Shadow caster
The gnomon point of my sun dial is also very thin. In order to observe the shadow point as exactly as possible, I use an old and simple trick: by touching the gnomon-point with one of my fingers, the semishadow almost disappears locally and I can better observe the exact location. Jos Kint 50°59'N - 3°39E Jos Kint--- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Shadow caster
Dave,
Speaking of a bright sun spot, consider the following setup:
A
hemispherical convex mirror mounted above a (hopefully
clear) ceiling window so that you can see, from
inside the room, the bright reflection
of the sun
A
dial face, horizontal, on a table within view of the mirror
A
pinhole in a mask of the right size (will explain below),
horizontally suspended somewhere on a line between the
mirror and the dial face (placement explained below)
I've made
this sort of thing work at a distance roughly half of yours
using ...
A 26
inch diameter ellipsoidal weatherproof mirror
A
1/8 inch diameter pinhole
A
letter-size dial face.
If d =
diameter of mirror, w = longest dimension of the dial face, a=
distance above pinhole to the center of the mirror, b = distance
below pinhole to the dial face, L = a +b, and m = width of the
mask (large enough to keep the dial face in shadow from
extraneous light from the mirror), then
b =
L[w/(d + w)]
m =
w + (b/L)(d-w)
Assuming
for your application L = 10 ft, d = 26 inches, w = 13 inches,
we get b = 10/3 ft and m = 17 1/3 inches.
In other
words, if you hang a 17 1/3 inch diameter mask, with pinhole,
3.33 ft above the dial face, the mirror will reflect a sun spot
onto your 13 inch dial face that should not stray off the face
no matter the time of day or the day of year. (This assumes you
place the dial face approximately under the apex of the
south-facing hemisphere that has a clear view of the sun
throughout the day and year; if you place it much off from that,
the math would have to be elaborated to take the angle into
account.)
The spot is bright
enough to see in ambient light in my application. Further, a
discernible spot will be formed as long as the sun is bright
enough to cast a shadow.
Tom
On 3/4/2012 12:33 PM, Dave Bell wrote:
I agree that a
bright sun spot would be nice,
but I dont want to black out much of the window.
Unfortunately, over
the years, the
double-glazed window has grown a spotty diffusing layer
of something (mold? dried
rainwater seepage?) that makes it less than ideally
clear. I tried a solid, 2
inch spot of aluminum foil, and the shadow is barely
visibly 8 feet from the
glass.
I dont think the
glazing is something I
can disassemble for internal cleaning
Ill print some
positive and negative zone
plates in different sizes this week, and see how they
work.
Dave
From: Reinhold
Kriegler [mailto:reinhold.krieg...@gmx.de]
Sent:
Sunday, March 04, 2012 11:51
AM
To: 'Dave
Bell'
Subject:
AW: Shadow caster
I thought so!!!
Actually: Inside the house a
little sunspot is much more fun than a tiny dizzy and
diffuse shadow point
Reinhold
* **
*** * ** ***
Reinhold
R. Kriegler
Lat.
53 6' 52,6" Nord; Long.
8 53' 52,3 Ost; 48 m . N.N. GMT +1
(DST +2) www.ta-dip.de
http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18
http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html
http://www.ta-dip.de/dies-und-das.html
-Ursprngliche
Nachricht-
Von: Dave
Bell
[mailto:db...@thebells.net]
Gesendet:
Sonntag, 4. Mrz 2012
20:35
An: 'Reinhold Kriegler'
Betreff:
RE: Shadow caster
Yes,
I
remember Johns work well, particularly for gnomon
points, so the shadow isnt
truncated.
I
was
thinking about putting a shadow spot on one of my l
Shadow caster
Several times, there have been discussions about how to improve the shadow cast by a point nodus. I partially recall some conclusions regarding the optimum diameter vs, throw length, and some thoughts about adding an annular ring to take advantage of diffraction. Can anyone help remind me? Thanks! Dave Sent from my iPhone --- https://lists.uni-koeln.de/mailman/listinfo/sundial
AW: Shadow caster
John Carmichael has done excellent research work about this question! Just contact him! Regards! Reinhold Kriegler * ** *** * ** *** Reinhold R. Kriegler Lat. 53° 6' 52,6 Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N. GMT +1 (DST +2) www.ta-dip.de http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html -Ursprüngliche Nachricht- Von: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] Im Auftrag von David Bell Gesendet: Sonntag, 4. März 2012 18:50 An: sundial@uni-koeln.de Betreff: Shadow caster Several times, there have been discussions about how to improve the shadow cast by a point nodus. I partially recall some conclusions regarding the optimum diameter vs, throw length, and some thoughts about adding an annular ring to take advantage of diffraction. Can anyone help remind me? Thanks! Dave Sent from my iPhone --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Shadow caster
I agree that a bright sun spot would be nice, but I don’t want to black out much of the window. Unfortunately, over the years, the double-glazed window has grown a spotty diffusing layer of something (mold? dried rainwater seepage?) that makes it less than ideally clear. I tried a solid, 2 inch spot of aluminum foil, and the shadow is barely visibly 8 feet from the glass. I don’t think the glazing is something I can disassemble for internal cleaning… I’ll print some positive and negative zone plates in different sizes this week, and see how they work. Dave _ From: Reinhold Kriegler [mailto:reinhold.krieg...@gmx.de] Sent: Sunday, March 04, 2012 11:51 AM To: 'Dave Bell' Subject: AW: Shadow caster ☺ I thought so!!! Actually: Inside the house a little sunspot is much more fun than a tiny dizzy and diffuse shadow point… Reinhold * ** *** * ** *** Reinhold R. Kriegler Lat. 53° 6' 52,6 Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N. GMT +1 (DST +2) www.ta-dip.de http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html http://www.ta-dip.de/dies-und-das.html -Ursprüngliche Nachricht- Von: Dave Bell [mailto:db...@thebells.net] Gesendet: Sonntag, 4. März 2012 20:35 An: 'Reinhold Kriegler' Betreff: RE: Shadow caster Yes, I remember John’s work well, particularly for gnomon points, so the shadow isn’t truncated. I was thinking about putting a shadow spot on one of my living room roof windows, particularly with the approaching Equinox. But over an 8 to 15 foot throw, the shadow would be fairly diffuse. I’m going to try printing some large (1 to 3 inch outside diameter) zone plates on transparency film, and see how they work… _ From: Reinhold Kriegler [mailto:reinhold.krieg...@gmx.de] Sent: Sunday, March 04, 2012 10:05 AM To: 'David Bell'; sundial@uni-koeln.de Subject: AW: Shadow caster John Carmichael has done excellent research work about this question! Just contact him! Regards! Reinhold Kriegler * ** *** * ** *** Reinhold R. Kriegler Lat. 53° 6' 52,6 Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N. GMT +1 (DST +2) www.ta-dip.de http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html -Ursprüngliche Nachricht- Von: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] Im Auftrag von David Bell Gesendet: Sonntag, 4. März 2012 18:50 An: sundial@uni-koeln.de Betreff: Shadow caster Several times, there have been discussions about how to improve the shadow cast by a point nodus. I partially recall some conclusions regarding the optimum diameter vs, throw length, and some thoughts about adding an annular ring to take advantage of diffraction. Can anyone help remind me? Thanks! Dave Sent from my iPhone --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Shadow caster
Dave Bell - The trouble with your double glazing is that it is no longer effectively sealed. Moisture is entering from the interior of your building, and condensing on the inside surface of the outer face. The remedy is to ventilate the gap in the double glazing to the exterior, so that the relative humidity in the gap is the same as the relative humidity in the outdoor air. Block the passage with gauze to prevent the entry of insects or dirt. The double glazing should then become clear. Its thermal insulation would barely be affected. Worth trying? John Lynes From: Dave Bell db...@thebells.net To: 'Reinhold Kriegler' reinhold.krieg...@gmx.de Cc: sundial@uni-koeln.de Sent: Sunday, 4 March 2012, 20:33 Subject: RE: Shadow caster I agree that a bright sun spot would be nice, but I don’t want to black out much of the window. Unfortunately, over the years, the double-glazed window has grown a spotty diffusing layer of something (mold? dried rainwater seepage?) that makes it less than ideally clear. I tried a solid, 2 inch spot of aluminum foil, and the shadow is barely visibly 8 feet from the glass. I don’t think the glazing is something I can disassemble for internal cleaning… I’ll print some positive and negative zone plates in different sizes this week, and see how they work. Dave From:Reinhold Kriegler [mailto:reinhold.krieg...@gmx.de] Sent: Sunday, March 04, 2012 11:51 AM To: 'Dave Bell' Subject: AW: Shadow caster ☺ I thought so!!! Actually: Inside the house a little sunspot is much more fun than a tiny dizzy and diffuse shadow point… Reinhold * ** *** * ** *** Reinhold R. Kriegler Lat. 53° 6' 52,6 Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N. GMT +1 (DST +2) www.ta-dip.de http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html -Ursprüngliche Nachricht- Von: Dave Bell [mailto:db...@thebells.net] Gesendet: Sonntag, 4. März 2012 20:35 An: ' Reinhold Kriegler ' Betreff: RE: Shadow caster Yes, I remember John’s work well, particularly for gnomon points, so the shadow isn’t truncated. I was thinking about putting a shadow spot on one of my living room roof windows, particularly with the approaching Equinox. But over an 8 to 15 foot throw, the shadow would be fairly diffuse. I’m going to try printing some large (1 to 3 inch outside diameter) zone plates on transparency film, and see how they work… From:Reinhold Kriegler [mailto:reinhold.krieg...@gmx.de] Sent: Sunday, March 04, 2012 10:05 AM To: 'David Bell'; sundial@uni-koeln.de Subject: AW: Shadow caster John Carmichaelhas done excellent research work about this question! Just contact him! Regards! Reinhold Kriegler * ** *** * ** *** Reinhold R. Kriegler Lat. 53° 6' 52,6 Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N. GMT +1 (DST +2) www.ta-dip.de http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html -Ursprüngliche Nachricht- Von: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] Im Auftrag von David Bell Gesendet: Sonntag, 4. März 2012 18:50 An: sundial@uni-koeln.de Betreff: Shadow caster Several times, there have been discussions about how to improve the shadow cast by a point nodus. I partially recall some conclusions regarding the optimum diameter vs, throw length, and some thoughts about adding an annular ring to take advantage of diffraction. Can anyone help remind me? Thanks! Dave Sent from my iPhone --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Shadow caster
Hi Dave, The key principle to remember is the sun has a diameter of half a degree of arc, (32'). The sharp umbra shadow projects as a cone, vanishing to nothing at a distance 1/Tan (32') or 107 times the width of the point. The penumbra shadow keeps growing wider and dimmer as the distance increases. Cone of darkness? Cone of light from an aperture? Same phenomenon, same math, same fuzziness past 107 times. Bill Walton and John Carmichael both did experiments to determine the effect of shapes and combinations to develop shadow sharpeners. These basically cut down the apparent diameter of the sun. As a start, check the Compendium 9-4 Dec 2002 for articles by Gianni Ferrari, Bill Walton and John Carmichael. -- From: David Bell db...@thebells.net Sent: Sunday, March 04, 2012 9:50 AM To: sundial@uni-koeln.de Subject: Shadow caster Several times, there have been discussions about how to improve the shadow cast by a point nodus. I partially recall some conclusions regarding the optimum diameter vs, throw length, and some thoughts about adding an annular ring to take advantage of diffraction. Can anyone help remind me? Thanks! Dave Sent from my iPhone --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Shadow caster
That’s what I’m afraid of. The room has three near-horizontal (2:12) roof windows, and in the high south wall above them, three vertical windows, from the same line. All three horizontal windows are blotchy and hazy, but not the vertical ones. The outside surface was quire dirty after the winter, but a good cleaning did little to improve clarity; just made the blotchiness more evident. Dave _ From: Roger Bailey [mailto:rtbai...@telus.net] Sent: Sunday, March 04, 2012 2:59 PM To: John Lynes; Dave Bell Subject: Re: Shadow caster It is likely worse than that, The seals often fail on double glazed windows. Vapour builds inside and overwhelms the desiccant in the aluminum spacer. The moisture tends to etch the glass. Replacement is often the only repair. Like most Canadians, I have been there and done that. Regards, Roger From: John Lynes mailto:jly...@iee.org Sent: Sunday, March 04, 2012 1:21 PM To: Dave Bell mailto:db...@thebells.net ; sund...@rrz.uni-koeln.de Subject: Re: Shadow caster Dave Bell - The trouble with your double glazing is that it is no longer effectively sealed. Moisture is entering from the interior of your building, and condensing on the inside surface of the outer face. The remedy is to ventilate the gap in the double glazing to the exterior, so that the relative humidity in the gap is the same as the relative humidity in the outdoor air. Block the passage with gauze to prevent the entry of insects or dirt. The double glazing should then become clear. Its thermal insulation would barely be affected. Worth trying? John Lynes From: Dave Bell db...@thebells.net To: 'Reinhold Kriegler' reinhold.krieg...@gmx.de Cc: sundial@uni-koeln.de Sent: Sunday, 4 March 2012, 20:33 Subject: RE: Shadow caster I agree that a bright sun spot would be nice, but I don’t want to black out much of the window. Unfortunately, over the years, the double-glazed window has grown a spotty diffusing layer of something (mold? dried rainwater seepage?) that makes it less than ideally clear. I tried a solid, 2 inch spot of aluminum foil, and the shadow is barely visibly 8 feet from the glass. I don’t think the glazing is something I can disassemble for internal cleaning… I’ll print some positive and negative zone plates in different sizes this week, and see how they work. Dave From: Reinhold Kriegler [mailto:reinhold.krieg...@gmx.de] Sent: Sunday, March 04, 2012 11:51 AM To: 'Dave Bell' Subject: AW: Shadow caster ☺ I thought so!!! Actually: Inside the house a little sunspot is much more fun than a tiny dizzy and diffuse shadow point… Reinhold * ** *** * ** *** Reinhold R. Kriegler Lat. 53° 6' 52,6 Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N. GMT +1 (DST +2) www.ta-dip.de http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html http://www.ta-dip.de/dies-und-das.html -Ursprüngliche Nachricht- Von: Dave Bell [mailto:db...@thebells.net] Gesendet: Sonntag, 4. März 2012 20:35 An: ' Reinhold Kriegler ' Betreff: RE: Shadow caster Yes, I remember John’s work well, particularly for gnomon points, so the shadow isn’t truncated. I was thinking about putting a shadow spot on one of my living room roof windows, particularly with the approaching Equinox. But over an 8 to 15 foot throw, the shadow would be fairly diffuse. I’m going to try printing some large (1 to 3 inch outside diameter) zone plates on transparency film, and see how they work… From: Reinhold Kriegler [mailto:reinhold.krieg...@gmx.de] Sent: Sunday, March 04, 2012 10:05 AM To: 'David Bell'; sundial@uni-koeln.de Subject: AW: Shadow caster John Carmichael has done excellent research work about this question! Just contact him! Regards! Reinhold Kriegler * ** *** * ** *** Reinhold R. Kriegler Lat. 53° 6' 52,6 Nord; Long. 8° 53' 52,3 Ost; 48 m ü. N.N. GMT +1 (DST +2) www.ta-dip.de http://de.youtube.com/watch?v=XyCoJHwzzjUfmt=18 http://www.ta-dip.de/dies-und-das/r-e-i-n-h-o-l-d.html http://www.ta-dip.de/dies-und-das.html -Ursprüngliche Nachricht- Von: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] Im Auftrag von David Bell Gesendet: Sonntag, 4. März 2012 18:50 An: sundial@uni-koeln.de Betreff: Shadow caster Several times, there have been discussions about how to improve the shadow cast by a point nodus. I partially recall some conclusions regarding the optimum diameter vs, throw length, and some thoughts about adding an annular ring to take advantage of diffraction. Can anyone help remind me? Thanks! Dave Sent from my iPhone --- https://lists.uni-koeln.de/mailman
