LEDs vs HPS efficiency

Factual Questions
1

My High Pressure Sodium security light needs a new bulb. I was researching LEDs, and they seem to be less efficient: a 70 watt HPS bulb produces around 6,000 lumens and the 70 watt LED fixtures produce around 4,000.

So why are street lights being replaced with LEDs. Or is a lumen not a lumen, maybe LEDs are better at throwing light down in a street light application.

The security light is a non-cutoff wallpack. I don’t think a cutoff light would light up the entire yard. Maybe just buying a new HPS bulb is the best option. (The fixture was installed in 1996 and this is only the second bulb.

2

HPS produces ugly light. LPS lamps (which they used around here [San Jose, CA]) is even worse, but they’ve been switching to white LED recently.

My basic understanding is that:

  • The much higher light quality means they can use less total light while maintaining equal or better perceptual illumination
  • The smaller and more manageable sources means that more of the light can be directed downwards, where it’s needed, as opposed to laterally or skyward
3

Are LEDs also better suited to more on/off cycles?

4

Yes.

Also, the output from LED lamps remains fairly constant over their (long) life. Any arc lamp, including HPS and LPS begin deteriorating as soon as they are turned on and the light output drops significantly.

5

Also, the figures you’ve quoted are not cutting-edge. Your LED efficiency is 57 Lumens/Watt, and there are commercial fixtures with much higher performance - over 100 Lumens/Watt.

6

Here’s the real scoop: (I’m an electrical engineer who primarily does roadway lighting):

#1) You can’t look at lumen output for a direct comparison. The delivered lumens from an LED will be greater than the delivered lumens from an HPS lamp of similar brightness. The optics on an LED roadway/area fixture (at least good ones) are precisely aimed for each diode. There is an acrylic or polycarbonate lens over each diode that aims the light on a more direct line. Therefore the efficiency of the optics is higher for LEDs.

#2) Due to the more precise aiming of light, the distribution of an LED luminaire is going to be much more uniform than an HPS area luminaire. The HPS will have a very bright spot (that eats up most of the lumen output) directly below the lamp, which then fades rather rapidly to the edge of the light distribution. The LED luminaire will not have a bright spot, but will be much more evenly lit, spreading the light out. Often, an LED luminaire with half the light output of an HPS will have similar minimum illumination. Average illumination may be lower, but it’s more or less irrelevant, since most of that average bump is from the hotspot below.

#3) As mentioned above, the color temperature of the lamp will also produce a whiter light, which makes it easier to discern things. There’s a whole range of discussion that can be had on scotopic vs photopic illuminance, but that’s a sidebar you don’t need to have. Just know that the non-orange light makes it easier to see.

Also, as mentioned, the LED you’re looking at is relatively inefficient. Many commercial area lights are around 85 lumens/watt delivered. Modern LEDs (bare) are around 161 lumens per watt, but most fixtures aren’t delivering that (and those are only in most recent updates). Still, as mentioned, you don’t need as much overall light to properly illuminate the same area with an LED fixture. Exact requirements are hard to detail, but typically I’ve been able to replace 200W HPS luminaires with 125-150W LEDs and get better light distribution.

As far as life goes, the LED is going to last more or less forever. The LED driver will generally fail well before the diodes. Life for an LED is measured by the L70, or the expected point that light drops below 70% of initial. Typical current lumianires are in the 50-100,000 hour range for L70, though recent redefining of how this is tested means that manufacturers can only extrapolate their testing so far. However most LEDs are performing WAY beyond their rated extrapolated life, such that many LEDs are really at about 90% of initial at the 50,000 hour range.

7

I guess basically what I’m asking is:
I have a 70 watt sodium wall-pack lighting my back yard. The acrylic is starting to yellow but it appears to be just an aesthetic issue.
It needs a new bulb which is not cheap (About $30.00 at the big orange store)
Any reason to look at LED fixtures at this time rather than buying another sodium bulb for the next 8 years.

8

Oh, and it’s left on all night every night and is just a security light, so I don’t really care about the warm-up time or color rendering.

9

You may want to think about whether you really need a security light in your backyard. Lighting up an outdoor area does not necessarily make that area more secure. In fact, it could make the area less secure. The reason is that burglars or other intruders don’t have to bring their own lights and they can hide in the shadows cast by any large object. If the area is not lit up, the intruders have to bring flashlights, which show up really well to anyone even casually glancing at the area.

Plus outdoor lights contribute to light polution.

10

Agreed. Big, bright security lights left on all night provide lots of dark shadows for bad guys to hide in. If security is what you’re looking for, might I suggest you look at a motion controlled light?

11

Very informative. Isn’t the aiming the difference between lumens and candlepower?

I have an bicycle light that puts out 600 lumens for about 1.5 hours. It only weighs 190 grams complete with internal battery. Amazing. As a comparison some car headlight low beams are only 700 lumens.

12

More or less. Candlepower gives you the amount of light per unit solid angle. Lumens is the total amount of light sent in all directions. You can get (almost) whatever candlepower you want from a light of any given luminosity just by concentrating the beam. Unfortunately, that makes it nearly useless as an advertising term; hence, all those “2 million candlepower” spotlights that didn’t actually put out much light.

13

Very simply, the reason that HPS lights put off more lumens than LEDs is that HPS lights are those horrible orange lights you see in parking lots, the ones that make it impossible to tell the difference between a blue car and a green one. HPS put off, if I recall correctly, only a single wavelength of light, making everything look about the same color.

14

JerrySTL, can you give the brand/model of the light? I’m in the market for just this kind of light.

Thx,
hh

15

I drove through that San Jose neighborhood Dr. Strangelove mentioned with the LPS lights at night once. it was awful. The lights were all about the same color as a yellow traffic light. If you were glancing to the side when a light switched from green to yellow, you’d never pick it out when you looked back. I almost came to a stop at a green light, it was so confusing.

16

It’s a NiteRider MiNewt Cordless 600. I use that on my handlebars. I also have a NiteRider MiNewt Cordless 150 that I wear on my helmet. Heavy up there but really nice. Be careful of what you buy as some versions do not come with the helmet mounting strap.

Oh! NiteRider recently replaced the MinNewt line with the new and improved Lumina line. It comes in a 650 lumen version but I don’t think that you can get a helmet strap with it.

There might be some good buys on the MiNewts as they’ve been discontinued.

17

Ha; I’ve never blown through a light, but I’ve come close. It’s especially bad on a long, straight street with lots of intersections in a row and interspersed street lights. It’s very easy to miss one if you glance away for a second. The two colors are totally indistinguishable to me.

The LPS lights have one nice advantage for a small local population: because it’s such a narrow spectrum of light (almost laser-like), it is easy to filter away. The nearby Lick Observatory therefore likes the LPS lights, since they can have dark skies after losing only a small part of the spectrum. The LEDs will be harder to filter.

18

LED street lights have significantly higher efficiency than common LED bulbs; we are talking 65-85 l/W versus 95-130 l/W. For higher wattage applications it becomes more economical to invest in a more efficient power supply. It is also probably a little more challenging to put a highly efficient driver into the tiny space inside a retrofit LED bulb, and in addition many LED bulbs are aiming to be as cheap as possible, almost “throwaway” in some cases. Lastly, another significant reason is that LED streetlamps typically have lower CRI. A 70 CRI LED is typically around 20% more efficient than an 80 CRI LED, all else equal.

One thought thought that I had, theoretically if you were combine low pressure sodium lamps with some with low wattage LED lighting covering the same area, it could be substantially more efficient (in terms of lumens per watt) than standard HPS bulbs. There are many applications where the up-front cost of switching to all LED would be too expensive, and low pressure sodium (LPS) is extremely efficient, nearly as good as the most efficient commercially available LED street lamps. In fact most of the LED fixtures going up right now are less efficient than LPS.

The LPS would produce most of the light, and a little bit of light would be thrown in from the LED just so things would not be entirely monochromatic, just enough that you could make out different colors under the lighting.