The combination of intensity and time can provide insight into light exposure.
To calculate light exposure over time, such as lux hours, two key pieces of data are needed: the light intensity at a given moment and the time period being measured. These calculations require some assumptions based on the sampling rate.
If a sample is taken every 10 to 15 minutes, there will not be second-by-second data about the lighting conditions, but the calculations will be fairly accurate if you average the illuminance readings over that period.
To calculate lux hours, first determine the average illuminance for each hour within the specified time range. This average represents the illuminance for one hour. Then, to find the cumulative exposure, sum the hourly averages over the entire time period. For example, over a four-hour span, if the hourly averages total 120 lux/hours, this represents the total exposure for that period, as shown in the following example.
| Hour 1 | Hour 2 | Hour 3 | Hour 4 |
| Avg lux: 20 | Avg lux: 40 | Avg lux: 40 | Avg lux: 20 |