while looking at spark lite found this rather ingenious/strange contraption Solar spark lighter

The principle is pretty old. I even found it in a survival manual more than a decade ago. I admit that it´s cool but it it´s very limited in use. So you may find items for fire starting that give you more value for the same bulk or price.
I´d say it primarily has a toy value. It seems to work best when you least need a fire.

ooooo, the Death Ray!!!!

Mythbusters was just on about this.

I had one similar to that 20 or 30 years ago. It was sold as a solar cigerette lighter novelty lighter. Yoe they do work. No they are not worth the weight and bulk. It is basicly a toy. You would be better off getting a credit card sized fesnel lens. A lot smaller. Weighs next to nothing. Can fit in your wallet. Magnifies small objects. And will start a fire quicker under the same conditions. But the biggest factor is the lens costs less than a buck. The last time I looked the price was $.49 for one versus almost $ 12.00.

The same principal is being used with some suscess to build solar cookers in developing nations. When 20% of your income is spent on fuel or your children are not going to school because they cut firewood all day, these things are like gold. The trick is to try using locally available materials to manufacture them. But they are working and being well received.

re: Solar Cookers.
My daughter travels frequently in Central America. After her last trip she described a solar cooker built into the side of a house. In the kitchen was an oven door on the wall. When she opened it, she found a well insluated cavity with a glass "outside wall" which of course faced the south. She reported that it worked very well and cost nothing to operate.