Optical Centre / Geometric Centre

[hickny]

Hi ALL,
I was wondering if there was a difference between the Optical Centre of a parabolic mirror ie the mathematiocal term is focus and the geometric centre ie the physical centre of the mirror.

Are they the same? Can they be different? How would I tell?
Thanks in advance.

[Wavytone]

Peter I wouldn't use either term - they're both open to interpretation - that you're even asking confirms this. 'Optical centre' is a bit meaningless. 'Geometric centre' or just plain 'centre' could just as easily mean the centre of the mirror surface, right where many place a collimating dot.

If you mean the focus, that is the correct word to use. If you mean the "centre of curvature", ie located at 2F in front of the surface, that is also unambiguous.

Note that 'focus' is strictly only valid in the context of a parabolic mirror, and it is also valid in the context of a complete lens. In the case of a mirror that is some other shape (spherical, ellipsoidal, hyperboloidal) 'focus' should be avoided; radius of curvature is more often used by the optical designer or mirror maker. Similarly for individual surfaces on a lens, the optical designer calculates (and specifies) the radii and this is also what the optician will measure when making one.



Hope this helps !

[blink138]

the optical centre and the geometric centre can mean two different things actually
the geometric centre is the physical central point of a lens...... however if you
add prism to an optical lens or mirror, that is to have one side of the glass thicker than the opposite side, or "wedge" it, then you have actually moved the optical centre of the system while the geometric centre remains exactly in the middle
so the optical centre and the geometric centre are not in the same spot
pat

[bojan]

Optical centre of the (optical) system in general is the theoretical point, that can be defined as the point on the optical axis of the system, which is exactly a focal length away from focus point, towards the lens.

So, according to that definition, for parabolic mirror, optical centre and the centre of the mirror (where point where collimating dot is placed) are the same.

[Wavytone]

'Optical centre' has two other meanings, altogether.

In optometry it refers to the spot on a lens where the front and back surfaces are plane parallel. When the lens is trimmed to fit a frame, the lens is arranged so that this point coincides with the wearers line of vision when looking straight ahead.

In optical manufacturing it also refers to the same spot on a lens where the surfaces are plane parallel. 'Optical centring' refers to a process where a lens is mounted on a spindle using soft pitch, and as it rotates slowly, the lens is shifted laterally to place the optical centre on the axis of rotation. The lens is then 'edged', ie the edge ground off, the result being a lens that has no wedge. This is used in the manufacture of objective, eyepiece and camera lenses. Optical centring is also used to align the elements before the are cemented.

The optical centre of a lens can be seen while it is rotating on a spindle by shining a light at it, looking down from above. If it is off the axis of the spindle, the spot will wobble from side to side visibly.

Hence as I indicated earlier the term has different meanings according to the context; if you really mean the focus or centre of curvature, better to refer to them as such.