The paper describes part of a larger research project into expert witness practice in rights to light litigation in England and Wales. It examines the origins of the so-called “grumble point” (a sky factor of 0.2 per cent) as the measure of daylight adequacy which is still used by expert witnesses today. Through an examination of archive material and historic literature it seeks to identify the rationale, and underlying scientific basis, for the adoption of this standard in the early part of the 20th century.

It finds that the use of the 0.2 per cent standard does not appear to be based on empirical investigations involving human perceptions of adequate light. No evidence exists of the investigations reputedly undertaken by Percy Waldram during the early 20th century. Waldram’s own writings suggest that the standard began as a “rule of thumb” and was only later justified by reference to other independent reports.

These generally do not support the use of the standard and, in any event, were soon superseded by other reports which concluded that it was too low.

The paper concludes that there is a lack of reliable evidence to justify the original adoption of the 0.2 per cent figure and that many of the assumptions underpinning modern rights to light practice are based on inaccurate information.