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There's a glut of gimmicky little "plasma lighters" on the
market, and I have to say I'm a fan. No butane (so you can
take 'em with you when you fly), no fluid to smell funny,
and they're not bothered by dampness.
At first blush, this would SEEM ideal for an
emergency/survival kit, too, except
that after being
forgotten for years, the battery is likely to be flat.
We've all started fires with a magnifying glass before, and
that technique is well proven. But I never seem to have full
sun when I need to start a fire!
So what I need is a plasma-lighter with a solar panel, to
"time-shift" the sun's energy. Gather it when it shines, be
ready with a spark after dark.
Probably equip this thing with a supercapacitor rather than
a lithium battery, to better handle life in a mostly-
discharged state. And of course, the old USB jack can stay,
since sometimes I do have juice available that way, and it's
way faster than leaving it to bask.
Lenses
https://en.wikipedi.../wiki/Lens_(optics)
Very simple. [8th of 7, Dec 16 2017]
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You just need a portable sun. |
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Begs the question, how small can the solar furnance focal point be ? |
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Theoretically, there's no lower limit; practically, it's limited by the
aberration in the optical path. |
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A catadioptic tertiary stage, using actively cooled, temperature
controlled surface silvered mirrors, can produce energy densities
comparable with fusion reactors and supercolliding accelerators. |
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//Theoretically, there's no lower limit// |
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Hang on. We've done this before. Last time, I was under the misunderhension that an image of the sun can be focussed down to any size; but this isn't the case. The image can only be focussed down until it is "in focus", at which point you can't make the light intensity any higher. Hence, the attainable energy density is limited, and the attainable power is limited by the size of the lens or mirror. |
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Incidentally, why isn't there a wind-up version of a car's cigarette lighter (ie, a filament which is heated by electricity that comes from cranking a handle)? |
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Not really an improvement but I wonder if a 3x3 grid of
deep IR emitting LEDs (#) could make a an optical image
with high enough warmth to be focused to an ignition point? |
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I'm not sure, but that problem seems similar to the problem of using LEDs for microscope illumination (a problem which I have recently encountered). The limitation is the energy density of the LED itself, which you can't exceed by focussing. So the maximum intensity of illumination is the same as the intensity immediately adjacent to the working part of the LED. |
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Lasers are probably what you want. |
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// The image can only be focussed down until it is "in focus", at which point you can't make the light intensity any higher. // |
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That's only if you want to create an image. |
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Do you want this explained using the New Cartesian optical convention, or the real-is-positive convention ? |
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Either way, you're confusing "focal point" with "focussed image" (projected real image). |
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Well, I can only go by what the inventor of the modern confocal microscope told me. I'm fairly positive he's real. His name's Brad. |
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(+) but far simpler to carry a rechargeable 9 volt battery and some steel wool. |
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My guess is that Brad was not trying to start a fire in his
microscope, but to see things with it. This may have altered his
working assumptions. |
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I cannot be sure he wasn't trying to start a fire, but he did address the topic of concentrating light above and beyond its intensity at its point of origin. In fact, if I remember correctly (which sometimes happens by accident), he addressed it in response to a question from me which had been occasioned by this issue arising hithertofore on this very Halfbakery. |
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He used lots of words such as "etendue", but the bottom line was that no system of mirrors, lenses or other contrivances can concentrate light beyond its starting intensity. |
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Of course not, because that would violate the laws of
thermodynamics (conservation of energy). |
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Consider a perfect* biconvex lens with a frontal area of 1m2 and
a focal length of 1 metre. |
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The sun shines normal to the planar axis of the lens at an
intensity of 1 kW/m2. |
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Thus the energy density at the front of the lens is 1 kW/m2. |
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At the rear surface of the lens, the average energy density is the
same, but is no longer uniform. |
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500 mm behind the lens, the light energy is formed into a cone.
The energy density 400 mm from the primary axis is zero (for a
theoretical perfect system); the entire 1kW is now focussed into
an area of 0.26 m2, giving an energy density of 1/0.26, or 3.84
kW/m2. |
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At 900 mm behind the lens, the area is reduced to 0.01 m2, thus
the energy density is 1/.01, or 100kW/m2; the off-axis energy is
notionally zero. |
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At 1000 mm the theroretical energy density ,not achevable in
reality, spikes asymptotically. You still have 1 kW, but
concentrated on a very small area. |
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*optically perfect, lossless achromatic doublet. |
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// Brad was not trying to start a fire in his microscope // |
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Oh, so not proper science, then. If you don't have to leave the
room now and again while you wait for the smoke to clear, you
aren't doing it right. |
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OK [8th], in this idealised model with the metre-wide lens, the
incident light seems to be arriving as parallel rays. I mean, if
they're all normal to a given plane, then presumably they're all
parallel to each other. So, one difference between the ideal and
the real would be that the real sunshine is not quite parallel, but
slightly divergent. In my imagination, there is a distance behind
the lens at which this slightly divergent light makes an image of
the sun which is not arbitrarily small, but has a lower limit related
to the apparent size of the sun as seen from the lens. However,
I'm not sure what formula would define this minimum image size. |
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// not quite parallel, but slightly divergent // |
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This is theoretically so, but considering the relative distances involved (1.4 x 10^11 m vs. 1m ) and the apparent angular magnitude of the sun, even in the non-ideal case, the divergence is insignificant. |
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//spikes asymptotically// I am not sure if you meant asymptotocally or not. Howevertheless, at 1m you will form a perfect image of the sun - disc of small but definite size. All your captured energy is spread out over this small disc - it is not infinitely concentrated. |
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That's the highest density you can achieve - at any other position, you simply have a defocussed image of the sun, which will be an even larger disc. |
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