Hi All,
Well, I have some serious forelock tugging and bowing and scraping to do. I got it wrong for brushed stainless steel. I decided to start a new thread as reflection is real with some materials.
As you can see from the new pics, brushed stainless steel does indeed reflect so be careful.
Obviously the rate of thermal conductivity of stainless is not great enough to absorb all of the energy and some is reflected.
It is obviously a lower amount of energy as you can see from the small hole in the pics, it barely charred the edges of the paper. But reflected energy, there is and enough to instantly burn a hole in the paper.
For the second pic, I removed the stainless and brought the focus up to burn the paper at low power and high speed and moved the circle off to the side a bit. I wanted to get a comparison between the diameters of the actual hole traced on to the stainless and the one burned by reflection. As you can see there is not much divergence.
The final test was to tape some paper to the underside of the glass in the lid. This produced a very faint scorch mark for the first pass. I did 5 more passes and third pic shows the pattern emerging. The beam is diffusing probably from the surface of the brushed stainless steel.
The interesting thing is the beam is not diverging much. The burn size is about double the actual circle size traced on to the stainless. That burn on the paper taped to the lid was made about 10" from the stainless surface.
With high thermal conductivity materials it seems reflection is not so much an issue, but low thermal conductivity, take GREAT care. The glass on the lid did get a little warm and I suspect that the scorch would have been a nasty burn on skin even on a single pass.