[mcfly_c-137]

The monochromatic light emitted from sodium lamps is also close to the peak sensitivity of the human eye. Colours are not distinguishable, but contrast is much enhanced compared to “cooler” light sources.

*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.

[jacobolus]

It's not especially close to the peak sensitivity of the human eye (in either bright or dim conditions), but that's entirely okay. The goal should be to not affect people's level of dark adaptation.

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.

[adrian_b]

> It's not especially close to the peak sensitivity

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.

[Chris2048]

> The goal should be to not affect people's level of dark adaptation.

Wouldn't that be red light? But night scenes illuminated in red light have the side effect of looking nightmarish..

[adrian_b]

Red light would be even better for affecting the dark adaptation, but it has other disadvantages, like much worse energetic efficiency and lower visual resolution.

Yellow light a.k.a. amber light around the sodium emission line is a good compromise between energy efficiency, visual resolution and dark adaptation.

[ijk]

That's not necessarily a downside for traffic safety, though. Though I imagine someone must have studied the effects of various wavelengths on drivers...

[TeMPOraL]

Advertisers definitely did - there's (some) money in billboards, but only as long as you don't kill your prospective customers.

[adrian_b]

There are 2 kinds of sodium lamps, low-pressure and high-pressure.

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.

[rapnie]

In my area and esp. in the countryside they have green led lighting on various roads as an innovation, with the reasoning that is both least disturbing to wildlife, and best for human vision to see sharply. The light color takes some getting used to, but I am quite a fan of it. Esp. when cycling at home at night through the fields it makes things seem extra serene and peaceful.

[eastbound]

Sodium lamps were deemed dangerous for driving” because they made it difficult for drivers to distinguish shapes, since they were different from day shapes. A kid in bright 1980ies colors (Little Red Hood) would look black under those lights.

LED was presented as a sharp improvement because of the large spectrum of white light.

[jacobolus]

The sodium lamps are in fact safer for driving, because they preserve drivers' night vision, which improves visibility into the shadows, and because they cause less glare.

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.

[duskdozer]

That would make sense. Otherwise I have no idea how people wouldn't have noticed how much more difficult it makes seeing anything outside of the sharp cutoff of the light cone (or, of course, for the person being dazzled on the other side).

[AriedK]

The power savings are minor btweeen LED and low presssure sodium lamps. The LED streetlights emit light along the full spectrum, the sodium lamps only at 589 nm. The LEDs are more controllable so smart dimming ( when there are no cars) is a perceived advantage.