for a radiant energy of at a frequency of , which corresponds to a standard air wavelength of rather than , which is the peak of the luminosity curve. The value of (λ) is at , so that a value of 683/ = 683.002 is the multiplicative constant. The number 683 is connected to the modern (1979) definition of the candela, the unit of luminous intensity. This arbitrary number made the new definition give numbers equivalent to those from the old definition of the candela. Improvements to the standard The CIE 1924 photopic V(λ) luminosity function, which is included in the CIE 1931 color-matching functions as the (λ) function, has long been acknowledged to underestimate the contribution of the blue end of the spectrum to perceived luminance. There have been numerous attempts to improve the standard function, to make it more representative of human vision. Judd in 1951, improved by Vos in 1978, resulted in a function known as CIE VM(λ). More recently, Sharpe, Stockman, Jagla & Jägle (2005) developed a function consistent with the Stockman & Sharpe cone fundamentals; their curves are plotted in the figure above. ISO standard The ISO standard is ISO/CIE FDIS 11664-1. The standard provides an incremental table by nm of each value in the visible range. Scotopic luminosity For very low levels of intensity (scotopic vision), the sensitivity of the eye is mediated by rods, not cones, and shifts toward the violet, peaking around for young eyes; the sensitivity is equivalent to or at this peak. The standard scotopic luminous efficiency function or V(λ) was adopted by the CIE in 1951, based on measurements by Wald (1945) and by Crawford (1949). Color blindness Color blindness changes the sensitivity of the eye as a function of wavelength. For people with protanopia, the peak of the eye's response is shifted toward the short-wave part of the spectrum (approximately 540 nm), while for people suffering deuteranopia, there is a slight shift in the peak of the spectrum, to about 560 nm. People with protanopia have essentially no sensitivity to light of wavelengths more than 670 nm. Most non-primate mammals have the same luminous efficiency function as people with protanopia. Their insensitivity to long-wavelength red light makes it possible to use such illumination while studying the nocturnal life of animals. For older people with normal color vision, the crystalline lens may become slightly yellow due to cataracts, which