neutral density filter test

This test differentiates functional (reversible) from organic (pathological) amblyopia. Visual acuity is measured with and without neutral density filters that reduce luminance equally across wavelengths. In organic amblyopia, acuity significantly decreases with filters, while in functional amblyopia, acuity is minimally affected. This test helps determine if reduced acuity is due to retinal/optic nerve pathology or suppression. Filters of increasing density are used until acuity diminishes.