Back focal plane (BFP) image of SPR angle reflectance data and demonstration of incident angle control of excitation light by digital light projector (DLP). A) The BFP is fully illuminated, at all incident angles available, by 620 nm excitation light off of a 45 nm gold coated coverslip under water media. The incident light is linearly polarized in the x-direction. The dark crescent shaped regions at the edges of the field are where the incident light passes through the objective lens at a high angle of incidence and is coupled into the surface plasmons of the metal film, thus reducing reflectance. The absorbance occurs at the sides of the field where the incident light is p-polarized, and not at the top and bottom of the field where the incident light is s-polarized. The red line traces the angular distribution of incident light onto the image plane from the center of the BFP (0°) to the periphery (60°). B) A line scan of the red line in A) in the p-polarized direction shows the angular dependence of reflectivity and SPR coupling. The green line depicts the angle of illumination that provides maximum SPR coupling (≈53.5° for 620 nm) to be used for SPR imaging. C) Same BFP image area as shown in A) except here the DLP is used to project a thin arc of light that contains the same radial incident angle (≈53.5° illumination angle shown) as depicted by the green line in B).