Plein air

En plein air (French pronunciation: ​[ɑ̃ plɛn ɛːʁ]), or plein air painting, is a phrase borrowed from the French equivalent meaning "open (in full) air". It is particularly used to describe the act of painting outdoors, also called French: peinture sur le motif ("painting of the object(s) or what the eye actually sees"), where a painter reproduces the actual visual conditions seen at the time of the painting. This method contrasts with studio painting or academic rules; those might create a predetermined look. En plein air can also be used to describe other activities where a person partakes in an outdoor environment.

Artists have long painted outdoors, but in the mid-19th century, working in natural light became particularly important to the Barbizon school, Hudson River School, and Impressionists. The Newlyn School in England is considered another major proponent of the technique in the latter 19th century. The popularity of painting en plein air increased in the 1840s with the introduction of paints in tubes (like those for toothpaste). Previously, painters made their own paints by grinding and mixing dry pigment powders with linseed oil.

The act of outdoor painting from observation has been continually popular well into the 21st century.

It was during the mid-19th century that the box easel, typically known as the French box easel or field easel, was invented. It is uncertain who developed it, but these highly portable easels with telescopic legs and built-in paint box and palette made it easier to go into the forest and up the hillsides. Still made today, they remain a popular choice (even for home use) since they fold up to the size of a brief case and thus are easy to store.

...
Wikipedia