<P> En plein air (French pronunciation: ​ (ɑ̃ plɛn ɛːʁ), French for outdoors, or plein air painting) is the act of painting outdoors . This method contrasts with studio painting or academic rules that might create a predetermined look . </P> <P> 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 . </P> <P> The act of outdoor painting from observation has been continually popular well into the 21st century . </P>

Which group of artist is well known for painting en plein air