LED Traffic light question

[Paul Hovnanian P.E.]

I'm sure most people here have seen LED traffic lights.

They appear to have come out with the red lamp assemblies first,
followed more recently by green and amber. The red lamps had red LEDs
(Duh!) behind a red tinted lens. When the green and amber units started
appearing, they were made with clear lenses. This is particularly
helpful in the case of green so that sunlight reflected from a green
lens doesn't create a false green signal. When de-energized, they appear
greyish in color.

However, I've seen some brand new units lately that use LEDs for all
three colors and I've noticed that only the red signal still uses a
tinted red lens. Why?

 

[Douglas G. Cummins]

I'm sure most people here have seen LED traffic lights.

They appear to have come out with the red lamp assemblies first,
followed more recently by green and amber. The red lamps had red LEDs
(Duh!) behind a red tinted lens. When the green and amber units started
appearing, they were made with clear lenses. This is particularly
helpful in the case of green so that sunlight reflected from a green
lens doesn't create a false green signal. When de-energized, they appear
greyish in color.

However, I've seen some brand new units lately that use LEDs for all
three colors and I've noticed that only the red signal still uses a
tinted red lens. Why?

The only reason I can think of for not having a colored lens is UV
resistance. A clear lens may inherently more easily prevent UV fading
(so long as you have some sort of anti-UV colorfast material) than a
colored lens. As far as I know, the only color requirement for traffic
signals is of the light and not the lens it passes through. The red
lens by its nature already blocks a lot of UV, so I would think it's
easier to make more UV resistant, and so may not have faded enough to
warant replacing compared to the yellow and green lenses.

I don't think having a clear lens necessarily prevents false signals
from sunlight. The reflectors for the LEDs (if the unit has any) would
not reflect a significant amount of sunlight toward drivers, so the
color of the lens is mostly moot for that point.

 

[TKM]

The only reason I can think of for not having a colored lens is UV
resistance. A clear lens may inherently more easily prevent UV fading (so
long as you have some sort of anti-UV colorfast material) than a colored
lens. As far as I know, the only color requirement for traffic signals is
of the light and not the lens it passes through. The red lens by its
nature already blocks a lot of UV, so I would think it's easier to make
more UV resistant, and so may not have faded enough to warant replacing
compared to the yellow and green lenses.

I don't think having a clear lens necessarily prevents false signals from
sunlight. The reflectors for the LEDs (if the unit has any) would not
reflect a significant amount of sunlight toward drivers, so the color of
the lens is mostly moot for that point.

There are chromaticity specifications for signal light colors. I thought
the lenses brought the LED colors into the specification range.

Terry McGowan

 

[Douglas G. Cummins]

There are chromaticity specifications for signal light colors. I thought
the lenses brought the LED colors into the specification range.

Yes, that is why I wrote, "the ... color requirement for traffic signals
is of the light and not the lens it passes through."

Ah, now that I think about it, matching different manufacturer's lenses
with the color of different LED heads can shift the signal's color
out-of-compliance. It's not so important with red because a deep red
LED with just about any red lens will still produce a deep red color.
That's not true with the extremely narrow allowance for yellow nor for
the blue-green. It would make sense to have a clear or neutral density
lens for the yellow and green LEDs.

 

[Jim]

Paul Writes

However, I've seen some brand new units lately that use LEDs for all
three colors and I've noticed that only the red signal still uses a
tinted red lens. Why?

Jim Replies:
The cities & counties have requirements on the traffic signals purchased.
Some communities require all of the signals to have a tinted lens and others
do not.

In Arizona I noticed none of the signals were tinted.
In New York City all of the lenses were tinted.

To meet the ITE standard colors - usually (almost always) the manufacturer
does not need a tinted lens. The color coodinates of the LEDs often fall
completely within the ITE boundires.

Sincerely,

Jim Creveling
SSLC

 

[Daniel J. Stern]

I don't know if your reference to "LEDs for all three color" and "red
signal" refers to a traffic light box that has 3 separate lenses for
the 3 colors, or all 3 colors come from a single lens.

Erm...is there really anywhere you've seen single-head units producing all
three colours from just one "lens"? the closest thing I've seen is the
restricted-turn arrows in Toronto. They display a green arrow, then some
time later the arrow changes to yellow, then turns off. This is
problematic enough, I imagine, for colourblind people. The notion of a
single head providing red-ball, yellow-ball and green-ball signals is a
little hard for me to swallow given general North American practices and
the provisions contained in the MUTCD.

You did say "that only the red signal still" ... emphasis on "only", so
it would seem to me you mean "why not the others". But maybe you mean
"why is the red lens still red tinted"? That I cannot say, but I would
think it would not hurt. And it could allow substitution of yellow LEDs
in the red section.

That wouldn't make sense. In the first place, installing yellow LEDs
behind a red lens would be a tremendous waste, for very much less red
light is produced by yellow LEDs than by red ones. In the second place,
the colourimetry of such an arrangement would be very much more difficult
to bring into compliance than a setup with red LEDs behind a red, clear or
ND lens.

Speaking of LED traffic lights, I've been wondering when they might do
single lens types, with shapes supplementing the colors. For example
for "stop" the red LEDs would be in the shape of a traditional octagon,
either solid, or just lighting it as a border of an octagon. Yellow
could be in the shape of an "X", while green could be un the shape of
an arrow (pointing forward, or to either side, as the traffic cases
may need).

OK, so you're just guessing/daydreaming, and not reporting on
single-head/3-colour traffic signals you've seen?

Some areas of Quebec use horizontally-arranged 4-head traffic signals in
which the two outermost heads are *square* reds, the left inboard is
yellow, and the right inboard is green. Other heads (e.g. turn arrows), if
present, are also located inboard of the dual square reds:

http://www.gribblenation.net/canpics/subs/pq-signals.jpg

DS

 

[Victor Roberts]

Some areas of Quebec use horizontally-arranged 4-head traffic signals in
which the two outermost heads are *square* reds, the left inboard is
yellow, and the right inboard is green. Other heads (e.g. turn arrows), if
present, are also located inboard of the dual square reds:

http://www.gribblenation.net/canpics/subs/pq-signals.jpg

Back in the late 1960's and 1970's Baltimore had a few
horizontal traffic lights in the area near Ft. McHenry.
These were three-head units where all heads were round. Very
confusing for a semi-color blind person like myself. I'm not
sure if they are still there.

--
Vic Roberts
http://www.RobertsResearchInc.com
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[John David Galt]

Erm...is there really anywhere you've seen single-head units producing all
three colours from just one "lens"?

It's very common for railroad signals to work that way.

 

[Daniel J. Stern]

Back in the late 1960's and 1970's Baltimore had a few horizontal
traffic lights in the area near Ft. McHenry. These were three-head units
where all heads were round. Very confusing for a semi-color blind person
like myself. I'm not sure if they are still there.

Such units are all over the place (right now, today) in Texas.

 

[Daniel J. Stern]

It's very common for railroad signals to work that way.

Sure, but *road* traffic signals?

 

[me]

It's very common for railroad signals to work that way.

Both with LED and traditional bulbs. I assume with LED's, they aren't using
separate lenses, but on the old bulb style (called "searchlight"), there
were 3
lenses inside the housing, and the proper color would drop in front of the
bulb. It was supposed to have a failsafe of the red lens, but I've seen at
least one case where the outer lens was smashed (vandalism), and it
prevented
the lenses from moving (it was stuck with the yellow lens in position)