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Cheap laser pointers


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About this observation
Peter Anderson
Time of observation
23/04/2017 - 08:00
Green Laser Pointer
Observing location
Brisbane Australia
Standard camera

I have often wondered about the spread of the beam of cheap hand held laser pointers. I set up the pointer to shine on a card at our front boundary 160 metres distant. The experiment was conducted near dusk so both the card and the beam were visible. I pointed the beam assisted by a telescope and my wife imaged the card at close range and in one case her shadow partially obscured the beam. This shows its intensity at that range. The size of the exit lens of the pointer is ~4.5mm.  At 150metres the bulk of the illumination is contained within an oval area around 10cm in average size. At 1km it would be at least 60cm across and I contend that at distances of several kilometers its illumination would simply fade into the background unless the night was quite dark.  Other units may have a more coherent beam, I don't know, but what appears to be a very 'tight' beam when using such a unit as an astronomical pointer, ain't necessarily so!  NEXT PROJECT: Checking that the light in the fridge 'goes off' when you close the door. (Don't laugh, I have done it... and admit that you have too!)

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