The invention relates to an imaging device for imaging a long object. The imaging device includes at least one vertical lens device which concentrates light from the long object in a vertical direction, and a plurality of lens elements which are arranged in the form of a matrix in at least one lens line behind the vertical lens device, the lens elements respectively concentrating light from the long object in a horizontal direction. In order to achieve optimum light intensity in a cost-effective manner, each lens element comprises at least one horizontal collecting layer which is transparent to the light from the long object and comprises a refractive index having a gradient in the horizontal direction.