Seeing Scan Lines

[nikokapo]

I have noticed that I can clearly see scan lines on a CRT TV if I use my peripheral vision. If I focus onto the screen directly (fovea 100% ) then I can just watch TV as usual with no noticeable scan lines but when I turn my head 80° to the right/left and try to see the screen, the refresh rate is very clear.

Does anybody know why would that be?
Could it be because of the disposition of cones and rods in the eye?

 

[millirad]

Well, no to put it simply. This is what is known as Chuck Norris disease. It afflicts one in a million fans of Chuck Norris. The cure is very easy really, as you must "chuck" a brick at the crt and you will see that the lines vanish instantly. Always wear appropriate laser safety goggles whilst attempting this procedure. And just as important if you see any diagonal lines in this post you must immediately throw a brick at you monitor, because Chuck Norris disease is nearly always fatal when seen on your computer screen. ///////// \\\\\\\ //////// \\\\\\\\

 

[nikokapo]

Well, no to put it simply. This is what is known as Chuck Norris disease. It afflicts one in a million fans of Chuck Norris. The cure is very easy really, as you must "chuck" a brick at the crt and you will see that the lines vanish instantly. Always wear appropriate laser safety goggles whilst attempting this procedure. And just as important if you see any diagonal lines in this post you must immediately throw a brick at you monitor, because Chuck Norris disease is nearly always fatal when seen on your computer screen. ///////// \\\\\\\ //////// \\\\\\\\

I forgot to mention I have the seventh sense, I see broken screens as if they were actually displaying what they should.

Jokes apart, does anybody know the reason of what I said in the OP?

 

[HIMNL9]

@ millirad: ROTFLMAO !!

seriously, it's a common thing, due to the structure of our retinal element ..... peripherial elements of the retinal matrix are much more sensitive to the light than central ones, that instead are less sensitive to the light, but more sensitive to the "contrast" and colors .....

let me try to explain better ..... retina is covered from 2 types of "sensors", call them this way, called cones and rods ..... cones are sensitive to the colors, where rods, instead, are sensitive only to the light level (or, better said, to the light level with a peak at 498nm) but not to the spectral composition, and are more sensitive of the cones ..... also, there are something like 110 or 120 millions of rods, distributed in a grid, where instead there are just approximatively 7 millions of cones, concentrated in the central area (the one where you see better)

Your brain, also, take part in that, cause when you look with the central area, it "compensate" the image, and also, close or open the iris as diaphragm, for limit the intensity ..... if instead you look with your "lateral vision field", the light hit just the rods, they cannot give you a clear image, and also there's less concentration of them in the peripherial area of the retina, but are more sensitive to the different levels of the light, and also more quick in resopnse, also if they work only as a b/w camera ..... this make your peripherial vision area more sensitive to low levels and quick fluctuations

It's also an old trick from astronomy, that one to use peripherial vision for see stars that you can't see looking them straight ..... looking at a lateral area, respect from where you think there's a weak star, if there's one, you see it's light with the peripherial zone, much better than with the central zone

Hope it's clear, you know my english is not so good, sorry.


Edit: there is also another thing to considerate, sorry i remembered it only now ..... from laterally, the light is passing the crystaline (lens inside the eye) in diagonal path ..... it act as a more magnificating lens, in this way (never tried to look through a thick glass lens straight, and then roll it diagonally ? ..... when you do it, it ofcourse distort, but also increase the magnification )

 

[nikokapo]

@ millirad: ROTFLMAO !!

seriously, it's a common thing, due to the structure of our retinal element ..... peripherial elements of the retinal matrix are much more sensitive to the light than central ones, that instead are less sensitive to the light, but more sensitive to the "contrast" and colors .....

let me try to explain better ..... retina is covered from 2 types of "sensors", call them this way, called cones and rods ..... cones are sensitive to the colors, where rods, instead, are sensitive only to the light level (or, better said, to the light level with a peak at 498nm) but not to the spectral composition, and are more sensitive of the cones ..... also, there are something like 110 or 120 millions of rods, distributed in a grid, where instead there are just approximatively 7 millions of cones, concentrated in the central area (the one where you see better)

Your brain, also, take part in that, cause when you look with the central area, it "compensate" the image, and also, close or open the iris as diaphragm, for limit the intensity ..... if instead you look with your "lateral vision field", the light hit just the rods, they cannot give you a clear image, and also there's less concentration of them in the peripherial area of the retina, but are more sensitive to the different levels of the light, and also more quick in resopnse, also if they work only as a b/w camera ..... this make your peripherial vision area more sensitive to low levels and quick fluctuations

It's also an old trick from astronomy, that one to use peripherial vision for see stars that you can't see looking them straight ..... looking at a lateral area, respect from where you think there's a weak star, if there's one, you see it's light with the peripherial zone, much better than with the central zone

Hope it's clear, you know my english is not so good, sorry.

Ok, but I don't get why rods are "faster" than cones in a way that I can clearly see scan lines. Or is it just because I'm not seeing colours with my peripheral vision and only contrast?

(Thanks for the answer, man).

 

[lasersbee]

Correct me if I'm wrong but... 90 degrees would have you facing to the
right or left of the screen...
Wouldn't 160 degrees be close to 180 which would almost put your back
facing the screen...:thinking:

Unless you have eyes like a gecko.....:crackup:

At night when I want to see a faint light far away... I need to turn my head
to be able to see it out of the corner of my eyes... not looking straight on...
I thought it was due to the fact that I don't have the eyes of a 20 year old
anymore...


Jerry

 

[nikokapo]

Correct me if I'm wrong but... 90 degrees would have you facing to the
right or left of the screen...
Wouldn't 160 degrees be close to 180 which would almost put your back
facing the screen...:thinking:

Unless you have eyes like a gecko.....:crackup:

At night when I want to see a faint light far away... I need to turn my head
to be able to see it out of the corner of my eyes... not looking straight on...
I thought it was due to the fact that I don't have the eyes of a 20 year old
anymore...


Jerry

You're right, sorry. Too much R^3 graphs last week
I'm correcting it right now.

 

[lamborgini8]

Already been answered above but just thought I would add my bit.

I am the same as you, if I look directly at a CRT screen it seams fine, yet if I look say 1ft to the side of it for example I can see the screen flickering as its refreshing. I can see it on some old flat screens as well...

Slightly different concept but if I am in a dark room and wave my phone about with the screen displaying something with the backlight I can see multiple 'images' which I am guessing is where the screen is refreshing.

 

[lasersbee]

If you could slow down the scan rate of a CRT... you would only see one small
"dot" (monochrome screens) that scans the entire screen...
Because of persistence of vision and the phosphor coating on the inside of the CRT
screen we see a full image while the electron beam "dot" scans the screen...

Jerry

 

[HIMNL9]

Ok, but I don't get why rods are "faster" than cones in a way that I can clearly see scan lines. Or is it just because I'm not seeing colours with my peripheral vision and only contrast?

Do you mean you see the flickering of the scanlines, or that you see the lines of the screen more clearly ?

If for the flickering, the eye usually have approximatively 1/20 of second of "persistence" that can change a bit from person to person, and also change for a disease (that i actually don't remember the name, sorry), and this is the reason for which you can see images in monitors, tv and cinema, cause the persistence of the cones (if we had no persistence, all that what we can see on a CRT was a point that shift quickly, and at cinema, a series of still frames like strobo images, as example) ..... rods have a different persistence, shorter, so you can notice the flickering more intense, if the light hit only them, and the brain don't compensate with the more high persistence of the cones.

At night when I want to see a faint light far away... I need to turn my head
to be able to see it out of the corner of my eyes... not looking straight on...
I thought it was due to the fact that I don't have the eyes of a 20 year old
anymore...

That is the trick that i was referring before ..... no, is not cause you don't have the eyes of a 20 years old , it work also with me that i'm 47 ..... but was working the same way when i was 20 too

 

[charliebruce]

In the dark and/or at the edge of my vision, I can also notice CCFL lamp pulses, CRT scan lines and pulsed-light devices up to around a few hundred hz. I'm no expert, but I don't believe that rods have the same persistence of vision that the central section of your vision does because they are responding to fewer photons. Interestingly, the human eye can respond to a single photon, but not unless there's a set number of them within 100ms of each other.

This page may be interesting: The Human Eye and Single Photons

Edit: POV was mentioned above, beat me to it :yabbem: And I do the same whenever I'm looking for stars (or high-flying aeroplanes to avoid when using a greenie).

 

[nikokapo]

rods have a different persistence, shorter, so you can notice the flickering more intense,

This was the answer I was looking for, thanks a lot man !

 

[millirad]

I'm glad someone knew the answer for you Niko, unlike myself. I didn't have a clue.