However, excessive UVR exposure accounts for only 0.1% of the total global burden of disease in disability-adjusted life years (DALYs), according to the 2006 World Health Organization (WHO) report The Global Burden of Disease Due to Ultraviolet Radiation. Excessive sun exposure can also cause cataracts and diseases aggravated by UVR-induced immunosuppression such as reactivation of some latent viruses. Both forms can damage collagen fibers, destroy vitamin A in skin, accelerate aging of the skin, and increase the risk of skin cancers. Sunburn is caused by too much UVB radiation this form also leads to direct DNA damage and promotes various skin cancers. UVA radiation (95–97% of the UVR that reaches Earth’s surface) penetrates deeply into the skin, where it can contribute to skin cancer indirectly via generation of DNA-damaging molecules such as hydroxyl and oxygen radicals. Most public health messages of the past century have focused on the hazards of too much sun exposure. Today’s scientists have come to a similarly dichotomous recognition that exposure to the ultraviolet radiation (UVR) in sunlight has both beneficial and deleterious effects on human health. For the ancient Greeks and Romans, Apollo was the god of medicine and healing as well as of sun and light-but Apollo could bring sickness as well as cure. * calculate a correction for that effect.Each day, Apollo’s fiery chariot makes its way across the sky, bringing life-giving light to the planet. * Fairchild and Pirrotta came up with a formula to * That's called Helmholtz-Kohlrausch effect. * Saturated colors appear brighter to human eye. Return radiansToDegrees(Math.atan(b / a)) + xBias * Converts a and b of Lab color space to Hue of LCH color space. * Converts Lab color space to Luminance-Chroma-Hue color space. "D65" is a standard 6500K Daylight light source. Pseudocode: function sRGBtoLin(colorChannel) The color in the CIE L*a*b* color space. Where V´ is the gamma-encoded R, G, or B channel of sRGB. sRGB (computer standard) for instance requires a power curve of approximately V^2.2, though the "accurate" transform is: Step One:Ĭonvert all sRGB 8 bit integer values to decimal 0.0-1.0 vR = sR / 255 Ĭonvert a gamma encoded RGB to a linear value. Because apparently it was lost somewhere. To determine perceived lightness, first convert gamma encoded R´G´B´ image values to linear luminance ( L or Y ) and then to non-linear perceived lightness ( L*) Gamma or transfer curve (TRC) is a curve that is often similar to the perceptual curve, and is commonly applied to image data for storage or broadcast to reduce perceived noise and/or improve data utilization (and related reasons). It is not to be confused with linear luminance. Luma ( Y´ prime) is a gamma encoded, weighted signal used in some video encodings (Y´I´Q´). However some color appearance models do have a value, usualy denoted as "Q" for perceived brightness, which is different than perceived lightness. (That is, linear to perception but therefore non linear to light).īrightness is a perceptual attribute, it does not have a "physical" measure. Perceived lightness is used by some vision models such as CIELAB, here L* (Lstar) is a value of perceptual lightness, and is non-linear to approximate the human vision non-linear response curve. It can be a relative measure, Y as in CIEXYZ, or as L, an absolute measure in cd/m 2 (not to be confused with L*). Luminance is a linear measure of light, spectrally weighted for normal vision but not adjusted for the non-linear perception of lightness. What follows is a correct and complete answer:īecause this thread appears highly in search engines, I am adding this answer to clarify the various misconceptions on the subject. relative to perception), Euclidian distances require a perceptually uniform Cartesian vector space such as CIELAB. The IEC standard for sRGB specifies a threshold of 0.04045 it is NOT 0.03928 (that was from an obsolete early draft).HSV and HSL are not even remotely accurate in terms of perception. Perceived lightness (L*) is nonlinear as is human perception.Luminance (L or Y) is linear as is light.sRGB must be LINEARIZED before applying the coefficients.The only answers that are accurate are the and answers, and in support The other answers (including the accepted) are linking to or citing sources that are either wrong, irrelevant, obsolete, or broken. The "Accepted" Answer is Incorrect and Incomplete
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