by tonytran2015 (Melbourne, Australia).
You may already have in your possession some excellent lenses for making fires. Knowing about them will help you make fire quickly from sunlight when facing an emergency. All lenses described in this blog can make a cigarette smoulder in less than 30 seconds and can light up that cigarette afterwards.
1. Concentration of radiating heat flux by a lens.
The radiating heat flux from the Sun is concentrated by a factor C given by
C = Pi*d*d/(Pi*f*a*f*a) =
C = (d/f)*(d/f)*(1/a)*(1/a).
where d and f are respectively the diameter and focal length of the lenses and a is the angular diameter of the Sun (a = 0.5*3.14/180radian = 0.0087 radian).
The angular diameter of the Sun cannot be changed. It is obvious that we have to increase the ratio (d/f), which is called the aperture number of the lenses, to increase the heat flux concentration. The deciding factor for success is the aperture ratio of the mirror or lens system.
C = 13200*(d/f)*(d/f).
This concentration of heat flux is remarkably high for any lens system with D/f of more than 1/3 and can be used to ignite properly prepared tinder to make fire in survival situations.
The calculated concentration of radiating heat flux can only be achieved using precision optics. Any imperfection on the surface of the lens disperses the image of the Sun and reduces the concentration drastically.
2. A transparent perfect sphere.
Figures 1 & 2 : A small clear sphere.
You may have a small clear glass sphere somewhere in your household either as a decorative item, or as a bottle stopper, etc…
The small transparent sphere I use here is a small (25.4 mm diameter) clear quartz sphere often used as an item of curiosity and often called a “crystal sphere” by fortune tellers.
Calculations using geometrical optics show that (for n=1.5) the focal point is about 0.5 radius outside the surface of the ball and the equivalent focal length of the sphere is about 2r×(3/4)= 1.5r
Even if only rays of distance less than 0.5 radius (from the central ray) converge on the spot, the aperture of the sphere is still
D/f = (2*0.5r)/(1.5r) = 1/(1.5),
a high value for aperture.
At an aperture value of 2/3, the concentration of radiating heat by sunlight is
C = 13200*(D/f)*(D/f) = 6000.
The actual aperture number of this clear sphere is higher than 1/1.5 and the sphere can be used as a lens to light up cigarettes using sunlight.
Figures 3 : Lighting a cigarette by a small 25mm clear quartz sphere.
The above photo also demonstrates the danger of leaving clear glass balls on any combustible surface. When the afternoon Sun comes down to an elevation of 41degree (= arcsin 0.6666), the image of the Sun is exactly on the combustible material and combustion becomes a real possibility !
The concentration factor of C = 6000 is only realized with a perfect sphere. For any body of revolution of nearly spherical shape, the concentration of radiating heat flux is much lower, and may come down to C = 100 when there is some appreciable astigmatism. For this reason, PRECISION IS MORE IMPORTANT THAN SIZE for transparent spheres.
I prefer having a small perfect sphere to a large approximate sphere.
3. A watchmaker double staged ocular.
Figures 1 & 2 : A watch maker monocular.
Figures 3 : Lighting a cigarette by a watch maker monocular.
A monocular is a compound magnifying lens used by watchmakers to see small details of watch movements. It is a simple version of the oculars used for each eye in common sport binoculars.
Every first aid kit for hikers should have this light weight and useful device and a pair of sharp tweezers for detecting and removal of hurting spikes or splinters sticking in the skins.
The lens nearer to the eye of a proper ocular has a large diameter to give the eye a wide field of view. The double stage makes tiny object has a large image at infinity.
When sunlight travels in the reverse direction from the back (big) end to the front (small) end it will focus at a tiny spot outside the ocular and about 20mm from the front lens.
The aperture ratio D/f of this compound lens is about 1/2.
If the dark end of a cigarette is placed at that bright tiny spot smouldering will begin in less than 1 second on a sunny day.
If the oculars of your binoculars are thread removable then you can also use them in emergency but keeping in mind that taking apart a pair of binoculars will allow dirt to contaminate it.
The concentration factor of C = 13200*(d/f)*(d/f) is only realized with a lens having perfect spherical surfaces. For any lens with only approximate shape, the concentration of radiating heat flux is much lower, and may come down to C = 100 when there is some appreciable astigmatism. For this reason, PRECISION IS MORE IMPORTANT THAN SIZE for any monocular.
I prefer to have a small quality monocular than a large one with low quality.
4. Aspherical condenser lenses.
Figures 1 & 2: Different views of a fused quartz aspherical lens.
I was lucky to be given a fused quartz, thick aspheric lens. Its aperture ratio is about D/f=1/1.5 while other thin spherical, glass lenses have a ratio of less than 1/2.5.
The concentration of solar heat flux is much higher when using it than when using an ordinary magnifying glass.
Figure 3: A one battery LED torch with a thick aspherical lens at the front.
The front lenses of zoom focus LED torches are low cost substitutes for such aspherical lens. They do work exceptionally well and are even unbreakable since they are made from (acrylic ?) plastic.
The disadvantage of using any such aspherical lens for making fire is that it is heavy and it has very short distance from its flat side to the bright focal point. Sunlight coming to this bright focal point from many widely separated directions and it is difficult to direct them all to the trough of the dimple at the center of the end of a cigarette. I found that it is less easy to light up a cigarette using an aspherical lens than using a good monocular of the same diameter.
Again, it is important to also note that PRECISION IS MORE IMPORTANT THAN SIZE for any aspherical lens.
I prefer to have a small quality aspherical lens than a large one with low quality.
5. A flexible Fresnel lens for wallet.
Figure 1: A thin flexible Fresnel lens for wallet. Concentric grooves can be noticed at the right hand corners of this picture.
The lens is usually made of thin, flexible, soft clear plastic of the size of credit cards. This type of lenses is sold as wallet sized magnifying glasses for map reading.
A Fresnel lens has high aperture ratio and can be used to light up cigarettes with ease. However it needs to be properly cleaned after each use as it is easily scratched.
6. Method of lighting up a cigarette using a small lens.
1. A cigarette or its imitation made up from rolled up toilette tissue sheets with darkened ends seems to be the readily available suitable tinder sticks for making fire using sunlight.
2. A conical dimple should be made at the dark end of a cigarette. The depth of the dimple should be about the size of its radius. That is a conical concave surface should be made out of the dark flat tip of the cigarette. This tiny concave surface reduces radiating heat loss from the fire to be started at its trough.
3. A precision lens is used to focus sunlight onto the dark end of a cigarette. PRECISION IS MORE IMPORTANT THAN SIZE as the high concentration of sunlight depends on precision of the lens surface.
4. The axis of the lens system should point exactly at the Sun to have maximum amount of sunlight converging on the focal point.
5. Sunlight should be focused on one point on the surface of the dimpled end of the cigarette to blacken it. Smoke should be seen arising from the spot within 10 seconds after focusing. Other points of the surface should then be smothered to have all the surface gradually blackened.
6. Sunlight is now focused on the trough point of the dimpled surface. Smoke will be seen and the trough will glow red when assisted by gentle wind blowing toward the other end of the cigarette.
7. The cigarette should now be smoked or blown externally so that hot fume from the flame end travels toward its other end to heat up the adjacent zone to ready it for combustion.
8. A strong red glow indicates that the cigarette has been burning. It can now be used to start up a fire.
. tonytran2015, Making fire and lighting cigarettes with sunlight, survivaltricks.wordpress.com, Making fire and lighting cigarettes with sunlight, posted on February 27, 2016
. tonytran2015, Mirror for making fire using sunlight, survivaltricks.wordpress.com, Mirror for making fire using sunlight, posted on April 13, 2016
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