In the May 2020 I set up some light sensors and a data logger to record the sunlight above my house in Switzerland. I thought it would be interesting to see how this changes during the day and across the seasons. And I thought it may be useful for sun printing or wet plate collodion.

These charts show the average UV and visible light intensity against the time of day for each month from June to December.

Methodology

The sensors point roughly 45º upwards and north, north west. There is window glass between the sensor and the outside, of similar thickness to a contact printing frame. The sensors are sufficiently high not to be blocked by shadows.

The data logger recorded the UV and visible light intensity once per minute. This means that the value shown for 09:00 in July is the average of all measurements made between 09:00 and 10:00 on every day in July.

The results were analysed using Microsoft Excel to produce a monthly average measurement for each hour of the day.

The times are Central European Time (CET) not adjusted for summer time.

Observations

The most striking thing for me is the uniform shape of the UV intensity curves. They are much more consistent than the visible light curves.

Also of interest is that the range of UV intensity through the year is noticeably wider than that of the visible light. The peak UV varies by about 1.5 stops while the peak visible light varies by a little below 1 stop.