Thursday, December 3, 2009


On vacation in Paris last month we visited the Musee D'Orsay to see some of the classics of Impressionism. One of my current favorites (and one of the most expensive paintings ever sold) is Renoir's Bal du Moulin de la Galette:

What I particularly like about this painting is Renoir's careful depiction of sunlight dappling through trees. Look at the man in the foreground, facing away from us, with the curious round dabs of sunlight across his back. If you focus on this point and think about it, the appearance of the dapples seems a little strange, yet holistically it feels right.

In fact, what Renoir has captured here is a true optical curiosity. As discussed by Marcel Minnaert in Light and Color in the Outdoors, sunlight dappling through trees often produces these beautiful, perfectly round or elliptical spots of light:

As Minnaert points out: "the surprising thing is that all these images have the same shape, although it is obviously impossible that all the holes and slits in the foliage happen to have the same shape."

This happens because holes in foliage that are sufficiently small result in dapples whose shape is dominated by the image of the sun, i.e. a disc, rather than the shape of the hole. If the sun were a mere point source rather than having a significant diameter, we would not see this effect. (I believe that the shape of the spot of light produced is essentially just the image of the sun, scaled by the projection throw length and then convolved with the shape of the hole, neglecting diffraction effects...)

This effect is most dramatic when something distorts or obscures the image of the sun. A classic example is the change in the appearance of tree dapples during a solar eclipse: as the sun becomes obscured, its image becomes crescent shaped, as do its dapples through the trees:

Update: new eclipse photo, this time at annular stage: