Re: F/0.5 possible?

Dave Martindale wrote:snip

I've seen a version of that in use for a not-quite-photographic
application. It uses a glass sphere that's probably around n=1.5, so
the image is formed a short distance outside the sphere. The sphere
itself was about 3 inches in diameter. A strip of paper printed with
time of day markings is mounted in a holder that curves the paper around
the sphere for about 180 degrees of wrap, while holding the paper in the
focal surface of the sphere. The arc of the paper holder is tilted by
the latitude of the place where the device is mounted.

So the sphere forms an image of the sun on the paper surface, if the sun
is not blocked by clouds. The sphere collects enough energy and focuses
it in a small enough spot to burn the paper. As the sun moves through
the sky during the day, the image moves along the paper strip, and later
looking at the strip you can tell whether the sun was visible and
roughly how strong the sunlight was as a function of time throughout the
day.

There was one of these mounted on top of the heating plant building at
an agricultural research station where my father worked, and he changed
the paper strips every day for many years. Eventually, it was replaced
with some sort of electronic monitoring/recording device, and the
optical gadget was removed. I wish I knew where it went.

Dave

We almost had a fire one time from that effect. We were living in Lompoc CA near Vandenberg AFB. The town water was not fluorided, the base water was. For our kid I would fill up gallon glass jugs of water when I was on base.

We kept the jugs in the kitchen near a south wall. We had used newspapers stacked on floor near the bottles. For days we smelled what smelled like something burning but could not find source of smell.

One sunny day I happened to notice bright spot on the papers when I glanced in that direction. And there was smoke coming from it. And- other trails of brown across the paper!. The bottle filled with water (n= 1.33) was making a focus spot about two inches behind the bottle, on the papers, and marching across the paper as sun moved. The focus was not quite sharp enough to actually ignite the paper, but it was close- enough to brown the paper and raise a little smoke. We sure moved those bottles after that! As an optics guy I should have foreseen that and never stored them in the sun :-(
.

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