The article discusses the relationship between color sensations and reflectance in the visual system, challenging the assumption that color is determined by the amount of light reaching the eye. The authors propose the Retinex theory, which suggests that the visual system uses three independent cone systems, each sensitive to long-, middle-, and short-wavelength light, to form separate images of the world in terms of lightness. These images are compared to generate color sensations, which are independent of the flux from objects. The authors describe a mathematical scheme for generating lightness numbers, which are the biological correlates of reflectance, and demonstrate that color sensations are not dependent on the flux-wavelength distribution. They use a series of experiments, including the Mondrian pattern, to show that color sensations are determined by the reflectances of the objects, not by the illumination. The article also introduces the concept of edges as a source of lightness information and describes a physical model of the retinex mechanism, involving pairs of photoreceptors that detect ratios of luminances across edges to determine reflectances. The model is implemented in an electronic device that reproduces the Mondrian pattern, demonstrating the independence of lightness from illumination changes. The authors conclude that the retinex theory provides a framework for understanding how the visual system can determine reflectance in scenes with unknown illumination and reflectance.The article discusses the relationship between color sensations and reflectance in the visual system, challenging the assumption that color is determined by the amount of light reaching the eye. The authors propose the Retinex theory, which suggests that the visual system uses three independent cone systems, each sensitive to long-, middle-, and short-wavelength light, to form separate images of the world in terms of lightness. These images are compared to generate color sensations, which are independent of the flux from objects. The authors describe a mathematical scheme for generating lightness numbers, which are the biological correlates of reflectance, and demonstrate that color sensations are not dependent on the flux-wavelength distribution. They use a series of experiments, including the Mondrian pattern, to show that color sensations are determined by the reflectances of the objects, not by the illumination. The article also introduces the concept of edges as a source of lightness information and describes a physical model of the retinex mechanism, involving pairs of photoreceptors that detect ratios of luminances across edges to determine reflectances. The model is implemented in an electronic device that reproduces the Mondrian pattern, demonstrating the independence of lightness from illumination changes. The authors conclude that the retinex theory provides a framework for understanding how the visual system can determine reflectance in scenes with unknown illumination and reflectance.