandescent fixtures
based on the data included in this review there could be a noticeable
difference between the brightness of the two types of fixtures.
LED
The light output of blue elevated LED fixtures also varies from manufacturer
to manufacturer. The lowest seem to be in the 6-8 candela range and some
are in the 12-15 candela range. One fixture has a peak in excess of 30
candelas. Assuming the human eye can discern a 50% variation in light
output we can readily see that we can have non-uniform lighting with regard
to brightness when different manufacturer's LED fixtures are mixed on
the same circuit or if incandescent and LED fixtures are mixed.
Proposed maximum lighting level
There is a need for uniformity of light output between the various types
of elevated taxiway edge lights. A solution is to impose a maximum main
beam candela that reduces the candela spread currently being observed.
The minimum level could also be raised to help narrow the spread between
the maximum and minimum. However, keeping the output range on the low
side is consistent with what has historically been the case with the use
of the incandescent lamp. The sea of blue phenomena becomes more evident
the more light we spread around the airport. A range between 2 and 8 candela
is a suggestion that would meet the historical aspects of elevated taxiway
lighting and be consistent with a large percentage of what we are finding
on the taxiways today. If this range is not sufficient for today's operations
then the range could be bumped up but low visibility operations should
find in-pavement taxiway centerline lights providing the primary added
guidance.
The FAA has drafted an Engineering Brief in an attempt to bring more
uniformity between incandescent and LED taxiway lighting at brightness
levels steps 1 and 2. This is to be accomplished by specifying a percentage
of maximum light output that is applicable for steps 1 and 2. However,
this method is only a solution if we assume the maximum light output of
all elevated taxiway edge lights is equivalent and we know based on information
in this paper that this is not the case.
L861T chromaticity
Since the L861T uses a broad spectrum incandescent light source with
a blue absorption filter we get a light which has a predominant aviation
blue wave length but also contains many other colors. Even in aviation
blue where the predominant wavelength is aviation lighting blue the other
colors can add a hue to the color which is perceived as changes in color
even though the dominant wavelength has changed little. Differences in
filter color, filter temperature and filament temperature (no checks are
made at lower brightness levels) are all factors in determining what shade
of aviation blue you are going to see at any particular time. The LED
source is a pure monochromatic light source with a narrow frequency spectrum.
The major LED manufacturer Lumileds offers three aviation blue color bins
between 460 and 470nm with each bin containing approximately 3.3nm of
spectrum. It is doubtful the human eye can distinguish the color difference
over this 10nm portion of the spectrum. A wider spectrum which exists
for aviation green or amber and which contains more bins due to this wider
spectrum definition will present to the viewer a perceived change in color
if we pick bins which are not close to each other.
The bottom line
is that we will see more variation in the color of elevated taxiway edge
lights which have an incandescent light source than we will with a properly
designed LED source. The LED fixture because of its monochromatic characteristic
does provide us with a more uniform color of signal across the aviation
blue spectrum.
Enter our forum area and state
your opinion.
|
