Reliability and validity of the Actiwatch and Clouclip for measuring illumination in real‐world conditions

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Purpose: To compare real‐world measures of illumination obtained with the Actiwatch‐2 and Clouclip‐M2 with ‘gold standard’ photometry measures and to evaluate the ability of Actiwatch‐2 to correctly identify photometer‐defined conditions: scotopic (≤0.01 lux), mesopic (0.02–3 lux), indoor photopic (>3–1,000 lux) and outdoor photopic (>1,000 lux); and Clouclip to correctly identify photometer‐defined conditions within its operating range (>1 lux). Inter‐device reliability of Clouclip for illumination and viewing distance measures was also investigated. Methods: A Hagner‐S2 photometer was used as reference. Measures of illumination were obtained from a range of real‐world conditions. To investigate inter‐device reliability, five Clouclips were simultaneously exposed to varied light conditions and object distances. Results: Strong correlations existed between illumination measured with the photometer and both Actiwatch‐2 (ρ = 0.99, p < 0.0001) and Clouclip (ρ = 0.99, p < 0.0001). However, both devices underestimated illumination compared to the photometer; disparity increased with increasing illumination and was greater for Actiwatch‐2 than Clouclip measures. Actiwatch‐2 successfully categorised illumination level (scotopic, mesopic, indoor and outdoor photopic) in 71.2% of cases. Clouclip successfully categorised illumination levels as scotopic/mesopic (≤3 lux) and indoor and outdoor photopic in 100% of cases. Mean differences and limits of agreement (LOA) were 430.92 ± 1,828.74 and 79.35 ± 407.33 lux, between the photometer and Actiwatch‐2 and photometer and Clouclip, respectively. The Intra‐class Correlation Coefficients for illumination and viewing distance measured with five Clouclips were 0.85 and 0.96, respectively. Conclusion: These data illustrate that different Clouclip devices produce comparable measures of viewing distance and illumination in real‐world settings. Both Actiwatch‐2 and Clouclip underestimate illumination in the field compared to gold standard photometer measures. The disparity increases at higher levels of illumination and the discrepancy was greater for Actiwatch‐2 measures. For researchers interested in categorising light exposure, Clouclip classifies illumination levels >2 lux more accurately than Actiwatch‐2 but cannot discriminate between scotopic and low mesopic light.
Original languageEnglish
Pages (from-to)1048-1059
Number of pages12
JournalOphthalmic and Physiological Optics
Issue number5
Early online date13 Aug 2021
Publication statusPublished (in print/issue) - Sept 2021

Bibliographical note

Funding Information:
Funded by the Department for the Economy, Northern Ireland.

Publisher Copyright:
© 2021 The Authors. Ophthalmic and Physiological Optics published by John Wiley & Sons Ltd on behalf of College of Optometrists.


  • Actiwatch
  • Clouclip
  • light exposure
  • myopia
  • near work
  • sensor technology


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