[doninover (OP)]

Would anyone know which coloured light is hardest to see at night from a distance?
There's a lot of conflicting information out there, so I'm finding it difficult to find an answer on this.

[Colin2B]

It isn’t really related to distance, but to the sensitivity of the 2 types of receptor in the retina. The cones are the colour receptors sensitive across the range of colours but need quite a bit of light to work, the rods are better at night - low light - but are less sensitive to the red end. So as it gets darker you will start to see the reds getting duller and at night blues are easier to see - assuming the same level of illumination.
As regards lights in the distance, it will depend on the relative level when the light reaches your eye, so a really bright red light will be easier to see than a dull blue one and vise versa, but if they are the same brightness the bluer one will be more easily seen.

If you want to know more tech detail look up Purkinje effect.

[doninover (OP)]

Thanks for that.. That's exactly the kind if information I was looking for!

[chris]

We answered a related question on the show recently: https://www.thenakedscientists.com/articles/questions/why-are-blue-lights-harder-see

[doninover (OP)]

A related question yes, but not the same question.  I believe show about blue light was in relation to detail, but not about the sensitivity to blue light.  The article you sent me initially seemed to explain it well (highlighting the cones insensitivity to red light).

[chris]

Your question: "which coloured light is hardest to see at night from a distance?"

The question answered in the link I sent you: "While walking my dog in the evenings during the holiday season this year I noticed I was unable to focus on and clearly resolve the dark blue LEDs in Christmas tree lights or fairy lights. For all the other colours I could resolve the shape and even the texture of the individual light bulbs, but with these deep royal blue LEDs all I could see was a hazy blur. Why is that?"

I struggle to see a distinction, but the point is that shorter wavelengths - like blue light - will be scattered a lot more by the air and airborne particles between you and the light source compared with longer wavelengths like red light. This is why cars put the red lights on the back, rather than white, and why driving with full beam engaged in fog makes it harder to see the road ahead compared with dipped beams, because more light is scattered back at you, reducing the contrast with the road objects ahead. The other point made in the piece I linked to is that the eye is intrinsically less sensitive to blue light and less good at resolving it because the photoreceptor density is lower, making it harder to see.

[Colin2B]

The difficulty in focussing blue light is due do imperfections in the lens of the eye (chromatic aberation) which means that different colours focus at different points on the retina.
Spatial discrimination is really important to animals with 3d vision and in humans the red & green cones take priority in determining the focus point, leaving blue to fend for itself. You can sometimes see this occuring with a point lght that contains red & blue eg some tv standby lights, the red is in focus and the blue shows as a fuzzy halo.

EDIT whoops, sorry just noticed @chris has sent you a link which should explain all

[Janus]

Dominic says:

Would anyone know which colour of light is hardest to see from a distance at night?
There's a lot of conflicting information out there so I'm wondering if any of you know the true answer to this.

What do you think?

According to this link, as light levels drop, the sensitivity to the Red end of the spectrum diminishes.
https://www.nde-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/lightresponse.htm
As light levels drop, the cones in your eyes which are responsible for your color vision and are less sensitive to light start to become unresponsive, and you become more reliant on the rods which are not able to distinguish color.  Thus as it gets darker, you start to lose your color vision and start seeing only shades of gray. 
But all in all, the light response for both cones and rods fall faster towards the red end of the spectrum. This make sense as red light is the least energetic end of the spectrum. 

[alancalverd]

It's not a simple question!

Normal daylight (photoptic) vision covers the wavelength range 450 - 750 nanometer with maximum sensitivity at 555 nm (green).

As the target light dims, the retinal cone cells (which respond to and analyse the entire spectrum) become less effective and vision is dominated by the rods. These have only a single response (no color discrimination) which peaks at a slightly shorter wavelength (500 nm, blue-green) than the photoptic peak, so we "lose" objects at the red end of spectrum. After some  20 minutes of adaptation to a dark ambient,  the overall sensitivity of the eye increases but color perception, spatial resolution and intensity resolution are degraded (scotoptic vision).

The obvious question is why we use red light to illuminate the cockpit area when sailing, flying or driving at night. The reason is that reversion from scotoptic (poor resolution, high sensitivity)  to photoptic (high resolution, low sensitivity) vision occurs very rapidly (5 seconds) on exposure to blue or white light, and at night it is more important to detect a faint object (particularly a moving one, and rods are better at detecting movement than cones)  than to analyse it.  A dim red light will not revert the eye to photoptic mode so we remain at maximum sensitivity.

Some railway signal lamps and runway markers are blue . These are easier than green to distinguish from red  under intermediate conditions, particularly for drivers with some forms of color blindness, but blue light is more dispersed by mist, so they need to be brighter - it's a difficult balancing act!

[Colin2B]

We’ve ended up with the same question in 2 parts of the forum so I’ve merged them

[Tomassci]

I think that would be blue. Because raylegh scattering . Order red because cone insensivity. (Red stars are also dimmer)

[Bored chemist]

The difficulty in focussing blue light is due do imperfections in the lens of the eye (chromatic aberation) which means that different colours focus at different points on the retina.

If the only light you are looking at is blue, the eye can focus it just fine.
Chromatic aberration is only an issue if you are trying to focus more than 1 colour at a time.

[Colin2B]

The difficulty in focussing blue light is due do imperfections in the lens of the eye (chromatic aberation) which means that different colours focus at different points on the retina.

If the only light you are looking at is blue, the eye can focus it just fine.
Chromatic aberration is only an issue if you are trying to focus more than 1 colour at a time.

That's true. If your extract of my quote was all I wrote I would have expanded, but it was just opening to 2nd part of reply.