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Diffraction Limiting a Mk I Eyeball
On Jan 6, 8:25*am, Helpful person wrote:
On Jan 6, 3:23*am, "N_Cook" wrote: Helpful person wrote in message ... On Dec 30 2010, 8:06 am, Martin Brown ||| wrote: I was doing a Christmas lecture earlier in the the week on "Light and Colour". I had an unexpectedly strong positive audience reaction from a very simple and cheap demonstration using 0.1-0.2mm diameter pinholes in aluminium foil held to the eye. Materials are so cheap and prep so easy that everyone can see this with their own eyes. And it is far more convincing to them than any number of fancy laser based demos! This is roughly equivalent to having a pupil stopped down to 1/10 or 1/20th of the normal 2mm daylight size and is small enough to give a very clear stable Airy disk on point sources like Christmas tree lights whilst still retaining colour vision. The bullseye appearance of point sources came as a big surprise to those without a science background. Edges are obviously soft and bright areas surrounded by a darker halo.. The main light source was a desktop quartz halogen with foil hung over the front and a 2mm hole in line with the bulb filament. This gave very pleasing views of the diffraction pattern of the pinhole. But it was the coloured Christmas lights that people found most striking. And by swapping pinholes (which were of somewhat variable size) with their neighbours the audience quickly realised for themselves that the smallest pinholes had the biggest and faintest diffraction patterns. It also increases depth of field allowing very fine print to be read with relative ease when held suitably close to the eye. I discovered in the process that a few young children in the audience with otherwise normal vision could read 2pt text with the unaided eye! I hope this is of use to others in showing convincing diffraction effects to a lay audience using minimal cheap hardware. NB the real Royal Institution Christmas science lectures are titled "Size Matters" and are being televised on BBC4 at 8pm this week. Regards, Martin Brown You can also easily demonstrate the effect of aperture on MTF (modulation transfer function). *Look at cubism type pictures or pixelated pictures on TV (used to hide identity). Using one eye, squint at the object. *This reduces the aperture, cutting out the high frequencies and removing the sharp boundaries. The "original" picture can be seen quite clearly. www.richardfisher.com &&&&&&& But on the other hand if the light level is high enough and you've forgotten your reading glases - you can read small print by "stopping down" an eye by viewing through the central hole between bent-over thumb and first skin fold of the index finger of a partial fist-making. Limiting to the central least distorted part of the eyeball It's not a matter of "less distorted" or aberrations it's a matter of spatial filtering. www.richardfisher.com I know nothing of the science, but I have used this trick many times. I use thumb, index, and social fingers to make a tiny triangle peep hole. It allows near focus on objects almost in contact with the cornea. I just tested it on my monitor, and I can see the 3 colored bands (couldn't quite make out the individual pixels). |
#12
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Diffraction Limiting a Mk I Eyeball
On 06/01/2011 15:25, Helpful person wrote:
On Jan 6, 3:23 am, wrote: Helpful wrote in message ... On Dec 30 2010, 8:06 am, Martin Brown||| wrote: I was doing a Christmas lecture earlier in the the week on "Light and Colour". I had an unexpectedly strong positive audience reaction from a very simple and cheap demonstration using 0.1-0.2mm diameter pinholes in aluminium foil held to the eye. Materials are so cheap and prep so easy that everyone can see this with their own eyes. And it is far more convincing to them than any number of fancy laser based demos! This is roughly equivalent to having a pupil stopped down to 1/10 or 1/20th of the normal 2mm daylight size and is small enough to give a very clear stable Airy disk on point sources like Christmas tree lights whilst still retaining colour vision. The bullseye appearance of point sources came as a big surprise to those without a science background. Edges are obviously soft and bright areas surrounded by a darker halo. You can also easily demonstrate the effect of aperture on MTF (modulation transfer function). Look at cubism type pictures or pixelated pictures on TV (used to hide identity). Using one eye, squint at the object. This reduces the aperture, cutting out the high frequencies and removing the sharp boundaries. The "original" picture can be seen quite clearly. www.richardfisher.com &&&&&&& But on the other hand if the light level is high enough and you've forgotten your reading glases - you can read small print by "stopping down" an eye by viewing through the central hole between bent-over thumb and first skin fold of the index finger of a partial fist-making. Limiting to the central least distorted part of the eyeball It's not a matter of "less distorted" or aberrations it's a matter of spatial filtering. He is correct. When the pinhole aperture is small enough the eye lens geometry becomes irrelevant provided that it is transparent. It is not for nothing that eye tests are conducted in semi darkness to better measure the overall lens properties accurately at maximum aperture. Spatial filtering of the lens is frequency domain filtering in the final image which makes the diffraction effects stronger but largely eliminates all geometrical lens abberations. It is the pinhole imaging approximation that matters when the aperture is so greatly curtailed. Regards, Martin Brown |
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