If a Protanope sets the mixture field to pure 670 nm, how will they set the test field?

A protanope, who has a deficiency in the red photoreceptors (the long-wavelength cones), perceives colors differently than individuals with normal color vision. When presented with pure 670 nm light, which is a wavelength within the red spectrum, a protanope will struggle to perceive it accurately due to their reduced sensitivity to these longer wavelengths.

To balance this issue in a color mixing task, a protanope would need to adjust the test field to a lower intensity or a different wavelength that would allow them to perceive a hue closest to what is being presented in the mixture field. By setting the test field lower, they compensatively harmonize the visual output to create a perception that aligns more closely with the intended mixture. This is a necessary adaptation given their altered color perception, as maintaining the same or higher setting would not yield an accurate visual experience to achieve a match with the pure red light.