The definition of blindness varies from one country to the next. The ONCE (National Organization of Spanish blind people) defines blindness as ´the total absence of vision or simple light perception in one or both eyes´. The ONCE also specifies that blind people are ´those persons that are unable to see anything at all, or those who have only a slight light perception (able to distinguish between light and darkness, but not the shape of objects´. In optical terms, the degree of blindness is measured by the patient’s visual acuity and visual field. To become a member of ONCE, the following limits have been established: visual acuity equal to or less than 1/10 on the Wecker scale, or visual field reduced to the central 10 degrees, or lower, in the best eye.In the UK, the legal definition for blindness is 'so blind as to be unable to perform any work for which eyesight is essential' The optical profession has defined this definition further by using a value that can be measured by standard tests: A visual acuity of 3/60 or less or 6/60 or worse with markedly restricted fields.