 By the end of this presentation you should be able to describe the World Health Organization's classification of visual impairment. This classification is based on Section H54 of the ICD-10, the International Classification of Diseases. ICD-10 is the standard diagnostic tool for epidemiology, health management and clinical purposes. It was endorsed by the 43rd World Health Assembly in May 1990 and it came into use in WHO Member States in 1994. Visual acuity is an objective quantitative measure of visual function and it is one of the first clinical examinations in eye care. It can be carried out in various settings and conditions using a range of standardized charts, distances and illumination. Various charts are used across the world. The most commonly used are the Snellen chart, Landle C, E chart and the Logmore chart. The characters on the charts, known as opto types, are arranged in a standardized way to test distance vision. The most recent arrangement is the Logmore chart. This is based on proportional spacing, geometric progression, sensory letters and five letters to each line. Visual acuity measurements made using one chart can be converted to other chart measurements. For example, a visual acuity of 660 on a Snellen chart is 0.1 on a decimal scale and 1.0 on a logmore scale. In order to understand the epidemiology of visual impairment, we need to use an agreed definition of visual acuity. This will allow us to compare studies and also to measure impact. The World Health Organization has agreed on the use of four levels of visual function, as described in the ICD-10. These four levels are mild or no visual impairment, moderate visual impairment, severe visual impairment and blindness. Each of these four levels has a clear quantitative measure. Moderate visual impairment and severe visual impairment are grouped together under the term low vision. And when low vision is taken together with blindness, it represents all visual impairment. Measurement of visual acuity is based on distance vision or visual field assessment. A Snellen or equivalent chart is used at a distance of 6 metres or 20 feet in a well illuminated environment. The patient reads across and down the chart as far as they can. Each eye is tested independently and recorded. WHO categories are based on the visual acuity of the better eye and presenting vision. Pinhole is not used when using this method. Presenting visual acuity provides an understanding of refractive need and is important, particularly for epidemiological surveys and for planning. The WHO's four levels of visual acuity are categorised and measured as follows. Category 0 is mild or no visual impairment. In this category vision is equal to or better than 618 using Snellen chart measurements. Category 1 is moderate visual impairment. In this category vision is worse than 618 but equal to or better than 660. Category 2 is severe visual impairment. In this category vision is worse than 660 but equal to or better than 360. The final category is Category 3 blindness. In this category vision is worse than 360. It's important to remember that visual status categories are based on visual acuity as measured in the better eye. Blindness is also used if the visual field is less than 10 degrees from the point of fixation. In summary, visual acuity measurement is an objective measure of visual function and it is used to categorise and define visual impairment in epidemiological research, public health planning and screening programmes. Finally, by using the ICD-10 standard definition and categorisation of visual impairment we are able to compare data and understand the epidemiology of visual impairment.