A method for determining a distance D sensor between an image sensor and a face of a driver of a vehicle, comprising: arranging a point light source at a known geometric relationship with respect to the image sensor, characterized by : said point light source positioned such that spokes of a steering wheel of the vehicle at least occasionally cast a shadow in the face; determining a distance D light source between the point light source and the face based on an image acquired by the image sensor of the face including the shadow, and a geometric relationship between said point light source and said steering wheel; and determining the distance D sensor based on said distance D light source and said known geometric relationship between the point light source and the image sensor, wherein the distance D light source either is determined by: - detecting, in the image, a width (R) of a shadow of a spoke of the steering wheel, and - calculating the distance D light source as D light source = d × R / r , where d is a known distance between the point light source and the steering wheel, r is a known width of said spoke, and R is the detected width, or is determined by: - detecting, in the image, a position P of a shadow of a specific contour of a spoke of the steering wheel, and - calculating the distance D light source as D light source = d × P / p , where d is a known distance between the point light source and the steering wheel, p is a known position of the contour, and P is the detected position, wherein the position p and the detected position P are both determined with respect to an optical axis of the point light source. said point light source positioned such that spokes of a steering wheel of the vehicle at least occasionally cast a shadow in the face; determining a distance D light source between the point light source and the face based on an image acquired by the image sensor of the face including the shadow, and a geometric relationship between said point light source and said steering wheel; and determining the distance D sensor based on said distance D light source and said known geometric relationship between the point light source and the image sensor, wherein the distance D light source either is determined by: - detecting, in the image, a width (R) of a shadow of a spoke of the steering wheel, and - calculating the distance D light source as D light source = d × R / r , where d is a known distance between the point light source and the steering wheel, r is a known width of said spoke, and R is the detected width, - detecting, in the image, a width (R) of a shadow of a spoke of the steering wheel, and - calculating the distance D light source as D light source = d × R / r , where d is a known distance between the point light source and the steering wheel, r is a known width of said spoke, and R is the detected width, or is determined by: - detecting, in the image, a position P of a shadow of a specific contour of a spoke of the steering wheel, and - calculating the distance D light source as D light source = d × P / p , where d is a known distance between the point light source and the steering wheel, p is a known position of the contour, and P is the detected position, - detecting, in the image, a position P of a shadow of a specific contour of a spoke of the steering wheel, and - calculating the distance D light source as D light source = d × P / p , where d is a known distance between the point light source and the steering wheel, p is a known position of the contour, and P is the detected position, wherein the position p and the detected position P are both determined with respect to an optical axis of the point light source. <EOS>