That doesn't seem right to me. Sodium (and mercury) vapor lamps are the color they are due to physics, and were chosen because they're very efficient (and long lasting). Low-pressure sodium is the best and worst of these; essentially monochromatic but fantastic efficiency. Their only advantage, color-wise, is that the light can be filtered out easily (they used to be widely used in San Jose because Lick Observatory could filter out the 589 nm light).
*edit: but it’s the overwhelmingly larger lifespan (20-30k hrs) that led to the wide adoption as streetlights. And I guess, the same is true for the change to led today, because of less power consumption.
LED was presented as a sharp improvement because of the large spectrum of white light.
What they aren't good for is LED manufacturers' bottom line, and the lighting industry spent a lot of lobbying money to entice friendly politicians to heavily subsidize them with public infrastructure budgets, with those subsidies then misleadingly sold to the public as "efficient" and "environmentally friendly".
They're also not very good for reading the newspaper or doing critical color analysis. Thankfully such tasks do not need to be done at night in the middle of the street.
If you use shorter ("bluer") wavelengths, as happens with white LEDs which consist of a blue LED + phosphor, it causes people's eyes to become bright adapted and effective night vision is ruined, causing people to have much worse vision in the shadows.
Also, if you use bluer light, the lights themselves cause dramatically more glare in peripheral vision, because the shorter-wavelength-sensitive "S" cone cells and rod cells are mostly absent from the fovea (center of the retina), and prevalent in the outer areas of the retina. This is why LED headlamps on cars are so obnoxious for drivers going the opposite direction.
Also, the LEDs clobber people's circadian rhythms and are extremely disruptive to wildlife.
Finally, the light pollution caused by the LEDs is much worse for seeing the stars, which is maybe not as important as the other harms, but still kind of sad.
Wouldn't that be red light? But night scenes illuminated in red light have the side effect of looking nightmarish..
Yellow light a.k.a. amber light around the sodium emission line is a good compromise between energy efficiency, visual resolution and dark adaptation.
The sensitivity at sodium light is above 75% of the peak human vision (photopic) sensitivity.
This is a very small difference in light sensitivity. For example in the case of many sources of red light or blue light the sensitivity can be 5 to 10 times lower than the peak sensitivity.
Moreover, a perfect source of white light cannot achieve a better sensitivity than around 37%, i.e. less than half of the efficiency of an ideal source of monochromatic light at the sodium emission line.
Therefore the fact that currently LED lamps and low-pressure sodium lamps have about the same energy efficiency is caused by the LED lamps having a higher photonic efficiency and a lower threshold voltage (caused by a P-N junction voltage instead of the ionization potential of sodium), which compensate the disadvantage of using white light. A monochromatic LED lamp with the same color as the sodium lamps could have an energy efficiency at least double over the white LED lamps.
The low-pressure lamps emit monochromatic light and they have not only the advantage of long life but they are also the only other source of light that matches the energy efficiency of converting electrical energy to light of the LED lamps.
So replacing low-pressure lamps with LED lamps does not produce any significant economic effects, it was justified only by the supposed advantage of enabling color vision.
However in many places high-pressure sodium lamps have been preferred, which have a wider spectrum, so they allow some very poor color discrimination. The high-pressure lamps have a lower efficiency than LED lamps, so replacing them was justified by energy savings.
Outdoors at night, I prefer the monochromatic low-pressure sodium lamps, but sadly LED lamps have replaced them in most places.
1. For a sufficient gas pressure in the lamp, the substance must be either a gas or a metal with low boiling temperature, so that it will be vaporized by an electrical discharge.
2. The gas must not react chemically with the enclosure and with the electrodes, which prevents the use of most gases except noble gases and metallic vapors. Except for noble gases and metallic vapors, the lamps using other substances must not have electrodes, so they need a more complex and less efficient electronic system for producing a high-frequency AC discharge, e.g. using a magnetron from microwave ovens.
3. The ionization potential must be low for a good energy efficiency. Alkaline metals have low ionization potentials and low boiling temperatures, so they are better than noble gases and other metals.
4. The color of the light must be one where the sensitivity and the visual acuity are high. This narrows the choice to yellow light, i.e. to sodium, between the alkaline metals.
Among alkaline metals, sodium is the cheapest, so it was a logical choice.
However, the fact that it produces light of a suitable color was a happy coincidence. If sodium had produced violet light, like potassium, and potassium had produced yellow light, potassium would have been chosen for lamps.
So among the criteria for choosing sodium for lamps, the color of the light was as important as cost, ionization potential and vapor pressure.
> Their only advantage...
How are you coming to this conclusion?! Their warmer has very meaningful effects on processing, attention and other visual effects as is the point of the discussion in the first place. It's not clear what makes you so sure that color differentiation is essential and the other effects are irrelevant.
No I absolutely don't know what matters. But it seems neither do you.
Way better work than whoever it is handling this LED nonsense. Why we can't find a diode that we can use to simulate the old spectra would be a fun research project.
But different phosphors have different efficiency and price. LED lamps were first introduced for interior lighting, where sun-like spectrum is welcome. Such LEDs were produced en masse and relatively cheaply. So street lighting naturally used them, because municipalities usually look for the cheapest viable option.
We likely could produce high-power narrow-spectrum orange LEDs if there was a large market for the economies of scale to kick in. You can buy deep orange LED lamps today (look for color temperature 1800K or 1600K, "amber"), but they are more expensive, because they are niche.
We can make LED light appear to be any given colour by mixing multiple LEDs. But mixed colour isn't the same as pure colour, made from a single spectra of light. Nor is it the same as true broad spectrum light - like we get from black-body radiation like the sun, or a tungsten bulb.
Its hard to tell the difference just by looking at a light. But different kinds of lights - even lights which look the same colour - will change what objects actually look like. And they probably have different effects on our sleep cycle and our low light vision. I was in a room once lit only by sodium vapour lights. The lights were yellow, but everything in the room (including me) appeared to be in greyscale. It was uncanny.
This is part of the reason why LED lights are still looked down on by a lot of old school photographers and film makers. Skin doesn't look as good under cheap LED lights.
Only green LEDs have worse efficiency, because they must be made with semiconductors for which optimum efficiency is attained at either lower or higher light frequencies.
Lamps using high-efficiency amber LEDs with about the same color with sodium lamps could be made at an energetic efficiency at least double to that of white LED lamps.
The double factor comes from the visual sensitivity being double for the light at sodium color than for ideal white light.
In reality the energetic efficiency of such LED lamps should be more than double, because they do not have losses caused by conversion through fluorescence.