**What is an Optical Cross?**

- 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 2
^{nd}meridian will be 70°. - So, One Meridian is 160° & Another Meridian is 70°

**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:__

- 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.

**Check Our Courses:****Ophthalmic Instrumentation****,****Clinical Refraction**,**Contact Lens****,****Binocular Vision****,****Dispensing Optics****,****MCQs in Optometry****Download our App “****Optometry Notes & MCQs****” from Google Play Store.**