Smaller sized but powerful spot Fresnel lens measuring 280mm x 280mm, this size fresnel lens is great for experimenting with and seeing what spot fresnel lenses are capable of in bright sunshine,they are excellent solar concentrators and can be used in the construction of solar cookers and ovens which can be made incredibly cheaply,free plans for these can be found very easily on the internet,a solar cooker in its most basic form can be a cardboard box,some polystyrene insulation,silver foil to line it and a see through plastic lid or window, left in the sun it will slowly cook food during the day,by using a fresnel lens instead of a clear plastic or glass window sunlight can be concentrated to a spot inside the cooker,preferably the black cooking pot (must be black) which means the food inside it will cook much more efficiently and quicker,this size fresnel lens is ideal for use in a small to medium sized solar oven or cooker for concentrating sunlight to cook thecontents that bit faster, its also great for camping and barbecues as it can be used to light fires very quickly in bright sunshine,just concentrate the sunspot on a piece of crumpled newspaper and it will go to flame in a few seconds, wood can also be burnt although it will take much longer to go to flame with this size fresnel lens,it will certainly char it pretty well though, fresnel lenses can also be used with homemade copper pipe solar water heaters or solar air heaters to boost their output and they can also be used with small photovoltaic solar panels to boost their current output though extreme care must be taken if you want to try this as they can cause photovoltaic solar panels to overheat and be destroyed,the specs for these particular industrial fresnel lenses are as follows,Size:280*280mm,Thickness: 2mm ,Material: PMMA,Groove pitch: 0.5mm ,Focal length: 220mm.WARNING : Fresnel lenses can be very dangerous if used incorrectly,they should never be left outside uncovered when unattended and definitely should never be left leaning against a wooden fence or building in the sun,they should be kept indoors covered up or laid flat when not in use so as not to create a potential fire hazard,they can cause serious burns if a sunspot is made on open skin or clothing so care must be taking when using them,basically treat it like a mini blowtorch whenever its sunny ! Price includes Uk delivery.