It helps to evaluate refractive error(Refractive Defect) of the eye.

It is widely measured today by digital instruments. Especially in children, this tool cannot be used.

In this case, the refraction error of the eye is measured with the help of a retinoscope and a ruler (skiascopy ruler) with a lens between ± 0.50 D and ± 20 D. This process is called skiascopy.