A room is considered to have adequate daylight if a target illuminance level is achieved across a required percentage of the working plane, for at least 50% of the daylight hours over a typical year.

This approach, defined within BS EN17037:2018, uses Climate-Based Daylight Modelling (CBDM) as one method to assess daylight performance under realistic sky conditions, providing a more accurate evaluation than traditional methods.

BS EN17037:2018 defines three recommended performance levels for assessing daylight in interior spaces:

• Minimum

• Medium

• High

Each level sets specific illuminance targets to ensure sufficient daylight across the space:

• Target Illuminance (ET)
  This illuminance level must be achieved for at least 50% of the daylight hours, across 50% of the working plane area.

• Minimum Target Illuminance (ETM)
  A lower threshold of illuminance, which should be achieved for at least 50% of daylight hours, across 95% of the space. This ensures that almost the entire space receives an acceptable minimum level of daylight.

For top-lit spaces, both the ET and ETM requirements apply to 95% of the area, rather than the standard 50%/95% split, providing more uniform daylight coverage.

Lighting Analysts provide detailed CBDM analysis to assess and report against these requirements, helping project teams achieve the desired performance level.

BS EN17037:2018 provides two recognised methods for calculating daylight provision within buildings:

• Method 1: Daylight Factor (Hybrid Approach)

• Method 2: Climate-Based Daylight Modelling (CBDM)

Our Recommendation

At Lighting Analysts, we strongly advise against using Method 1. The hybrid Daylight Factor approach often significantly underestimates internal illuminance levels and can lead to incorrect window and rooflight sizing, ultimately compromising daylight quality.

As pioneers in the use of Climate-Based Daylight Modelling (CBDM), we recommend Method 2, which delivers a far more accurate, climate-specific assessment of daylight performance and importantly considers the impact of building orientation.

Example Comparison — Method 1 vs Method 2

We analysed a simple test room in eight principal orientations using both methods:

Key Findings:

• The Daylight Factor approach underestimated daylight levels by 42% compared to the north-facing CBDM result, and by 72% compared to the south-facing result.

• This simple analysis demonstrates >100% increase in internal daylight levels between north and south orientations — a critical factor entirely overlooked by the hybrid daylight factor approach, which results in the same internal illuminance for all orientations!