With a spectacle prescription of -3.00 -1.50 x 180 and K readings of 43.50 @180 and 46.50 @ 90, what would be the residual astigmatism?

To determine the residual astigmatism, one needs to analyze the spectacle prescription and the corneal curvature measurements (K readings). The spectacle prescription is given as -3.00 -1.50 x 180. This indicates that there is a spherical component of -3.00D and a cylindrical component of -1.50D with a 180-degree axis.

The K readings indicate the flat and steep meridians of the cornea: 43.50 diopters at 180 degrees, and 46.50 diopters at 90 degrees. The difference between these two readings gives an amount of corneal astigmatism, which can be calculated as follows:

1. Calculate the K astigmatism:

[

K \text{ astigmatism} = K_{\text{steep}} - K_{\text{flat}} = 46.50 - 43.50 = 3.00D

]

This astigmatism is aligned at 90 degrees (the steep meridian).

2. Combine the sphere and cylinder from the spectacle prescription:

The total refractive power in the horizontal (180 degrees) meridian would be: