# Why Does a Street Lamp Have a Halo?

• T@P
In summary, The conversation discusses a question about seeing a halo around street lamps from far away. The theory proposed is that it is caused by diffraction from the circular "hole" of the eye. Suggestions for testing the theory include using a circular object to decrease the size of the aperture and calculating the expected size of the halo. Other factors such as pupil size and light source may also affect the halo. Different explanations such as lens flare or eye lashes are also mentioned. Measuring the angular size of the halo is suggested, but it may be difficult due to the changing size of the halo and the distance to the lamp. It is noted that the halo is not at the lamp, but in the eye if the result is diffraction.
T@P
ok i asked the lamp halo question a while ago and i got a pretty insatisfactory response. i can't find the original post, but i have a theory as to why it happens.

and just to clarify the original question was why is it that when you look at a street lamp from far away you see like a halo ish thing around it.

my reason is like this: your eye is essentially a circular 'hole' and you look through it, so there is diffraction. and the diffracted thiny looks (in 2d) like a blob of light with rings around it that are really dim. so maybe you see the lamps like ring thingies around it? sorry if my terminology is terrible

any thoughts?

Sounds reasonable... How would you test your idea?

Your original post is here .

thanks danger...

as to testing it, well, i actually don't know ...

maybe if you put a little circular thingy in front of your eye to decrease the size of the aperture, then the result will be more obvious...?

T@P said:
maybe if you put a little circular thingy in front of your eye to decrease the size of the aperture, then the result will be more obvious...?
I've had vision problems most of my life, so I don't know if this is normal or not, but just squinting my eyes makes stroby, steaky auras around all light sources.

Testing:

You might like to see if you can calculate from theory how large a halo you should see, and see if it compares with your experimental results. You'll need some way of measuring halo size. I think the measurement you'll want is the angle between the lamp and the halo, measured at your eye.

If the size of your pupil changed, how should that affect the halo?
Can you predict the affect quantitatively as well as qualitatively?
How could you change the size of your pupil? (Drugs? Eyedrops? Extra light?)
Could you measure the size of your pupil and the size of the halo at the same time?

If you use your thumb to blot out the light source and the halo remains unchanged then it is probably caused by diffraction/reflection of water in the air as Danger pointed out.

I'm sure the lenses in your eyes, even glasses, or a window could produce a corona (halo), but by blocking the source you should be able to tell which is happening.

actually GOD___AM i did try blocking out the light source, and indeed the halo was blotted. as to putting drops in my eyes... i don't think so :) but actually danger squinting does make those funny strobe like streaks of light. i really don't know why.

as to meassuring it, well, it would be hard in ideal conditions. see by moving the size of it changes, so even if you do get a ruler up there, you would have to hold it and read off the tiny numbers from like 5 meters away. not to mention that this is a full size street lamp, and it would be strange at best if you got a ruler up there.

just eyeballing it is kinda hard, but qualitatively at least i can say for sure that as the distance to the lamp decreases, the halo decreases. which (i think) is ok with my theory. also its hard to see it when there's more light around (the particular street lamp where i noticed this is sort of isolated from the rest, but when you look towards others from far away you really don't see a halo.

edit: oh and another reason i didnt really like Danger's idea (no offense) was that by moving closer the halo changes in size, which is pretty weird if there are like 2 people near it, cause like for person a the air diffracts/refracts/whatever in a certain size and the air like b more and increases the effect or something.

The strobe like streaks of light are most likely your eye lashes if you are squinting. If you do a google search on coronas you will find a lot of pages explaining what is happening.

What you seem to be describing is a lense flair. You see these in movies sometimes when the sun hits the camera lense at the right angle. Heres a pic of one,
click They are much more dynamic and move around and change size with camera position. I'm guessing since your eye is a lot like a camera lense the same kind of effect can happen.

T@P said:
even if you do get a ruler up there, you would have to hold it and read off the tiny numbers from like 5 meters away. not to mention that this is a full size street lamp, and it would be strange at best if you got a ruler up there... ...oh and another reason i didnt really like Danger's idea (no offense)
All that you need to do to measure it, although it will be rough, is to measure the angular size (how many degrees of arc it covers). You can do this more or less by sighting along a protractor or a couple of sticks taped together at one end. Measure the distance to the light from where you're standing, then use regular trianglation math to figure out the 'actual' size.
And certainly no offense taken. I'm pretty much just thinking out loud myself, and going by things that I've experienced in the past.

T@P said:
as to meassuring it, well, it would be hard in ideal conditions. see by moving the size of it changes, so even if you do get a ruler up there, you would have to hold it and read off the tiny numbers from like 5 meters away. not to mention that this is a full size street lamp, and it would be strange at best if you got a ruler up there.

If the result is diffraction, then the angular size should be the same, and the halo is not at the lamp, it's in your eye.
You don't need to measure it at the lamp. Hold the ruler at arm's length and measure it there.

oh and another reason i didnt really like Danger's idea (no offense) was that by moving closer the halo changes in size, which is pretty weird if there are like 2 people near it, cause like for person a the air diffracts/refracts/whatever in a certain size and the air like b more and increases the effect or something.
Well, that's the way a lot of these things work. For example, you can sometimes see a halo around the shadow of your head on the sidewalk. No-one else sees it - they see halos around their own head. This could be the same sort of thing.

ok thanks for the help. qualitatively at least it seems feasable. oh and btw GOD___AM your link sends me to a forbidden page, but point well taken :)

actually, you know those funny like rings you see in movies when the camera looks at the sun? like theyre not aounrd the sun or concentric, theyre in like a string sort of diagonally across it? is that the same effect?

T@P said:
ok thanks for the help. qualitatively at least it seems feasable. oh and btw GOD___AM your link sends me to a forbidden page, but point well taken :)

actually, you know those funny like rings you see in movies when the camera looks at the sun? like theyre not aounrd the sun or concentric, theyre in like a string sort of diagonally across it? is that the same effect?

Yes that's the effect in the pic that I posted. They must have blocked it for too many hits Here's a link to the page http://www.deviantart.com/deviation/17105570/ It shows multiple rings (I'm guessing) caused by thickness of the lense? Could be a filter in front of the lense too as there seems to be a much smaller spot right above the bottom one. How far the flair is off center from the source has to do with the angle of the camera. As the sun moves to the center of the frame the flair will too.

yea that's exactly what i was thinking of. cool thanks for the pics

## 1. Why does a street lamp have a halo?

The halo around a street lamp is caused by a phenomenon known as light diffraction. When light passes through a small opening, such as the gap between the lamp and its protective casing, it spreads out and creates a halo effect.

## 2. Is the halo around a street lamp harmful?

No, the halo around a street lamp is purely a visual effect and does not pose any harm. It is a natural occurrence caused by the way light behaves when passing through small openings.

## 3. Why do some street lamps have a larger halo than others?

The size of the halo is dependent on the size of the gap between the lamp and its casing. If the gap is larger, the halo will be larger. Additionally, the type of light bulb used in the street lamp can also affect the size of the halo.

## 4. Can the halo around a street lamp be controlled or eliminated?

Yes, the halo effect can be minimized by using a smaller gap between the lamp and its casing or by using a different type of light bulb. However, it cannot be completely eliminated as it is a natural occurrence caused by light diffraction.

## 5. Does the halo around a street lamp affect its brightness?

No, the halo does not affect the brightness of the street lamp. It is simply a visual effect caused by the way light behaves when passing through small openings and does not impact the actual brightness of the lamp.

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