Response to light with laterally varying photon energy distribution is based on position, such as position on a detector of a respective light spot or intensity maximum for a wavelength or photon energy subrange. A layered structure such as a coating over the detector can produce the laterally varying distribution, such as due to a laterally varying transmission property. A differential output or quantity can be obtained using sensing results from the detector and can then be used to monitor a light source's wavelength. The light source can, for example, be a pulsed or continuous laser, in which case an optical component between the light source and the detector can be structured to prevent inhomogeneities such as speckle and also reflection back to the laser that could cause feedback. A tunable light source can be tuned in response to positions at which its light is detected.