Would it be possible to install filters on LED lights to reduce blue light?

[RJDG14]

I came across this article talking about a study about the negative effects of LED streetlights on human and animal health due to the fact that much of the light they emit is blue light which can decrease melatonin levels, affecting sleep patterns:

https://www.theguardian.com/environment ... mal-health

The majority of LED streetlights seem to give off white coloured light, unlike the cream-orange (high pressure sodium) or orange (low pressure sodium) that older streetlights gave off. Something I was wondering would be if there would be if it would be possible to install filters on new LED lights so that they give off a cream/yellow instead of a white colour, thereby likely reducing the amount of blue light they give off? As far as I know, this could be done very easily (and probably even retrofitted to existing ones) and would cost virtually nothing extra. I believe such filters are already used in a lot of household LED bulbs.

[jnty]

I came across this article talking about a study about the negative effects of LED streetlights on human and animal health due to the fact that much of the light they emit is blue light which can decrease melatonin levels, affecting sleep patterns:

https://www.theguardian.com/environment ... mal-health

The majority of LED streetlights seem to give off white coloured light, unlike the cream-orange (high pressure sodium) or orange (low pressure sodium) that older streetlights gave off. Something I was wondering would be if there would be if it would be possible to install filters on new LED lights so that they give off a cream/yellow instead of a white colour, thereby likely reducing the amount of blue light they give off? As far as I know, this could be done very easily (and probably even retrofitted to existing ones) and would cost virtually nothing extra. I believe such filters are already used in a lot of household LED bulbs.

It would cost extra, because by definition the filter would reduce the amount of light being emitted by the lamp and require an increase in power for the same brightness (or tolerance of decreased brightness.) I understand that warmer LED lights basically work roughly in the way you suggest, filtering out undesirable wavelengths from the full selection produced, and therefore take more power to run and generally, therefore, were not chosen for streetlights.

[RJDG14]

I came across this article talking about a study about the negative effects of LED streetlights on human and animal health due to the fact that much of the light they emit is blue light which can decrease melatonin levels, affecting sleep patterns:

https://www.theguardian.com/environment ... mal-health

The majority of LED streetlights seem to give off white coloured light, unlike the cream-orange (high pressure sodium) or orange (low pressure sodium) that older streetlights gave off. Something I was wondering would be if there would be if it would be possible to install filters on new LED lights so that they give off a cream/yellow instead of a white colour, thereby likely reducing the amount of blue light they give off? As far as I know, this could be done very easily (and probably even retrofitted to existing ones) and would cost virtually nothing extra. I believe such filters are already used in a lot of household LED bulbs.

It would cost extra, because by definition the filter would reduce the amount of light being emitted by the lamp and require an increase in power for the same brightness (or tolerance of decreased brightness.) I understand that warmer LED lights basically work roughly in the way you suggest, filtering out undesirable wavelengths from the full selection produced, and therefore take more power to run and generally, therefore, were not chosen for streetlights.

According to that article, it sounds like the Isle of Wight found a way to alter the bandwidth that their LED lights use, thereby making them warmer.

[JohnnyMo]

My understanding is LEDs bulbs can be made to emit whatever light spectrum is desired. The assumption is the nearest approximation to normal daylight is best for vision. As the article points out what is best for vision may not be best for our mental health or the environment.

The atmosphere scatters blue light, which is why the sun appears yellow and the sky blue. As the sun sets the path through the atmosphere increases therefore the amount of blue light we receive decreases. Our ancestors all the way back to the first mammal and beyond have used that drop in blue light to know night is coming; prepare for sleep.

The orange (low pressure sodium) that older streetlights gave off was simply the most light that could be produced with the least power at the time and little regard to the spectrum (quality of light if you prefer).

[SteelCamel]

OK, I stand to be corrected if someone has better information. But LEDs are basically monochromatic - even more so than sodium lights. There's no such thing as a "white" LED. There's a couple of ways round that - you can mix different colours of LEDs (which colour-changing lights do) or you can use phosphors (commonly used in white "LED" lamps). A phosphor absorbs light of one colour and emits light of a different colour - so you can shine your LED on an appropriate mix of phosphors and it will change your monochromatic light into "white".

However, there's a limitation. Bluer photons have more energy than redder ones, and phosphors can only emit photons with less energy than the ones hitting them. So they can only make light redder, never bluer. Since you want some blue in the mix, the driving LED has to be blue. The phosphor then converts some of the blue to fill in the yellow part of the spectrum. Also, since the phosphor is converting photons to lower-energy ones, it's discarding some of the energy in the process. Convert most of the blue to yellow, and you'll throw away a lot more energy than is you only converted a bit of it. Look at the ratings for domestic bulbs sometime - "warm white" bulbs are noticeably less efficient than "cool white" ones. And you can't just make the LED redder, as then you won't get any blue at all and you'll have yellow-green lamps. So the most efficient "white" lamps are a rather blue-white. Orange-white ones would be significantly less efficient.

Monochrome yellow-orange like the old sodium lights would actually be even more efficient than blue-white ones - but these were considered unacceptable as the lack of colour differences made it harder to distinguish things.

[Vierwielen]

I went to a lecture on LEDs recently. It appears that a "white-light" LED is really three separate LEDs that uise different semiconductors working in parallel. ELectrical eningeers might recall that germanium diodes have a voltage drop of 0.3 V and silicon diodes have a voltage drop of 0.6 V. These diodes emit infra-red light. The approximate formula linking wavelength (in nanometres) and voltage is

lambda x V = 1234.

The value "1234" is an approximation, but it is easy to remember. Anybody who has played around with red LEDs will know that they have diodes that have a 2.2 V drop across them (approx). This results in light with a wavelength of about 600 nm (which is red light). If different material is used for the diode, the voltage drop (and hence the wavelength of the emitted light) is different.

Back to your question about filters - Using filters is counter-productive: if you are producing too much blue light, change the design of your LED rather than using energy producing blue light which you are going to reduce by using a filter.

[DB617]

I suppose the filtered light gets converted to heat, too. Now you have a system which produces waste light, and has excess cooling requirements or (more likely in the public sector) fails prematurely. It really does feel like cool temperature lights were a choice made by the industry. Was it cheaper or did they think they would be an improvement?