Angelo Alaimo and Michael Seliske are two ECE students who have had several co-op placements within the consumer imaging industry. Over our terms, we both learned the fine details and behind the scenes aspects of digital photography, and would like to share some of that insight and passion with you. Over this term’s Iron Warrior issues, we plan to bring you information related to imaging technology and what you learn might surprise you.
A photograph is defined within Webster’s dictionary as “the art or process of producing images by the action of radiant energy and especially light on a sensitive surface.” It’s a pretty generic definition, but imaging and, as an extension, digital imaging is a complex field that requires many components to work together in order to produce a final image that can be stored and viewed later. At the forefront of this system is the understanding and mimicry of the human visual system.
To begin, stop and think on how one could describe what the colour red is to an alien species. One might start by quantitatively explaining that it’s a small portion of the light spectrum with a wavelength between 635-700 nanometres, but that doesn’t really mean anything to the alien. One’s next thought might be more along the lines of “red is just red” but red is actually the colour that humans perceive when light of those wavelengths enter our eyes, and so it could look very different to someone with a different visual system then ours.
The human visual system is based on two different types of receptors within our eyes. These receptors are called rods and cones, and they function differently from each other. First are rods, which have higher sensitivity to light than cones but are monochromatic, which means that they cannot distinguish colour. Try it out yourself – go into a dark area and notice that it’s difficult to see colour. You’re essentially seeing just a black and white image. Second are cones, and there are three different types which are sensitive to a different spectral range of visible light. The ‘L’ cone is sensitive to “long” wavelengths (red), The ‘M’ cone is sensitive to “medium” wavelengths (green), and the ‘S’ cone which is sensitive to “short” wavelengths (blue). Each of the cone’s spectral sensitivities overlaps one another in order to allow us to perceive more colours.
Graph illustrating spectral overlap
Once light is collected by the rods and cones, the brain decodes the data and allows us to see a colour image. The distribution of the rods and cones is not even across the retina, with a large concentration of cones contained in the fovea, which is about 4 to 8 degrees from the optical centre of the retina.
The basic set up of the human eye and how it perceives the world is a large component of imaging. However, it’s impossible to mimic the eye completely – for example reproducing colours so that they are properly viewed by humans can be difficult. There are two ways of delivering colour to the eyes. One is active colour which produces the light and delivers the specific wavelengths to the eye (computer monitors, LCD screens) and the other is passive colour which filters out wavelengths and only allows the desirable wavelengths to be reflected to the observer (any printed media). Although these modes of colour reproduction are totally different, there has been a lot of research regarding the ability to reproduce a colour using any type of device, and that is the basis for many colour scientists’ research.
There is so much information regarding how the human visual system affects the imaging world, but it is too much to include in a short article. If you enjoyed this short introduction, please take a look online for more information on the human visual system and how it works. In the next issue, we’ll be discussing the basics of how an image sensor is used to create a digital image.
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