# Optics of perceived shadows on LCD monitor

After changing my desktop wallpaper on my LED-lit LCD monitor to one with a dark corner, I started seeing shadows on where the clock widget is while looking sideways at the screen. These shadows move with the turn of my head and disappear when I look straight. My question has two parts:

1. Is this normal with LCD screens or is it a result of the fact that I wear very thick glasses?
2. Does this effect have a name, and where can I read more about it?

A diagram illustrating light propagation would be very much appreciated.

Below is the image in question. I see dark blue shadows of the big numbers while looking sideways at them

• Do you see similar things if you printed a screen shot and looked at that? Thick glasses can certainly give rise to ghosting - especially when you are viewing at an angle. Hard to know what is going on without more experiments on your part. Commented Aug 21, 2015 at 16:26
• @Floris I don't see shadows on the paper printout, but it happens on the phone screen if I'm holding it far enough. Commented Aug 21, 2015 at 16:32
• What is the distance between the number and its shadow? Is there a relationship between the direction of the shadow and the position of the screen relative to your eye (for example: "when it is to my left the shadow appears to the right, shifted about twice the thickness of the line"). Does the spacing of the shadow depend on the distance? Commented Aug 21, 2015 at 17:03
• @Floris The shadow is the same size as the digit, the distance between them increases as I move away from the screen. It is shifted around twice the thickness of the line when I am around one arm's length away, but that also varies depending on the angle I'm looking at it. Directionwise, if my head is turned to the left, the shadows are on the left of the numbers, they move to the right when I turn in that direction Commented Aug 21, 2015 at 17:16
• Could be chromatic aberration from your glasses. If you use a drawing program to draw a thin purple circle on a black background and look at it from an angle, does the circle split into a red and (faint) blue circles slight offset from each other? Might have to play with the color a bit for maximum effect. This happens to me with my glasses and I see a bit of the effect in your screenshot. Commented Aug 23, 2015 at 3:35

Lenses can exhibit chromatic aberration due to a color (wavelength) dependent index of refraction in the lens material. Chromatic aberration can cause the three colors from the red-green-blue (RGB) display to separate when viewed off the lens axis:

Left: on axis. Right: off axis.

In less pronounced cases, this might lead to a ghosting of images. Definitely causes me problems when I word in CAD programs and don't view the screen straight on.

• Well done. I had not paid attention to two critical words in the question: "I see dark blue shadows..." Commented Aug 23, 2015 at 14:02

I am hypothesizing based on the information you gave in the question and clarifying comments.

The fact that the distance between the "shadow" (or ghost) and the number increases with distance tells me that the shadow is produced by your eye+glasses, rather than the screen itself. Most likely, you are seeing reflections from the front/back surface of your glasses: these reflections would overlap with the image when you are looking straight at the source, and shift as you are looking from the side.

If that is correct, you would see a similar thing with street lights at night.

The reason you see this with the screen and not a piece of paper may be due to the contrast ratio - "light" on the screen vs "black" can be quite a large ratio, so even a faint "echo" of the number will be visible. In particular, the screen is usually brighter than other objects in the room (it has to be, in order to see things properly). By contrast, a print out may just not have the same contrast - and definitely doesn't usually have the brightness, since it is illuminated by the rest of the environment. Thus, the ghost would be much dimmer for the paper than for the screen.

• Seems plausible. Yes, I do see two reflections of point light sources on my glasses so frequently that I'm used to it, but it's the first time I notice it on a screen and with so much clarity. I should have linked the two. Thank you Commented Aug 21, 2015 at 18:26