PS_Color - The radiance spectrum of the light reaching the...

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1 The radiance spectrum of the light reaching the eye from a surface is the product of the irradiance spectrum falling on a surface and the reflectance function of the surface.
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2 The radiance spectrum of a surface is the product of the reflectance function of the surface and the irradiance spectrum of the light falling on the surface.
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3 Five Difficult Problems for the Visual System Context problem Objects often appear in a complex and varying context of other objects, making recognition of objects difficult. Viewpoint problem Objects are rarely seen from the same viewpoint, making recognition of objects difficult. Category complexity problem The specific objects that define a category are often quite different, making object categorization difficult. Illumination problem The illumination of scenes is highly variable, making recognition of objects and materials difficult. Depth problem The images in the eyes are two-dimensional projections of the three-dimensional world, thus the third dimension (depth) is lost and must be figured out. A big part of the illumination problem is that the light reaching the eye from an object is greatly affected by the spectrum of the light falling on the object. I will come back to this a little at the end, but first we need to discuss the basics of color vision.
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4 Illustration of the distribution of the three types of cone photoreceptors in the human fovea. See, for example, Roorda, A. and Williams, D.R. Nature , 397, 520-522 (1999).
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5 Spectral sensitivities of the three classes of cone photoreceptor.
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6 The response of a receptor is the product of the intensity of the light and the sensitivity of the receptor. The thick arrow and the dashed arrow pointing to the left show the response of the receptor to a particular wavelength (one that looks orange to us). The black curve shows what the response of the receptor would be at each wavelength, assuming each wavelength presented to the eye has the same physical intensity.
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7 Here we see that a blue wavelength and an orange wavelength of the same intensity produce the same response. This illustrates the principle of univariance: a receptor’s response only signals how much total light is absorbed, not which wavelengths are in the light. Having only one type of receptor would make telling the difference between lights impossible on the basis of wavelength alone. For example, switching between the orange and blue light in this case would not produce a change in the response of the receptor.
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8 A numerical example Wavelength Intensity Sensitivity Response 450 nm 200 0.5 100 625 nm 200 0.5 100 550 nm 200 1.0 200 550 nm 100 1.0 100 A numerical example of calculating the response from the intensity of a pure wavelength and the spectral sensitivity curve of the receptor. Notice that with only one receptor type, every wavelength can be adjusted in intensity to produce the same response as any other wavelength. Therefore, at night when only the rod
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PS_Color - The radiance spectrum of the light reaching the...

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