Method for assessing ambient light during night mode image acquisition

1. A method for assessing an ambient light level during video acquisition with a video camera, the method comprising:

acquiring a stream of images with the video camera in a night mode, with an IR illuminator having a first illumination output level, wherein the video camera is operably connected with the IR illuminator, and has a day mode in which an IR-cut filter is arranged in front of an image sensor and the night mode in which the IR-cut filter is not arranged in front of the image sensor;

reducing an output level of the IR illuminator from a first illumination output level to a predetermined illumination output level during acquisition of a sequence of a predetermined number of consecutive image frames within the image stream, wherein the reduction of the output level of the IR illumination is a sudden change;

assessing a measure representative of an ambient light level from an evaluation of the sequence of image frames during which the output level of the IR illumination is reduced from the first illumination output level to the predetermined illumination output level, under a condition that a contribution of visual light is unchanged while the output level of the IR illumination is reduced from the first illumination output level to the predetermined illumination output level and while the measure representative of the ambient light level is assessed, by computing a contribution of the IR illumination to a measured value by the image sensor using a known reduced amount of the output level of the IR illumination from the first illumination output level to the predetermined illumination output level, and by computing a contribution of the ambient light to the measured value by the image sensor excluding the contribution of the IR illumination; and

using the assessed measure as input to a controller for the IR-cut filter, and switching the video camera to the day mode in response to the assessed measure passing a threshold value.

2. The method of claim 1 , further comprising increasing the output of the IR illuminator following the reduction in the output level.

3. The method of claim 1 , wherein the measure representative of the ambient light level is selected from the group comprising: a measure of current exposure, a measure of signal-to-noise ratio, and a measure of a light intensity, or any combination thereof.

4. The method of claim 1 , comprising processing the sequence of image frames acquired with reduced IR illumination such that their brightness resembles that of the surrounding frames.

5. The method of claim 1 , wherein the sequence of image frames is removed from the image stream prior to leaving an encoder associated with the video camera.

6. The method of claim 1 , wherein each frame in the sequence of image frames is marked so as not to be shown in a later display of the image stream.

7. The method of claim 1 , wherein the predetermined illumination output level corresponds to a level between 0 and 100% of the first illumination output level.

8. The method of claim 7 , wherein the predetermined illumination output level corresponds to zero output from the IR illuminator.

9. The method of claim 1 , wherein the sequence of image frames is a single image frame.

10. The method of claim 1 , wherein the IR illuminator comprises several different IR-LEDs, and a control of the IR illuminator is performed on a selection of the several different IR-LEDs.

11. The method of claim 1 , wherein the method is timed to start using image statistics.

12. The method of claim 1 , further comprising checking an operation mode of the video camera, wherein adjustment of the IR illuminator is postponed based on the operation mode of the video camera.

13. The method of claim 1 , wherein the assessment is performed on a subsection in the sequence of image frames, the subsection being a specific area of each image frame in the sequence.

14. A video camera configured to perform the method of claim 1 , said video camera comprising:

an image sensor;

an IR-cut filter arranged in front of the image sensor in day mode and not arranged in front of the image sensor in night mode;

an IR illuminator, said video camera being connectable to said IR illuminator, and

a control circuit for the IR illuminator, said control circuit comprising a trigger circuit for reading an output trigger signal from the image sensor and for timing a control of the IR illuminator to an operation of the image sensor read-out.

15. The method of claim 1 , wherein the predetermined illumination output level is above zero.