The requirement that the z ¯ ( λ ) {\displaystyle {\overline {z}}(\lambda )} function be zero above 650 nm means that the line connecting Cg and Cr must be tangent to the gamut in the region of Kr.
z ¯ ( λ ) {\displaystyle {\overline {z}}(\lambda )} 函数在650 nm处为零的要求意味着连接Cg和Cr的直线必须是Kr区域内的色域的切线。