Light detection circuit

An electronic light detection circuit for detecting an intensity modulated light signal on a photosensitive element under a backlight condition, comprising:

• said photosensitive element, including a light position detector for detecting a striking position of the light signal spot within a detection window;
• an amplifier with high input resistance connected to an output of said photosensitive element;
• and
• backlight suppression circuitry connected to the output of the photosensitive element in parallel to the amplifier, the backlight suppression circuitry including an electronic active resonator structure with at least one operational amplifier, configured to provide a load impedance to an output of the photosensitive element with:
• a low load impedance for low frequencies for suppression of natural and artificial backlight-saturation of the photosensitive element, and
• a high load impedance at the frequency of intensity modulation, wherein the low load impedance is lower than the high load impedance for backlight suppression by draining low frequency photo-induced currents from the output of the photosensitive element.

A method for evaluating a light spot in a backlight condition by a photosensitive element in a leveling laser receiver, comprising:

• applying the light spot to the active surface of the photosensitive element to achieve a modulated light intensity on the photosensitive element;
• generating at least one electrical signal dependent on the light spot intensity and the backlight condition;
• evaluating the electrical signal, including amplification of the electrical signal with a high input impedance amplifier and a subsequent conversion from analog to digital;
• and
• actively draining a low frequency part of the electrical signal resulting from backlight by an active resonator structure at the output of the photosensitive element, having a low impedance for low frequencies, from zero to twice the mains frequency, for avoiding backlight saturation of the photoelectric element by draining a low frequency part of the electrical signal, while having a high impedance at modulation frequency for maintaining a high level of a modulation frequency part of the electrical signal for evaluation wherein the low impedance is lower than the high impedance, resulting in a reduced draining for the modulation frequency part than for the low frequency part.