Sir,
In response to the comment by Mahroo et al.1 on determining the accuracy of higher cut-off values of light meter readings to determine time spent outdoors vs indoors, we have evaluated the accuracy of cut-offs higher than 1000 Lux of 1200 and 1500 Lux.
The Intra Class correlation for the cut-off values of 1200 and 1500 Lux showed higher correlations between the light meter and diary recordings during the week in a school term and school holidays, compared with cut-offs of 800 and 1000 Lux (Table 1).
Table 1. Intra Class Correlation co-efficients (ICC) for cut-off values of 800, 1000, 1200, and 1500 Lux during the school term and school holidays.
| Cut-off light intensity (in Lux) | ICC—school term | ICC—school holidays |
|---|---|---|
| 800 | 0.1 (−0.13, 0.32) | 0.23 (−0.07, 0.49) |
| 1000 | 0.21 (−0.02, 0.42) | 0.28 (−0.02, 0.53) |
| 1200 | 0.26 (0.03, 0.46) | 0.28 (−0.02, 0.53) |
| 1500 | 0.29 (0.07, 0.49) | 0.25 (−0.05, 0.50) |
Thus, we agree with the proposal that the best cut-off value for evaluating outdoor vs indoor activities is 1500 Lux instead of 1000 Lux.
The authors declare no conflict of interest.
References
- Mahroo OA, Gavin EA, Williams KM, De Smit E, Hammond CJ, Morrison DA. Potential effect of ‘cut-off intensity' on correlation between light meter measurements and time outdoors. Eye. 2013;27 (8:990–991. doi: 10.1038/eye.2013.88. [DOI] [PMC free article] [PubMed] [Google Scholar]