Abstract

Analysis of the optics of photorefractively computed ray tracing shows that, for short camera-to-subject distances, the function relating image size to defocus of the eye is not symmetrical for errors of focus in front of and behind the camera. This asymmetry is exploited in the new method of isotropic photorefraction, in which the supplementary cylinder lenses of the original orthogonal photorefractors are replaced by defocusing of the camera lens itself. By comparing photographs taken with the camera focused in front of and behind the subject, the sign of the eyes’ defocus (myopic or hyperopic relative to the camera) can be determined. The axis of any astigmatism is readily apparent as the direction in which the photorefractive images are elongated. The method is well adapted for the refractive screening of infants and young children.

© 1983 Optical Society of America

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References

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  1. H. C. Howland and B. Howland, “Photorefraction: a technique for study of refractive state at a distance,” J. Opt. Soc. Am. 64, 240–249 (1974).
    [CrossRef] [PubMed]
  2. O. Braddick, J. Atkinson, J. French, and H. C. Howland, “A photorefractive study of infant accommodation,” Vision Res. 19, 1319–1330 (1979).
    [CrossRef] [PubMed]
  3. We refer to instances when the subject focused at a closer distance than that of the photorefractor as myopic errors and to instances of focusing further than the photorefractor as hyperopic errors. These do not necessarily imply that with relaxed accommodation the subject would be myopic or hyperopic, respectively.
  4. This theoretical treatment ignores the influence of aberrations of the eye. Of these, longitudinal chromatic aberration results in the formation of red and blue color fringes around the photorefractive images (see Fig. 10). Spherical aberration and coma may cause some increase in the size of the photorefractive images, but it may be estimated that in most eyes this represents less than 0.1-D equivalent defocus.
  5. J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).
  6. J. Atkinson and O. J. Braddick, “The use of isotropic photorefraction for vision screening in infants,” Acta Ophthalmol. Suppl. 157, 36–46 (1983).
    [PubMed]
  7. J. Atkinson, O. J. Braddick, K. Durden, P. G. Watson, and S. Atkinson, “Screening of 6–9 month old infants for refractive errors using photorefraction,” Br. J. Ophthalmol. (to be published).
  8. J. Atkinson and O. Braddick, “Vision screening: the relation of refractive error to strabismus and amblyopia,” Behav. Brain Res. (to be published).
  9. V. Dobson, H. C. Howland, C. Moss, and M. S. Banks, “Photorefraction of normal and astigmatic infants during viewing of patterned stimuli,” Vision Res. (to be published).
  10. H. Howland and N. Sayles, “Photorefractive measurements of astigmatism in infants and young children,” Invest. Ophthalmol. (to be published).
  11. H. C. Howland, “Infant eyes: optics and accommodation,” Curr. Eye Res. 2, 217–224 (1983).
    [CrossRef]
  12. H. C. Howland and N. Sayles, Photorefractive studies of normal and handicapped infants and children,” Behav. Brain Res. (to be published).
  13. J. Atkinson, “Assessment of vision in infants and young children,” in Proceedings of the Second International Workshop on the “At Risk” Infant (Brooks, Bakersfield, Calif., to be published).
  14. K. Kaakinen, “A simple method for screening of children with strabismus, anisometropia or ametropia by simultaneous photography of the corneal and fundus reflexes,” Acta Ophthalmol. 57, 161–171 (1979).
    [CrossRef]
  15. K. Kaakinen, “Simultaneous two flash static photoskiascopy,” Acta Ophthalmol. 59, 378–386, 1981.
    [CrossRef]
  16. H. C. Howland, “The optics of photographic skiascopy,” Acta Ophthalmol. 58, 221–227 (1980).
    [CrossRef]

1983 (2)

J. Atkinson and O. J. Braddick, “The use of isotropic photorefraction for vision screening in infants,” Acta Ophthalmol. Suppl. 157, 36–46 (1983).
[PubMed]

H. C. Howland, “Infant eyes: optics and accommodation,” Curr. Eye Res. 2, 217–224 (1983).
[CrossRef]

1981 (2)

K. Kaakinen, “Simultaneous two flash static photoskiascopy,” Acta Ophthalmol. 59, 378–386, 1981.
[CrossRef]

J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).

1980 (1)

H. C. Howland, “The optics of photographic skiascopy,” Acta Ophthalmol. 58, 221–227 (1980).
[CrossRef]

1979 (2)

K. Kaakinen, “A simple method for screening of children with strabismus, anisometropia or ametropia by simultaneous photography of the corneal and fundus reflexes,” Acta Ophthalmol. 57, 161–171 (1979).
[CrossRef]

O. Braddick, J. Atkinson, J. French, and H. C. Howland, “A photorefractive study of infant accommodation,” Vision Res. 19, 1319–1330 (1979).
[CrossRef] [PubMed]

1974 (1)

Atkinson, J.

J. Atkinson and O. J. Braddick, “The use of isotropic photorefraction for vision screening in infants,” Acta Ophthalmol. Suppl. 157, 36–46 (1983).
[PubMed]

J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).

O. Braddick, J. Atkinson, J. French, and H. C. Howland, “A photorefractive study of infant accommodation,” Vision Res. 19, 1319–1330 (1979).
[CrossRef] [PubMed]

J. Atkinson, O. J. Braddick, K. Durden, P. G. Watson, and S. Atkinson, “Screening of 6–9 month old infants for refractive errors using photorefraction,” Br. J. Ophthalmol. (to be published).

J. Atkinson and O. Braddick, “Vision screening: the relation of refractive error to strabismus and amblyopia,” Behav. Brain Res. (to be published).

J. Atkinson, “Assessment of vision in infants and young children,” in Proceedings of the Second International Workshop on the “At Risk” Infant (Brooks, Bakersfield, Calif., to be published).

Atkinson, S.

J. Atkinson, O. J. Braddick, K. Durden, P. G. Watson, and S. Atkinson, “Screening of 6–9 month old infants for refractive errors using photorefraction,” Br. J. Ophthalmol. (to be published).

Ayling, L.

J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).

Banks, M. S.

V. Dobson, H. C. Howland, C. Moss, and M. S. Banks, “Photorefraction of normal and astigmatic infants during viewing of patterned stimuli,” Vision Res. (to be published).

Braddick, O.

J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).

O. Braddick, J. Atkinson, J. French, and H. C. Howland, “A photorefractive study of infant accommodation,” Vision Res. 19, 1319–1330 (1979).
[CrossRef] [PubMed]

J. Atkinson and O. Braddick, “Vision screening: the relation of refractive error to strabismus and amblyopia,” Behav. Brain Res. (to be published).

Braddick, O. J.

J. Atkinson and O. J. Braddick, “The use of isotropic photorefraction for vision screening in infants,” Acta Ophthalmol. Suppl. 157, 36–46 (1983).
[PubMed]

J. Atkinson, O. J. Braddick, K. Durden, P. G. Watson, and S. Atkinson, “Screening of 6–9 month old infants for refractive errors using photorefraction,” Br. J. Ophthalmol. (to be published).

Dobson, V.

V. Dobson, H. C. Howland, C. Moss, and M. S. Banks, “Photorefraction of normal and astigmatic infants during viewing of patterned stimuli,” Vision Res. (to be published).

Durden, K.

J. Atkinson, O. J. Braddick, K. Durden, P. G. Watson, and S. Atkinson, “Screening of 6–9 month old infants for refractive errors using photorefraction,” Br. J. Ophthalmol. (to be published).

French, J.

O. Braddick, J. Atkinson, J. French, and H. C. Howland, “A photorefractive study of infant accommodation,” Vision Res. 19, 1319–1330 (1979).
[CrossRef] [PubMed]

Howland, B.

Howland, H.

H. Howland and N. Sayles, “Photorefractive measurements of astigmatism in infants and young children,” Invest. Ophthalmol. (to be published).

Howland, H. C.

H. C. Howland, “Infant eyes: optics and accommodation,” Curr. Eye Res. 2, 217–224 (1983).
[CrossRef]

J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).

H. C. Howland, “The optics of photographic skiascopy,” Acta Ophthalmol. 58, 221–227 (1980).
[CrossRef]

O. Braddick, J. Atkinson, J. French, and H. C. Howland, “A photorefractive study of infant accommodation,” Vision Res. 19, 1319–1330 (1979).
[CrossRef] [PubMed]

H. C. Howland and B. Howland, “Photorefraction: a technique for study of refractive state at a distance,” J. Opt. Soc. Am. 64, 240–249 (1974).
[CrossRef] [PubMed]

H. C. Howland and N. Sayles, Photorefractive studies of normal and handicapped infants and children,” Behav. Brain Res. (to be published).

V. Dobson, H. C. Howland, C. Moss, and M. S. Banks, “Photorefraction of normal and astigmatic infants during viewing of patterned stimuli,” Vision Res. (to be published).

Ingram, R. M.

J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).

Kaakinen, K.

K. Kaakinen, “Simultaneous two flash static photoskiascopy,” Acta Ophthalmol. 59, 378–386, 1981.
[CrossRef]

K. Kaakinen, “A simple method for screening of children with strabismus, anisometropia or ametropia by simultaneous photography of the corneal and fundus reflexes,” Acta Ophthalmol. 57, 161–171 (1979).
[CrossRef]

Moss, C.

V. Dobson, H. C. Howland, C. Moss, and M. S. Banks, “Photorefraction of normal and astigmatic infants during viewing of patterned stimuli,” Vision Res. (to be published).

Pimm-Smith, E.

J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).

Sayles, N.

H. C. Howland and N. Sayles, Photorefractive studies of normal and handicapped infants and children,” Behav. Brain Res. (to be published).

H. Howland and N. Sayles, “Photorefractive measurements of astigmatism in infants and young children,” Invest. Ophthalmol. (to be published).

Watson, P. G.

J. Atkinson, O. J. Braddick, K. Durden, P. G. Watson, and S. Atkinson, “Screening of 6–9 month old infants for refractive errors using photorefraction,” Br. J. Ophthalmol. (to be published).

Acta Ophthalmol. (3)

K. Kaakinen, “A simple method for screening of children with strabismus, anisometropia or ametropia by simultaneous photography of the corneal and fundus reflexes,” Acta Ophthalmol. 57, 161–171 (1979).
[CrossRef]

K. Kaakinen, “Simultaneous two flash static photoskiascopy,” Acta Ophthalmol. 59, 378–386, 1981.
[CrossRef]

H. C. Howland, “The optics of photographic skiascopy,” Acta Ophthalmol. 58, 221–227 (1980).
[CrossRef]

Acta Ophthalmol. Suppl. (1)

J. Atkinson and O. J. Braddick, “The use of isotropic photorefraction for vision screening in infants,” Acta Ophthalmol. Suppl. 157, 36–46 (1983).
[PubMed]

Curr. Eye Res. (1)

H. C. Howland, “Infant eyes: optics and accommodation,” Curr. Eye Res. 2, 217–224 (1983).
[CrossRef]

Doc. Ophthalmol. (1)

J. Atkinson, O. Braddick, L. Ayling, E. Pimm-Smith, H. C. Howland, and R. M. Ingram, “Isotropic photorefraction: a new method for photorefractive testing of infants,” Doc. Ophthalmol. 30, 217–223 (1981).

J. Opt. Soc. Am. (1)

Vision Res. (1)

O. Braddick, J. Atkinson, J. French, and H. C. Howland, “A photorefractive study of infant accommodation,” Vision Res. 19, 1319–1330 (1979).
[CrossRef] [PubMed]

Other (8)

We refer to instances when the subject focused at a closer distance than that of the photorefractor as myopic errors and to instances of focusing further than the photorefractor as hyperopic errors. These do not necessarily imply that with relaxed accommodation the subject would be myopic or hyperopic, respectively.

This theoretical treatment ignores the influence of aberrations of the eye. Of these, longitudinal chromatic aberration results in the formation of red and blue color fringes around the photorefractive images (see Fig. 10). Spherical aberration and coma may cause some increase in the size of the photorefractive images, but it may be estimated that in most eyes this represents less than 0.1-D equivalent defocus.

J. Atkinson, O. J. Braddick, K. Durden, P. G. Watson, and S. Atkinson, “Screening of 6–9 month old infants for refractive errors using photorefraction,” Br. J. Ophthalmol. (to be published).

J. Atkinson and O. Braddick, “Vision screening: the relation of refractive error to strabismus and amblyopia,” Behav. Brain Res. (to be published).

V. Dobson, H. C. Howland, C. Moss, and M. S. Banks, “Photorefraction of normal and astigmatic infants during viewing of patterned stimuli,” Vision Res. (to be published).

H. Howland and N. Sayles, “Photorefractive measurements of astigmatism in infants and young children,” Invest. Ophthalmol. (to be published).

H. C. Howland and N. Sayles, Photorefractive studies of normal and handicapped infants and children,” Behav. Brain Res. (to be published).

J. Atkinson, “Assessment of vision in infants and young children,” in Proceedings of the Second International Workshop on the “At Risk” Infant (Brooks, Bakersfield, Calif., to be published).

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Figures (11)

Fig. 1
Fig. 1

Path of an individual ray through the optics of the eye and photorefractor.

Fig. 2
Fig. 2

Examples of functions relating the image size at the film plane to the distance at which the subject’s eye is focused. Solid line: values obtained from computer ray tracing as described in text. Dashed line: linear function given by Eq. (A3) of Ref. 1. (a) Functions for the +1.5-D orthogonal photorefractor at a camera-to-subject distance of 1.5 m, 6-mm eye pupil. (b) Functions for the +1.5-D orthogonal photorefractor at a camera-to-subject distance of 0.75 m, 6-mm eye pupil. The vertical dotted–dashed line shows the position of the fiber-optic source. (The camera-to-subject distances stated refer to the film plane of the camera; the fiber-optic source is 11 cm closer to the subject.)

Fig. 3
Fig. 3

Functions relating image size at the film plane to the distance at which the subject’s eye is focused, obtained by computer ray tracing with various simplifying assumptions (curve A–D) and from Eq. (A3) of Ref. 1 (curve E). Functions are calculated for the +1.5-D orthogonal photorefractor at a camera-to-subject distance of 0.75 m; eye pupil diameter of 6 mm.

Fig. 4
Fig. 4

Paths of three rays through the eye and camera optics in photorefraction, for various positions of focus of the eye. One of the three rays A, B, C gives the extreme point of the image on the film for any focus of the eye. In all four diagrams the camera is focused on the same point O between eye and camera, and I indicates the image of the source in the eye. (a) The eye is focused on a point E closer than the camera focus O; F is conjugate with this point within the camera; the dimension df of the photographed image is determined by ray A. (b) The eye is focused on E, which coincides with camera focus O; df is determined by rays A and B. (c) The eye is focused on E, which lies between the fiber-optic source and the camera focus O; df is determined by ray B. (d) The eye is focused on the fiber-optic source itself; df is determined by rays B and C.

Fig. 5
Fig. 5

The camera attachment for isotropic photorefraction consists of a fiber-optic light guide with a right-angle tip (Ealing Corporation) mounted in a tube that screws onto the front of the camera lens (f/1.2; 50 mm). The outer end of the fiber-optic guide is clamped against the face plate of a small photographic electronic flashgun.

Fig. 6
Fig. 6

Functions relating image size at the film plane to the distance at which the subject’s eye is focused, for isotropic photorefraction with a defocused camera lens. (Camera-to-subject distance, 1.5 m; eye pupil diameter, 6 mm). Functions are shown for the camera lens focused 0.5 D in front of the subject (i.e., 0.86-m distance, dashed line) and 0.5 D beyond the subject (6.0-m distance, solid line). Functions were obtained by computer ray tracing. The near-horizontal outer portions of the functions are caused by vignetting of the returning light by the entrance pupil of the camera lens.

Fig. 7
Fig. 7

Isotropic photorefractive images from a 6-month-old infant showing a position of focus moderately hyperopic with respect to the camera distance of 0.75 m. (a) Photograph with camera focused at the subject’s distance of 0.75 m, showing brightly transilluminated pupils. A photograph of this type permits measurement of pupil size and hence quantitative interpretation of images taken with the camera defocused. (b) Photograph with camera focused beyond the subject, at 1.5 m. (c) Photograph with camera focused closer than the camera-to-subject distance, at 0.5 m. The fact that the image in (b) is smaller and less diffuse than in (c) indicates that the subject’s focus is hyperopic relative to the camera. The pattern illustrated here is characteristic of an emmetropic or slightly hyperopic individual with accommodation relaxed by cycloplegia.

Fig. 8
Fig. 8

Isotropic photorefractive images from a 6-month-old infant showing a position of focus myopic with respect to the camera distance of 0.75 m. Photos (a), (b), and (c) were taken with camera focused at 0.75, 1.5, and 0.5 m, respectively. Larger, more-diffuse images in (b) compared with (c) indicate the myopic position of focus (accommodation relaxed by cycloplegia).

Fig. 9
Fig. 9

Isotropic photorefractive images from two astigmatic infants. Photos (a) and (b) show the same infant, with camera focused at 1.5 and 0.5 m, respectively; (c) and (d) show a second infant, with the same two focus settings. (Camera-to-subject distance, 0.75 m in all cases.) In each case the long axis of the blurred pupil image shows the meridian of greatest defocus relative to the camera. For both infants, the long and short axes of the images are shorter with the 1.5-m focus setting than with the 0.5-m setting, indicating that both meridians are hyperopically focused relative to the camera (accommodation relaxed by cycloplegia).

Fig. 10
Fig. 10

Appearance of isotropic photorefractive pupil reflexes of subject with mixed, against-the-rule astigmatism with camera focused behind subject. Because of the interaction of the chromatic aberration of the eye with the subject’s astigmatism, a virtual image of the red light in the horizontal meridian and the blue light in the vertical meridian exists behind the subject and close to the plane of focus of the camera. Accordingly, red light and blue light are scattered in the orthogonal meridians, giving rise to reddish and bluish reflexes. When such chromatic separation is obtained, the reddish axis always indicates the negative cylinder axis of the correcting lens.

Fig. 11
Fig. 11

Theoretical distribution of light in the photorefractive image [from Eq. (12) of Ref. 1] for a small defocus of the eye (curve 1) and a larger defocus (curve 2). The diameters of the distributions are df1 and df2, but if there is an effective threshold level it for the light distribution as measured on the photographed image, the measured diameters will be m1 and m2.

Equations (2)

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d f = 2 F g p E a ,
radius = | B D { ( A - B - D + I ) [ P E - P / 2 + S H ) ( A - X ) ] - P 2 } | ,