How to use the RayOcular fundus view

Input

To correctly integrate fundus photographs in RayOcular’s fundus view, you need these parameters:

Camera view settings

• $FOV\ [\mathrm{deg}]$: field of view configured in RayOcular. We recommend starting with a value of 60 deg.
• $FS\ [\mathrm{cm}]$: film size configured in RayOcular. Set this to 1 cm to get an intuitive relation between angles and distances.

If you cannot find these parameters, make sure you set the View mode to Camera.

Biometry parameters

• $NP\ [\mathrm{cm}]$: Location of the second nodal point, posterior to the cornea apex. This parameter is patient-specific and can be calculated from biometrical measurements using e.g. Visisipy.
• $AL\ [\mathrm{deg}]$: Axial length of the eye, measured from the cornea apex to the retina.

Fundus photograph parameters

• $W_{px}\ [\mathrm{px}]$: The width of the photograph in pixels. RayOcular assumes fundus photographs are always 7.68 cm wide.

• $S_{photo}\ [\mathrm{px/deg}]$: Scale of the fundus photograph in pixels per degree. This parameter is used to map locations on the fundus photograph to eccentricities (or ray angles).

  How to determine $\ S_{photo}$

  The pixel-to-angle scaling factor can be determined in two ways:

  • Directly using a phantom based on an $f-\theta$ lens.

  • Using the central magnification ($M [\mathrm{px/mm}]$) calculated with PAROS. The scale $S_{photo}$ can be calculated from the magnification using this equation:

    $$S_{photo}\ [\mathrm{px/deg}]= \frac{ M\ [\mathrm{px/mm}] }{ \arctan \frac{ 0.1\ [\mathrm{cm}] }{ AL - NP\ [\mathrm{cm}] } } \cdot 1\ [\mathrm{mm}]$$
  Important

  This parameter depends on the camera type and may depend on the refractive error of the eye.

Eye model parameters

• $R_{sclera,AP}\ [\mathrm{cm}]$: The anterior-posterior radius of the sclera in RayOcular’s eye model.
• $T_{sclera}\ [\mathrm{cm}]$: The thickness of the sclera in RayOcular’s eye model.

Calculations

Using these parameters, you can calculate the fundus photograph scaling that should be entered in RayOcular:

$$scaling\ [\mathrm{-}] = \frac{ W_{px}\ [\mathrm{px}] }{ S_{photo}\ [\mathrm{px/deg}] \cdot \frac{ FOV\ [\mathrm{deg}] }{ FS\ [\mathrm{cm}] } \cdot 7.68 \ [\mathrm{cm}]}$$

Furthermore, RayOcular uses a different definition of the second nodal point location. The nodal point location that should be entered in RayOcular ($NP_{RO}$) can be calculated with

$$NP_{RO} = NP + (2 * R_{sclera,AP} - AL) + T_{sclera}$$

You can now use these values to configure the fundus view:

• Enter $scaling$ under Photo alignment > Scaling
• Enter $NP_{RO}$ under Camera view settings > Nodal point dist. [cm]
• Set Camera view settings > Optics fit factor to 0.0.