Something to think about
A bright sunny day 80,000 lux
Overcast day 2000-5000 lux
Modern Office 500 lux
A typical cow building 50-60 lux Uk (winter daytime)
Evidence shows that milk output and feed intake is highest when light periods of 16-18 hours per day at 160-200 lux, and dark levels of 50 lux for 6-8 hours.
Dry cows however are different there is new work to say that they prefer long nights and shorter days (8 hours light and 16 hours dark), this has shown improvements in subsequent lactations (it effects mammary gland growth and immune function).