Refraction measurement will be done by the investigator in all 161 children. Visual acuity is checked, initial and after correction with proper correction. Vitamin D level is dosed from all the children in order to make correlations with the visual acuity, refractive status and risk for visual impairment.
The investigator want to determine the refractive status of 80 children with disabilities and of 81 healthy children from a witness group. The vitamin D level of the children will be dosed for making different correlations with visual acuity status.