±0.001 rounding discrepancy in specular daylight reflectance calculation

Related services Daylight reflectance

In the daylight reflectance test, the specular daylight reflectance is calculated as:

Specular daylight reflectance = Total daylight reflectance – Diffuse daylight reflectance

Due to the rounding operations, the reported specular specular daylight reflectance could be:

Specular daylight reflectance = Total daylight reflectance – Diffuse daylight reflectance ± 0.001

Shown below is an example result set, with this ± 0.001 discrepancy:

  • Total daylight reflectance = 0.216 (21.6%)
  • Diffuse daylight reflectance = 0.200 (20.0%)
  • Specular daylight reflectance = 0.015 (1.5%)

Some customers expect that the specular daylight reflectance should be 0.216 – 0.200 = 0.016 (1.6%) and suspect that there is a typo in the test report.

Our daylight reflectance test reports are generated by in-house software automatically. The ±0.001 discrepancy is not due to a calculation mistake or a typo, but due to the rounding operations, as illustrated in the table below:

Before roundingAfter rounding
Total daylight reflectance0.21560.216 (21.6%)
Diffuse daylight reflectance0.20040.200 (20.0%)
Specular daylight reflectance0.01520.015 (1.5%)

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