Sky Factor & Rights of Light

Obstruction & View

Sky visibility through window apertures

Sky Factor is the ratio of illuminance arriving through an unglazed window opening to unobstructed outdoor illuminance. Unlike Daylight Factor, it excludes reflected light — making it the correct metric for Rights of Light assessments in the UK.

DL-Light automates the traditionally labour-intensive Sky Factor calculation, mapping results per window and comparing before/after scenarios. A sky factor loss exceeding 0.2% of the original can trigger a Rights of Light dispute.

Sky Factor per window — no reflected light component
Rights of Light threshold check (0.2% loss test)
Before/after scenario comparison
Compatible with UK planning policy
Automates a traditionally manual calculation

Frequently asked questions

What is the Sky Factor?

Sky Factor is the ratio of illuminance at a window from an unglazed opening to the simultaneous unobstructed exterior illuminance under a CIE overcast sky. Unlike Daylight Factor, it excludes internally reflected light, making it the appropriate metric for UK Rights of Light assessments.

What Sky Factor loss triggers a Rights of Light dispute?

A Sky Factor loss greater than 0.2% (in absolute terms) of the original value can trigger a Rights of Light dispute under UK case law (the so-called 50/50 / Waldram method threshold).

Does DL-Light automate Sky Factor for UK Rights of Light?

Yes. DL-Light computes Sky Factor per window, supports before/after scenario comparison, and flags windows that exceed the 0.2% loss threshold — automating a calculation traditionally done by hand.

Going further

Read the full DL-Light user-guide reference for Sky Factor & Rights of Light: Windows guide · macOS guide.

Sky Factor & Rights of Light

Sky visibility through window apertures

Frequently asked questions

Start using Sky Factor today