Introduction: Photorefraction is the newest retinoscopy technique Its main application is the screening of infants and young children

1. Photorefraction
Mohammad Almasi - Mojtaba Rajabpour
Shamim Yaghoubian
Optometry S. in SBMU
2016
Photorefraction
of Eye
Mohammad Almasi
Mojtaba Rajabpour
Shamim Yaghoubian
Optometry S. in S.B.M.U
2016

2. Introduction
 Photorefraction is the newest retinoscopy technique
 Its main application is the screening of infants and young
children
 It is useful in the detection of anisometropia and strabismus
 In essence, a flash photograph is taken of both eyes
together, with the flash source near the camera
 Size and location of fundus reflection as seen in the pupil in
the camera determines refraction

3. Forms of Photorefraction
Photorefraction based on point-spread (on-axis) :
 Orthogonal
 Isotropic
Photorefraction based on retinoscopic-like (off-axis) :
 eccentric

4. How to do photorefraction ?
 In practice, the subject is seated in a darken
room (ambient light between 2 and 20 lx) at a
distance of 0.75 to 1.5 m from the camera
 The photographer ensures that the subject is
fixating a point near the camera lens before
the picture
 With infants this is usually done by the
phothographer placing his or her head near
the camera lens and establishing eye contact
with the subject

5. Point-spread method

6. Orthogonal photorefraction
 Orthogonal photorefraction was the first
pointspread photorefraction method to be
developed
 In this technique, the camera lens is focused on
the patient’s eyes
 A small flash source of light is mounted
centrally in front of the camera lens
 The light returning from the fundus to the
camera lens falls on four cylindrical lenses
arranged radially around the source at 90
degrees intervals

7. Orthogonal photorefraction
 The image on the camera plane is a cross
 The length of the cross arms is proportional to the size of the
point spread image, which in turn is proportional to the
refractive error relative to the camera
 Two meridians are measured simultaneously
 Photograph are taken with the lens elements at 90/180, at
45/135
 These two images are taken to give reasonable information
about astigmatism

8. Orthogonal photorefraction

9. Isotropic photorefraction
 A second pointspread method eliminates the need for the
cylander lenses of orthogonal photorefraction
 This method provides a means of assessing the sign of
defocus
 3 separate pictures are taken of the subject with a flash
source centred in the lens of the camera
 One picture is taken with the camera focused on the eyes
of the subject. This picture may be used to measure pupil
diameters
 Then two other pictures are taken with the camera focused
0.5 D in front of and behind the subject.

10. Isotropic photorefraction
 Myopic eye :
 In myopic eye, the real image of the
fundus reflex is between the camera lens
and the subject’s eye
 If the camera is focused in front of the eye,
this real image is in better focus at the
camera and appear smaller than it would if
the camera were to be focused beyond the
eye
 By comparison of the size and brightness
of the image, the sign of refractive error
can be ascertained

11. Isotropic photorefraction
 Hyperopic eye :
 In hyperopic eye, the virtual image of the
fundus reflex is located behind the subject’s
eye
 When the camera is focused behind the
subject, this virtual image will be in focus and
small bright spot will be recorded at the
camera’s image plane
 When the camera is focused in front of the
subject,this virtual image of the retina is now
defocused,and a difuse,large spot will be
recorded
 By comparison of the size and brightness of
the image, the sign of refractive error can be
ascertained

12. Rtinoscopic-like method

13. Eccentric photorefraction (photoretionscopy)
 2 versions of photoretinoscope are exist :
 A- One version of photoretinoscope consists of a
centered fiber light guide and a shield that
surrounds the light source.the shield provides an
eccentric knife edge to the source and a finite
‘eccentricity’ or distance between the knife edge
and the center of the photorefractor’s camera
 B- Infrared light-emitting diodes arranged in rows
that can be separately illuminated in sequence

14. Eccentric photorefraction (photoretionscopy)
If the pupil of the subject is photographed :
 In emmetropic eyes :
The retinal reflex is uniform red reflex in the
pupil (as seen as in this figure) →
 In ametropic eyes :
A whitish crescent will be captured within the
red reflex. This is a picture of the reflected blur
circle at the plane of the pupil

15. Crescent size
Crescent size indicates the magnitude of the refractive error, whereas
crescent location indicates the type of the ametropia
The size of crescent is dependent on:
 pupil size
 eccentricity of light source
 camera to subject distance
 distance at which the eye is focused

16. Eccentric photorefraction (photoretionscopy)
If the light source is placed below the camera
aperture and the pupil of the subject is
photographed :
 In myopic eyes :
The crescent will be appear in the bottom of the
pupil
 In hyperopic eyes :
The crescent will be appear in the top of the pupil

17. Video-retinoscopy
 The phtoretinoscopic technique has developed into video-
retinoscopy using rows of infra-red LEDs within the shield
 Rows can be illuminated sequentially, and the rate of movement of
the detected crescent measured t determine refraction
 A more recent development is to have all the LEDs on
simultaneously, and to measure the slope of image intensity at
right angle to the edge of the shield
 The slope is linearly related to the refractive error for a
considerable range

18. Advantages
 the photorefractive techniques use a very large camera to subject
distance (typically 1 m)
 They refract both eyes simultaneously
 They don’t require fixation of the subject
 They are optimally suited for noncooperative subjects (for example Infants
)

19. disadvantages
 The results of these methods are not very accurate
 The range of refractive error that can be measured by orthogonal
and isotropic photorefraction is limited

20. Currently Available Photoscreeners
 MTI
 PlusoptiX
 iScreen3000
 Spot

21. Refrences
 William J.Benjamin, Clinical Refraction, 618-626
 Mark Rosenfield, Nicola Logan,Optometry: Science, Technique AND Clinical
Management, Second Edition, 205-207
 Bruce Moore, Eye care for infants and young children,1st Edition, 52-53, 116-117
 W. R. BOBIER & O. J. BRADDICK, American Journal, Eccentric Photorefraction: Optical
Analysis and Empirical Measures
 Howland HC., Braddick O. Atkinson J. Howland B.; Optics of Photorefraction:
Orthogonal and Isotropic methods; J opt Soc Am. 1983 Dec;73(12):1701-8.
 W. Neil Charman, Visual Optics and Instrumentation,1991, 407-410
 http://eyewiki.aao.org/Photoscreening
 Howard C. Howland, Optometry and Vision Science, 2009, 603-606