A substrate processing system, comprising: a support assembly to support a substrate (230; 330; 400; 505); and an optical assembly (425; 525) to illuminate a portion of the substrate, the optical assembly comprising: a source of illumination configured to generate a light beam of at least one wavelength; a first optical homogenizer (426-1; 526-1) with an output numerical aperture, comprising: a first micro-lens array arranged to receive the light beam and output a first light beam; a second micro-lens array arranged to receive the first light beam and output a second light beam; and a first Fourier lens arranged to receive the second light beam and output a third light beam; the optical assembly being characterized in that it further comprises: a second optical homogenizer (426-2; 526-1) with an input numerical aperture, comprising: a third micro-lens array arranged to receive the third light beam and output a fourth light beam; a fourth micro-lens array arranged to receive the fourth light beam and output a fifth light beam; and a second Fourier lens arranged to receive the fifth light beam and output a sixth light beam to illuminate the portion of the substrate; wherein the output numerical aperture is within 5% of the input numerical aperture. a support assembly to support a substrate (230; 330; 400; 505); and an optical assembly (425; 525) to illuminate a portion of the substrate, the optical assembly comprising: a source of illumination configured to generate a light beam of at least one wavelength; a first optical homogenizer (426-1; 526-1) with an output numerical aperture, comprising: a first micro-lens array arranged to receive the light beam and output a first light beam; a second micro-lens array arranged to receive the first light beam and output a second light beam; and a first Fourier lens arranged to receive the second light beam and output a third light beam; the optical assembly being characterized in that it further comprises: a second optical homogenizer (426-2; 526-1) with an input numerical aperture, comprising: a third micro-lens array arranged to receive the third light beam and output a fourth light beam; a fourth micro-lens array arranged to receive the fourth light beam and output a fifth light beam; and a second Fourier lens arranged to receive the fifth light beam and output a sixth light beam to illuminate the portion of the substrate; wherein the output numerical aperture is within 5% of the input numerical aperture. a source of illumination configured to generate a light beam of at least one wavelength; a first optical homogenizer (426-1; 526-1) with an output numerical aperture, comprising: a first micro-lens array arranged to receive the light beam and output a first light beam; a second micro-lens array arranged to receive the first light beam and output a second light beam; and a first Fourier lens arranged to receive the second light beam and output a third light beam; the optical assembly being characterized in that it further comprises: a second optical homogenizer (426-2; 526-1) with an input numerical aperture, comprising: a third micro-lens array arranged to receive the third light beam and output a fourth light beam; a fourth micro-lens array arranged to receive the fourth light beam and output a fifth light beam; and a second Fourier lens arranged to receive the fifth light beam and output a sixth light beam to illuminate the portion of the substrate; wherein the output numerical aperture is within 5% of the input numerical aperture. a first micro-lens array arranged to receive the light beam and output a first light beam; a second micro-lens array arranged to receive the first light beam and output a second light beam; and a first Fourier lens arranged to receive the second light beam and output a third light beam; the optical assembly being characterized in that it further comprises: a second optical homogenizer (426-2; 526-1) with an input numerical aperture, comprising: a third micro-lens array arranged to receive the third light beam and output a fourth light beam; a fourth micro-lens array arranged to receive the fourth light beam and output a fifth light beam; and a second Fourier lens arranged to receive the fifth light beam and output a sixth light beam to illuminate the portion of the substrate; wherein the output numerical aperture is within 5% of the input numerical aperture. a second optical homogenizer (426-2; 526-1) with an input numerical aperture, comprising: a third micro-lens array arranged to receive the third light beam and output a fourth light beam; a fourth micro-lens array arranged to receive the fourth light beam and output a fifth light beam; and a second Fourier lens arranged to receive the fifth light beam and output a sixth light beam to illuminate the portion of the substrate; a third micro-lens array arranged to receive the third light beam and output a fourth light beam; a fourth micro-lens array arranged to receive the fourth light beam and output a fifth light beam; and a second Fourier lens arranged to receive the fifth light beam and output a sixth light beam to illuminate the portion of the substrate; wherein the output numerical aperture is within 5% of the input numerical aperture.