Perceived Brightness
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Index Vision concepts Reference Williamson & Cummins p177-178 | ||
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Index Vision concepts Reference Williamson & Cummins p173 | |||||
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Photopic Vision
Each wavelength has a relative value for the Standard Observer's sensitivity, the luminous efficacy at that wavelength, Vλ. The value of Vλ is designated as unity at 555 nm and decreases to zero at the ends of the visible spectrum. This is associated with the daylight vision of the human eye, also known as photopic vision. In lowlight conditions, the efficacy curve shifts toward the blue end of the spectrum due to the sensitivity of the eye. Chemical changes in the eye at night shift our vision to the scotopic range. This differentiation between light and dark vision is caused by the activity of the rods and cones in the retina, and their sensitivity to light. At 555 nm, this efficacy translates to a luminous flux of 683 lumens/W, and thus a fraction of that value at wavelengths to either side of the visible spectrum. This value is derived from the definition of the candela directly. |
Index Vision concepts Reference Williamson & Cummins p173 | ||
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Scotopic Vision
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Index Vision concepts Reference Williamson & Cummins p173 | |||
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