An initial reaction to the above question is that HMRC would be unlikely to allow such an expense in the accounts especially if the self-employed person wears glasses all the time.
The general rule for calculating the profits of a trade is that no tax deduction is allowed for expenses not incurred wholly or exclusively for the purposes of a trade.
In some cases however where an expense is incurred for more than one purpose it may be possible to claim a tax deduction for a proportion of the expense which is incurred for the purposes of the trade.
Perhaps an approach could be adopted similar to that in respect of the cost of a car that is used both for business and private journeys whereby costs incurred in buying and running the car are claimed in proportion to the car’s business usage on the basis there are distinctive periods of business use and private use.
As such it would appear that a valid claim could be made for at least a proportion of the cost of the glasses or contact lens. If the self employed person has glasses that are used wholly and exclusively for work then the full cost could be claimed as a business expense.