Mars appearance?

Discussion in 'Optometry Archives' started by Repeating Decimal, Aug 13, 2003.

  1. Repeating Decimal

    danek Guest


    A window screen is a metal mesh that is put over windows here in the
    states. It keeps the bugs out. Typically they are made up of a small
    diameter wire spaced about a mm on center. Better quality screens use
    smaller gauge wire and a tighter spacing. I have also seen them made from
    fine plastic thread. Observing planets, stars and street lights through
    them makes for some interesting side effects; star bursts, double images,
    false clouds etc.

    P. Danek
    danek, Aug 21, 2003
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  2. Repeating Decimal

    Han Sibot Guest

    Ten meters is impressive. Is your light source smaller than a LED?
    When my head moves sideways, the ghost mesh maintains a fixed position in
    space. The ghost mesh is a copy of the real mesh, in the plane at distance x
    in front of the eye (x = distance where you see sharp without glasses; in my
    case 16 cm). I am able to 'touch' a ghost mesh wire in that plane with my
    finger, and when my head moves sidewards that finger has the same apparent
    motion as the ghost mesh. You could probably verify this for yourself. (It
    is more convenient to move closer to the window screen than 10 meters, as
    the ghost mesh is sharper then).

    I agree that the apparent motion of speckle is similar. It is useful to
    remember that speckles too have a fixed position in space, it is a 3
    dimensional cloud of speckles. Without my glasses the only ones that I see
    sharp are those in the plane at distance x. If the scattering surface is
    further away, the sharp speckles *appear* to move opposite to my head
    motion, compared to the background of that scattering surface. I just did
    the experiment you asked for: if the scattering surface is closer than
    distance x, the sharp speckles appear to move in the same direction as my
    head. (A small laser pointer shining at a matte light bulb created a
    suitable speckle pattern.)

    Similarly, by looking through the window screen at a very small distance,
    like 1 cm, I am able to see the ghost mesh move opposite to my head
    (apparent motion compared to the actual window).
    Han Sibot, Aug 23, 2003
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  3. Repeating Decimal

    Han Sibot Guest

    I'm sorry, at that small distance the ghost mesh appears to move in the
    *same* direction as my head. I hate discussing apparent motion because it's
    a source of confusion, I can't remember it either. It is easier to keep in
    mind that it actually has a fixed position in space.
    Han Sibot, Aug 23, 2003
  4. Repeating Decimal

    Han Sibot Guest

    Sorry, no link. IIRC, the calculation of speckle is basically the same as
    calculating the interference maxima and minima in the space in front of the
    scattering surface. Select an observation point in space, and add the light
    rays from all points of the scattering surface towards that observation
    point. This calculation results in a 3 dimensional cloud of speckles with
    fixed positions in space.
    Han Sibot, Aug 24, 2003
  5. Repeating Decimal

    Han Sibot Guest

    Supposing that my model is correct, myopic people actually see a fairly
    sharp image of the lens of the eye by looking at a LED, or Mars, without
    glasses. Nonmyopic people are able to do the same thing by holding a
    magnifying glass in front of the eye. A wire mesh in front of the eye even
    projects a nice coordinate system on the lens.

    I don't know much about cataract, but I think that it sometimes is composed
    of regional opacities, and that these opacities absorb some of the
    brightness of the light rays passing through them. I would guess that a
    person with such a cataract is able to draw a detailed map of his opacities
    with densities, simply by looking at that LED image. I assume the
    inhomogenities seen by people without cataract can be ignored compared to
    those opacities.

    Repeating Decimal, do you think you would be able to draw a map of the dark
    areas in the LED image of your cataract eye?

    BTW, opacities near the boundary (the pupil) might appear like indentations
    of the disk that give the impression of multiple images with identical
    Han Sibot, Aug 25, 2003
  6. I really do not understand your arguments. I will also limit how hard I try.

    Using a small source will provide shadow rather than images. Given
    information about transmission and lighting, it is not difficult, in
    princple, to calculate light distribution on the retina. You just have to
    know or assume properties of the cataract and the eye's optical system. That
    is not a project I am going to undertake. I will be interested in your

    Meanwhile, I think that I do understand the speckles and their distributions
    well enough for my curiosity.Bill
    Repeating Decimal, Aug 26, 2003
  7. Just because the retina is illuminated, doesn't make the light pattern an
    image. An image is formed when ALL the light emanating from a point on the
    object goes to one point at the image. There are no perfect images because
    of diffraction and aberration, but often the approximation to an image is
    good enough.

    Repeating Decimal, Aug 26, 2003
  8. Repeating Decimal

    Han Sibot Guest

    Yes, within geometrical optics. On the other hand, for an image detector
    like the retina, any light pattern is an image. Anyway, it is a good point
    that in some of the posts the 'image' on the retina could have been replaced
    by 'light pattern' on the retina.
    Han Sibot, Aug 26, 2003
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