This Solar Burner Tube will collect sun light and focus it in a small point at the smaller end of the tube. You will have to coat the inner surface with a reflecting material. For me a reflecting tape worked well enough to melt black plastic. I guess, the Solar Burner Tube (h40fa18) can increase the sun light's energy density by a factor of 45.
Never look into the focused light spot. Regular sun glasses will not prevent eye damage. You will need a professional eye protection. Store it in a fire prove place like a metal box.
First check out "Maximum permissible exposure":
The power density of the sun on earth is about 0.1 W/cm^2. A maximum of 0.001 W is defined to be save in Laser Class 2 (pupil's surface = 0.39 cm^2). You should never look directly into the sun. And you should never look at a 45 times stronger light source, too.
Some calculations for version h40fa18 :
My Solar Burner Tube's effective area is A_top - A_bottom = 55.4 cm^2 - 10.2 cm^2 = 45.2 cm^2. Let's say the focus area size is about 1 cm^2. That's up to 4.52 W focused in 1 cm^2.
Well, tell me if there are mistakes in the calculations.
About the surface:
My printer doesn't produce a smooth surface. I always can feel the lines of the print. But a mirroring surface needs to be very smooth.
By Using a mirroring tape from the hardware store, I had my best results.
I uploaded some pictures of the narrow tube. I tried to cover the surface by a mirroring paint. But as you can see, the paint doesn't stick to it. The ripples lead to a scattering surface. There is no mirror effect by this coating.
Then I tried to melt the surface by a flame. But this leads to a deformation.