Fluorescent lights that flicker imperceptibly are used in up to 80% of classrooms due to misguided policy moves, the study of 90 classrooms said.

These should be replaced with higher frequency light bulbs or turned off - with natural light used instead, an education conference in London heard.

Poor classroom design and the position of equipment could also add to glare.

The findings could also help to explain why some children found reading easier when they use coloured overlays on top of books to reduce the brightness of the page, the study by a team fom the universities of Cambridge and Essex said.

We found that neither artificial light nor daylight could be adequately controlled

Dr Mark Winterbottom

The researchers measured the flicker and lighting levels in 90 classrooms at 11 secondary schools in six local authority areas.

They claimed that nine out of 10 classrooms were excessively bright.

Dr Mark Winterbottom said: "We found that neither artificial light nor daylight could be adequately controlled due to classroom design, positioning of equipment, and the poor state of Venetian blinds.

"A quarter of classrooms did not have blinds, and in half of those that did, blinds were either broken or at least one was missing."

'Safety advice'

He added that wall-mounted white boards and ceiling mounted projectors were also directing excessively bright light into pupils' eyes.

"Wherever possible, such boards should be tilted so that such reflected glare is directed towards the ceiling," he said.

A spokesman for the Department for Children, Schools and Families pointed out there was very comprehensive guidance on lighting and classroom design on its websites for teachers.

"These outline simple measures to take to ensure that equipment is being used appropriately and safely. In addition, guidance material has been distributed to all local authorities."

The research was presented at the annual conference of the British Educational Research Association at the Institute of Education in London.