A diffuse reflectance apparatus comprising: a light source configured to produce a beam of light; a diffuse reflectance mirror (52, 52') having a reflective surface (54, 54') and an aperture (50, 50') through which the beam of light passes to a target, wherein the diffuse reflectance mirror (52, 52') is positioned relative to the target such that a substantial portion of specular reflection from a surface of the target passes back through the aperture (50, 50') and the reflective surface (54, 54') collects a substantial portion of scattered light from the target and forms a second beam; and a detector configured to produce a signal in response to the second beam; characterized by: an optical assembly positioned to direct the second beam from the diffuse reflectance mirror (52, 52') to the detector, wherein the optical assembly comprises: a first flat mirror; a second flat mirror; and a concave ellipsoidal mirror, wherein the first flat mirror is positioned to reflect the second beam comprising the scattered light from the concave ellipsoid mirror to the second flat mirror, wherein the second flat mirror is positioned to reflect the second beam comprising the scattered light from the first flat mirror to the concave ellipsoidal mirror; and wherein the concave ellipsoidal mirror is positioned to reflect the second beam comprising the scattered light from the second flat mirror to the detector. a light source configured to produce a beam of light; a diffuse reflectance mirror (52, 52') having a reflective surface (54, 54') and an aperture (50, 50') through which the beam of light passes to a target, wherein the diffuse reflectance mirror (52, 52') is positioned relative to the target such that a substantial portion of specular reflection from a surface of the target passes back through the aperture (50, 50') and the reflective surface (54, 54') collects a substantial portion of scattered light from the target and forms a second beam; and a detector configured to produce a signal in response to the second beam; characterized by: an optical assembly positioned to direct the second beam from the diffuse reflectance mirror (52, 52') to the detector, wherein the optical assembly comprises: a first flat mirror; a second flat mirror; and a concave ellipsoidal mirror, wherein the first flat mirror is positioned to reflect the second beam comprising the scattered light from the concave ellipsoid mirror to the second flat mirror, wherein the second flat mirror is positioned to reflect the second beam comprising the scattered light from the first flat mirror to the concave ellipsoidal mirror; and wherein the concave ellipsoidal mirror is positioned to reflect the second beam comprising the scattered light from the second flat mirror to the detector. a first flat mirror; a second flat mirror; and a concave ellipsoidal mirror, wherein the first flat mirror is positioned to reflect the second beam comprising the scattered light from the concave ellipsoid mirror to the second flat mirror, wherein the second flat mirror is positioned to reflect the second beam comprising the scattered light from the first flat mirror to the concave ellipsoidal mirror; and wherein the concave ellipsoidal mirror is positioned to reflect the second beam comprising the scattered light from the second flat mirror to the detector.