There are four broad categories of vision impairment. The following descriptions are loose guidelines, and it should be noted that the individual will hold the information you require so always have that conversation. In general, though the following are a useful starting point

• Colour blindness: red-green impairment is the most common, affecting 5% of the male population and 1% of the female. One in twelve people using the web are colour blind.
• Mild vision impairment: may require larger font size, possibly a different background style and/or colour.
• Moderate vision impairment: may require the aid of screen magnification software.
• Blind/severe vision impairment: may require the aid of a screen reader.

For more information visit the RNIB website.