Optical Cross in Refraction: Everything You Need to Know.

What is an Optical Cross?

Optical Cross in Refraction
Optical Cross in Refraction
  • Optical cross is a Cross (“X”) used during retinoscopy to distribute the power meridian wise.
  • Two straight line is used to draw the Optical Cross at 90 apart from each other.
  • These two lines represent two principal meridians of eye.
  • Most people have vertical (90°) & Horizontal meridian (180°) as principal meridian. Thus, optical cross typically looks like Plus (+).
  • If principal meridians are not Vertical (90°) & Horizontal (180°) then generally we draw another pair of straight lines to represent the principal meridians and then it looks like a Star (*).
  • Vertical & Horizontal lines are then works as a reference mark.

Why Power is 90° apert from Axis?

Axis:

  • Axis indicates the position of Retinoscopic Streak.

Meridian:

  • Meridian is where the power is situated.
  • The difference between axis and meridian is 90°.

That means power present 90° apert from the axis.

  • In Streak retinoscopy, the retinoscopic reflex is “Streak Shape”.
  • Due to streak shape, it’s easier to check power in vertical meridian when the streak is horizontal and moves Up & Down.
  • Also, it’s easier to check power in horizontal meridian when streak is vertical and moves Right and Left.
  • That’s why power always situated 90° apert from axis.

Axis vs Meridian:

In keratometry, we don’t use any streak light source that’s why keratometric power is directly written without any cross, like

K1 = 44 D @ 90°

K2 = 46 D @ 180°

  • Means, Vertical meridian has 44D and horizontal meridian has 46D.
  • So, “X” means power situated in 90° apert from the given axis and “@” means power situated in the given axis.
  • Simply, “X” indicates the axis and “@” indicates the meridian.
  • So, -2.0 X 90 means, when retinoscopic streak were 90° or vertical, 180° horizontal meridian is neutralized with -2.0D trial lens.
  • 44D @ 90 means, vertical or 90 meridians has 44D power that is found in keratometry.

Spectacle Power to Optical Cross

How to identify principal meridians from Spectacle Power?

  • Now, let’s take a prescription power and distribute in optical cross.

                        OD = -2.0/-1.0 x 70°

  • Here, two principal meridians are 70° & 160°
  • As we already know meridian present 90 apert from the axis so one meridian will be:
    • Prescription Axis + 90° or 70° + 90° = 160°
  • We also know that difference between two meridian is 90°, so another meridian will be:
    • 160° + 90° = 250° or 160° – 90° = 70°.
  • Axis range of our eyes is 0 to 180, so 2nd meridian will be 70°.
  • So, One Meridian is 160° & Another Meridian is 70°
Spectacle Power to Optical Cross
Spectacle Power to Optical Cross

Let’s, draw these meridians in optical cross form.

  • -2.0D is spherical power so it will be in both meridians.
  • And cylinder power is -1.0D X 70° axis, so power will be here in 160°.
  • So, -2.0/-1.0 x 70° means, 70 meridian has -2.0D and 160 meridian has -3.0D.

Optical Cross to Spectacle Power:

Optical cross to Spectacle power
Optical cross to Spectacle power
  • If both meridians have same power in optical cross, then there is no astigmatism or cylinder power (Spherical Refractive Error).
  • If Power in both meridian is different, then astigmatism or cylinder power is there.
  • When cylinder power is there, one meridian is spherical, and another meridian is Cylinder.

Spherical Power:

  • Take any meridian as spherical meridian and write the power of that meridian  as spherical power.

Cylinder Power:

  • The difference between the power of cylinder meridian and spherical meridian will be cylinder power.
Spherical & Cylinder Meridian in Optical Cross
Spherical & Cylinder Meridian in Optical Cross

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