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Friday
Oct052007

Lecture 2 - light and vision

What sort of frames per second is needed for perception of smooth movement? In the UK, TVs refresh at 50 Hz (60Hz in the US). Apparently the picture only gets changed every other cycle, so that's 25 frames per second. Film is apparently 24 frames per second (which apparently causes some issues that we'll revisit in a later lecture... Should come back and link when we do!). Apparently at 20 frames per second you start to notice jerkiness, and 15 frames per second looks really jerky (I'm sure that could be a useful thing to know if you wanted that kind of effect).

A brief refresher on electromagnetic radiation (of which visible light is a tiny range in the middle):

wavespeed (in this case c) = frequency x wavelength

Visible light has a wavelength of between 400nm (blue) and 700nm (red). That's tiny, but there's still a huge difference in the wavelength of blue light compared to red. That makes it difficult to focus on red and blue at the same time (would it be easy if the wavelengths were  exactly double?). It also explains the sky being blue apparently, because the shorter wavelength of blue light means it's much more easily scattered, whereas the longer wavelength light comes straight to us from the sun. The sun therefore looks a lot more orange from Earth than it does from space (where there's no atmosphere to scatter the light!). And when the sun sets the light has to travel through more of the atmosphere, so longer wavelengths get scattered. Therefore the sky takes on the different colours, and the sun gets more and more red as the range of wavelengths that comes straight through to the eye gets smaller.

The sensors in the eye are split into two types - rods (which respond to light and dark) and cones (respond to colour). The cones are concentrated where we focus, while our peripheral vision is more dominated by the rods. At night the cones stop working (they need high light levels) so our night vision is dominated by the rods, which means you can often see things in your peripheral vision that you then can't see as clearly when you focus on them.

Rods react to a frequency range that overlaps mostly with blue and green. They do not respond to red light particularly. This can be useful for (for example) dials on a ship, where the dials can be displayed in red for the cones to pick up on, without disturbing the rods 'night vision' that can be maintained for looking out for things like iceburgs(!).

The primary colours are defined by the cones in the eye. The cones are responsive to three different frequencies. Blue, green and red. Apparently fish only react to blue and yellow, but on leaving the primordial soup it became more useful to split out the yellow into green and red. Because this is a relatively (in evolutionary terms) recent development red-green colourblindness is still fairly common.

The primary colours for active displays (those that create the light - e.g. monitors) are therefore red, green and blue. Passive displays work by reflecting light, showing colours by absorbing the other wavelengths. The primary colours for these are therefore cyan (absorbs red light), magenta (absorbs green light) and yellow (absorbs blue). This gives rise to the CMYK colour value (with K being for blacK - the pigments in ink being not sufficiently good to generate pure black so a separate cartridge is needed).

I need to do some follow up research on colour gamuts, because I wasn't entirely clear what the graphs he was showing us represented, or what the axes stood for. Meant to ask, but the queue of people waiting to talk to him was a little off-putting. I'm sure there are some resources out there!

The other way of measuring a given colour is to represent it as the chromacity (the colour defined by the main frequency of the light) the luminosity and the excitation. Saturation and brightness I think, but again, need to follow up on that.  

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