The "PINHEAD" Effect

Many people are familiar with "pinhole" projection and how, with a partial eclipse creates images of the solar crescent, E.g., naturally occurring with sunlight shining through the gaps between tree leaves, as in this example:

from the 2005 annular solar eclipse in Spain.

I have found, however, that not so many people are familiar with one of the aspects of diffraction by a circular aperture for anything but a point source.  If you take a circular aperture that is angularly large in extent compared to an illuminating extended (e .g., also circular) source, diffraction by the aperture will create a DARK image of the illuminating source in the center.  Not very dark (contrast is weak) but for the right ratio of sizes this can be seen visually, in particular if the source is the Sun, with angular diameter about half a degree.

Eclipse chaser John Beattie has dubbed this the "Pinhead" effect (to differentiate from the pin hole effect) and demonstrates with a combined pinhole and pinhead projection card during the partial phase of the 2012 annular eclipse in the photo below.

In the picture on the left side, John is holding his "pin head + hole" projection card in his unseen out-stretched right hand.  The card is just cardboard stock with four small pinholes - one each near each corner, and a large (about an inch) hole in the center -- to create the "pin head" image.  The images are projected onto a white card you see in his left hand.   With the Sun in partial eclipse, the diffracted pin head image is a DARK solar crescent.  This is a bit hard to see in the original photo due to the shallow contrast. (If you drag the image around display, as it is moving it may be easier to see).  In the image on the right I have applied ONLY a digital contrast re-stretch differently in the outer and inner parts on the image - to make this inverted (dark) crescent more readily seen.  Otherwise no monkey business.  

I often see eclipse chasers making pinhole projectors, but not often pin head projectors, and I suspect the effect may not be as commonly known to some.  So, here it is - for all the "pinheaded" eclipse chasers out there.

My thanks to John Beattie for this demonstration.


RETRUN to Glenn Schneider's Home Page