The CIE 1931 [[Color Space]] is a color space / standard which defines the relation between the [[Visible Spectrum]] and color perceived by the [[Human Eye]]. Given an incoming spectra $f(\lambda)$, where $f$ is a [[Function]] with [[Wavelength]] [[Domain]] $\lambda \in [0,\infty)$ and [[Codomain]] $f(\lambda) \in [0,\infty)$. $\huge \begin{align} L &= \int_{0}^{\infty} \bar{L}(\lambda)f(\lambda)\d \lambda = \braket{\bar{L},f} \\ M &= \int_{0}^{\infty} \bar{M}(\lambda)f(\lambda)\d \lambda= \braket{\bar{M},f} \\ S &= \int_{0}^{\infty} \bar{S}(\lambda)f(\lambda)\d \lambda= \braket{\bar{S},f} \\ \end{align}$ This can be thought of as an [[Functional Inner Product|Inner Product]] inside a functional vector space.