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Medical Forum / General / Vision / April 2005Variable amounts of prism in glasses from off centre eye movements.
Hi Can anybody throw any light on this one? I thought that modern glasses were corrected for prism effects but I have noticed that I am getting quite noticable amounts of what seems to be lateral chromatic abberration in opposite directions when I move my head to the left or right. These links might give you some idea what i am talking about. http://www.binoscope.co.nz/3d.htm http://www1.physik.tu-muenchen.de/~cucke/ftp/lectures/netduis.pdf It is normally hard to notice lateral chromatic abberration but longitudinal chromatic abberration is easy to detect with a Cobalt filter or mauve coloured filter and is used in sight testing for the red green test. I was using the cobalt filter and seeing a central red dot and blue outer circle and finding that when i move my head the red dot moves to the left side or right side of the blue circle. Since there is supposedly a 2 diopter difference between red and blue in the human eye it seems I am getting a significant prism effect to move all of the red dot to the outer edge of the blue circle. I get no change at all if i dont wear glasses. The blurred red circle stays exactly in the middle of the blurred blue for all head movements. Kind of fascinating but I dont like the sound of having built in prisms. Andrew All lens materials have chromatic aberration. Polycarbonate has the most. In general, chromatic aberration seems to increase with the index of refraction. Despite the fact that the index of polycarbonate isn't all that high, it does seem to be the worst offender. DrG It should also be mentioned that any ophthalmic lens can be thought of as two prisms -- either apex-to-apex in the case of minus lenses, or base-to-base in the case of plus lenses. This is why decentration produces image displacement. Also, any refractive medium causes a change in the velocity of light, with shorter wavelengths slowed more than longer wavelengths. DrG > It should also be mentioned that any ophthalmic lens can be thought of > as two prisms -- either apex-to-apex in the case of minus lenses, or [quoted text clipped - 4 lines] > > DrG While true in a first order approximation, such a description is not really very good. The lenses can be represented more accurately with a polyconic surface. A polyconic surfaces are used by the USGS for projecting maps at different latitudes. An even better approach is an expansion in Zernike functions which start out with piston, x-tilt, y-tilt (the prism values you guys use), followed by sphere (+/- lens), followed by xy and x^2-y^2 astigmatism. These are not your usual forms for astimatism used in prescriptions. Then come higher order aberrations. Bill > I thought that modern glasses were corrected for prism effects but I There are prism effects in all glasses. Opthalmic Optics 101. Some materials have more problems with this than others, most notably polycarbonate. The more off-center the line of site is, the greater the problem is. > Since there is supposedly a 2 diopter difference between red and blue > in the human eye Seems a little high to me. As one who uses bichrome often, I think the difference is more on the order of .5 D. I ususally find that adding -.25 to the neutral (balanced) finding, shifts preference to the green while adding +.25 to neutrality shifts preference to the red backgound. w.stacy,. o.d. >>Since there is supposedly a 2 diopter difference between red and blue >>in the human eye > > Seems a little high to me. William Stacey Wrote >>As one who uses bichrome often, I think the difference is more on the order of .5 D. Possibly for red to green only that is true. Using pure colour diodes red to green difference is 105nm. Red to blue is twice that at 230NM. Red to absolute far violet visible light is 260nm. I have never seen a violet diode but i have been told that the light refracts so strongly the human eye cannot focus on them clearly. When viewing a pure red green blue diode array my tests show that all people view blue as a starburst effect at best and often much worse than that. Strangle everybody sees the same array with binoculars or up close as completely in focus. >> Since there is supposedly a 2 diopter difference between red and blue >> in the human eye >Seems a little high to me. Seems as if the red to blue shift is in the order of .75D "If you plot the focal length of a simple lens as a function of wavelength, across the visible spectrum, you will find that the difference between the minimum focal length (for blue) and the maximum (red) is about one and a half percent of the average focal length. Thus, a simple lens of nominal focal length 1000 mm might have a focal length in the red of 1007.5 mm and in the blue of 992.5 mm." http://www.maa.mhn.de/Scholar/chromatic_aberration.html so 17mm * 100.75/100 = 17.1275 17mm * 99.25/100 = 16.875 With focal length directly proportional to Dioptric power of eye: 17.1275/.25 = 68.5 so 58D/68.5D difference = .84D > With focal length directly proportional to Dioptric power of eye: > > 17.1275/.25 = 68.5 so 58D/68.5D difference = .84D That's more consistent with experience. I also read the "2 diopter" value recently and thought it was overstated. -MT >I also read the "2 diopter" value recently and thought it was overstated. In fact 2 diopters is correct for the entire spectrum. Refs below. If you look thru a cobalt filter at a distant white light you can see there is a significant difference for red blue, but subjectively there is little difference in red green refraction compared to red blue refraction for an array of red green and blue pure color diodes. I have heard that Pure violet diodes are impossible to focus upon. Presumably its meant that only at reading distances do they become clearly focused. It seems (from other references) that we see clarity in all this blurr because boundaries in colour are detected as interpreted as sharp edge by retinal neural processing in the same way that unfocused grids appear to be more sharply focused than irregular patterns. http://research.opt.indiana.edu/Library/AchromatizingEye/AchromatizingEye.html "an eye correctly refracted for long wavelengths (700nm) will be myopic by approximately 2 diopters for short wavelengths (400nm) 3, 4, 5. Although this usually goes un-noticed in everyday situations, the eye's chromatic difference in refractive error (CDRx) is readily observable and exploited in the standard optometric red/green bi-chrome refraction test 6 which provides approximately 0.5 diopters difference between the typical red and green colors 7." Refs 4. Bedford RE, Wyszecki G. Axial chromatic aberration of the human eye. J. Opt. Soc. Am. 1957; 47: 564-565. 5. Howarth PA, Bradley A. The longitudinal chromatic aberration of the human eye, and its correction. Vision Res. 1986; 26: 361-366. Andrew > In fact 2 diopters is correct for the entire spectrum. For the entire "visible" spectrum this is true. When ''white'' light is used to project an object at the retina this "bandwith" is also used in adjusting the needed amount of accommodation. By focussing the eye to his focal point (no accommodation) the arrays of the red light (685 nm) are focussed sharp on the retina, by increasement in accommodation, the eye uses his chromatic aberation and use the more shorter wavelengts to focus sharp on the retina. > If you look thru a cobalt filter at a distant white light you can see > there is a significant difference for red blue, but subjectively there [quoted text clipped - 3 lines] > Presumably its meant that only at reading distances do they become > clearly focused. See the above  SignatureJan (normally Dutch spoken) >Hi > [quoted text clipped - 4 lines] >be lateral chromatic abberration in opposite directions when I move my >head to the left or right. Transverse, or Lateral chromatic aberration, which is a measure of the difference in prismatic deviation between the blue and red ends of the color spectrum, is calculated by LC = P / v where P is the prism of the lens, and v is the Abbe number. The prism is the distance in centimeters from the lens optical center multiplied by the lens power in diopters. The Abbe number of cr39 plastic is 57.8, polycarbonate is 30.0, and MR6 (1.6 index) is 37.0 (From "Spectral Transmittance of Lens Materials by Daniel Torgersen). According to Torgersen, amounts of LC over .16 prism diopters will reduce acuity by one line on the Snellen chart. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "An expert is a person who has made all the mistakes that can be made in a very narrow field." - Niels Bohr |
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