The present invention relates to a method and an apparatus for determining a three dimensional image of the a moving object by means of reflecting means such as mirrors and a planar beam of light such as a laser light. As the object enters the light a two dimensional image of the object is captured, wherein the image comprises both the reflection of the beam of light from the object and also the reflection from the reflecting means. By transformation procedure a two dimensional image is transformed to a two dimensional object image and based on plurality of such images a three dimensional image is defined from which the volume can be calculated. This can be used to determine the weight distribution of the object, which is important for portioning.