A filter for detecting changes in skin color. The filter includes a filter material capable of filtering a frequency range and configured to orthogonalize an overall red response and an overall blue response in response to a spectral power distribution of a given light condition. The overall red response is based on a first plurality of spectral responses for a first human perceived chromatic channel used primarily for detecting blood oxygenation in a human. The overall blue response is based on a second plurality of spectral responses for a second human perceived chromatic channel used primarily for detecting blood volume.
