Why Are High CCT LEDs Being Used in Outdoor Lighting?

[TKM]

Designers of the test and demonstration installations for outdoor lighting
and, particularly, roadway lighting seem to be in love with
high-chromaticity LEDs (5,000 K and higher). Why? O.K. so the lumen
output/watt of such LEDs is somewhat greater at the moment because of the
type of blue-light driven white-light phosphors being used, but phosphors
change and improve all the time. Blue-rich light is more glaring, scatters
more in the atmosphere which leads to greater light pollution and, from a
color standpoint, harks back to the days of clear mercury light sources.
What's the attraction?

Terry McGowan

 

[JB]

Designers of the test and demonstration installations for outdoor lighting
and, particularly, roadway lighting seem to be in love with
high-chromaticity LEDs (5,000 K and higher). Why? O.K. so the lumen
output/watt of such LEDs is somewhat greater at the moment because of the
type of blue-light driven white-light phosphors being used, but phosphors
change and improve all the time. Blue-rich light is more glaring,
scatters more in the atmosphere which leads to greater light pollution
and, from a color standpoint, harks back to the days of clear mercury
light sources. What's the attraction?

You said it yourself....Lumens. Or more specifically, Lm/$!

JB

 

[boxman]

Designers of the test and demonstration installations for outdoor lighting
and, particularly, roadway lighting seem to be in love with
high-chromaticity LEDs (5,000 K and higher). Why? O.K. so the lumen
output/watt of such LEDs is somewhat greater at the moment because of the
type of blue-light driven white-light phosphors being used, but phosphors
change and improve all the time. Blue-rich light is more glaring, scatters
more in the atmosphere which leads to greater light pollution and, from a
color standpoint, harks back to the days of clear mercury light sources.
What's the attraction?

Terry McGowan

My experience has been that it is mostly driven by what you already
referenced and that is that the higher CCT LEDs have the higher efficacy
(lumens per watt). The LED makers I have dealt with all indicated that
the gap in lumens/watt between higher CCT blue light and lower CCT light
will remain that way for the foreseeable future regardless of phosphor
development (the claim is the conversion to red light to lower the CCT
is more inefficient and represents a physical barrier to getting the
lower CCT efficiencies to match the higher CCT efficiencies. I'm no
expert on phosphor so I can't evaluate their claim FWIW).

A small change in efficacy of the LED even by a few percent can mean a
large change in fixture cost depending on the design. So even though
the gap seems small, it can make a big difference to the bottom line.
If you need 10,000 lumens for a particular application, you might use
100 100 lumen/watt leds, but you only need 91 110 lumen/watt leds.
Depending on circuit layouts etc. this could translate into a lower
rated power supply, less heat sink, less optical components etc. to get
the same function. They guy using 100 leds to get the same function
would be at a potentially large cost disadvantage to the guy with only
91 leds.

One other potential driver for high CCT light is that there has been
some talk by some manufacturers and application personnel of considering
scotopic lumens when dealing with outdoor lighting. The higher CCT
lighting then offers a huge advantage in scotopic output. Not sure
whether considering scotopic lumens has any merit in outdoor lighting
applications or not.

 

[Don Klipstein]

It's obvious that the LED community has become interested in
scotopic efficacy ONLY because LEDs are photopically
challenged.

My experience with outdoor lighting is that scotopic lumens count for
something. For example, for equal lux readings according to my light
meter, metal halide illumination appears to me brighter than high pressure
sodium illumination. I find this somewhat true even illuminating gray and
white surfaces where sodium's color-darkening spectrum does not reduce
apparent illumination of most colored illuminated objects.

Of course, scotopic lumens alone don't tell the whole story; visible
illumination efect is from some combination - where scotopic counts less
when photopic illuminance is greater.

- Don Klipstein ([email protected])

 

[boxman]

Designers of the test and demonstration installations for outdoor lighting
and, particularly, roadway lighting seem to be in love with
high-chromaticity LEDs (5,000 K and higher). Why? O.K. so the lumen
output/watt of such LEDs is somewhat greater at the moment because of the
type of blue-light driven white-light phosphors being used, but phosphors
change and improve all the time. Blue-rich light is more glaring, scatters
more in the atmosphere which leads to greater light pollution and, from a
color standpoint, harks back to the days of clear mercury light sources.
What's the attraction?

Terry McGowan

Terry,

Does your inquiry relate to this press release from the IDA?
http://docs.darksky.org/PR/PR_Blue_White_Light.pdf

I was vaguely aware of the issues with circadian rhythm regarding blue
light, but did not know the environmental impact was as strong as the
IDA release seems to imply. I would be interested in reading studies
that address the issue if you (or anybody else reading) know of any.

Just thinking off the top of my head, perhaps a more judicious use of
blue light would be a better solution than trying to restrict it's usage
altogether. Dimming when the space is not in use or limiting it's hours
of usage. It's not inconceivable for LEDs in particular to have a high
CCT source mixed with low CCT sources or other colors that could be
activated to lower the CCT during the night time to minimize circadian
rhythm impact etc. Obviously that's more expensive and more
complicated, but could be an option.