An apparatus, for use in synthesis of arrays of DNA probes, polypeptides and the like, comprising: (a) a substrate with an active surface on which arrays may be formed; (b) a flow cell enclosing the active surface of the substrate and having an input port and an output port (20, 21) for applying reagents into the flow cell; (c) an image former for projecting a high precision, two-dimensional light image onto the substrate active surface, comprising: a light source for providing a light beam; and a micromirror device for receiving the light beam from the source, the micromirror device formed of an array of electronically addressable micromirrors, each of which can be selectively tilted between one of at least two separate positions, wherein in one of the positions of each micromirror, light from the light source incident upon the micromirror is deflected away from an optical axis and in a second of the at least two positions of the micromirror, light from the light source is reflected along the optical axis; characterised in that the image former comprises: projection optics (60, 61, 63) for receiving the light reflected from the micromirrors along the optical axis and imaging the pattern of the micromirrors onto the active surface of the substrate, wherein the projection optics include two doublet focusing lenses and an adjustable aperture, the first doublet lens configured to pass light through the adjustable aperture located at the back focal plane of the first doublet lens and the second doublet lens configured to receive the light passed through the aperture and to focus that light onto the active surface of the substrate. (a) a substrate with an active surface on which arrays may be formed; (b) a flow cell enclosing the active surface of the substrate and having an input port and an output port (20, 21) for applying reagents into the flow cell; (c) an image former for projecting a high precision, two-dimensional light image onto the substrate active surface, comprising: a light source for providing a light beam; and a micromirror device for receiving the light beam from the source, the micromirror device formed of an array of electronically addressable micromirrors, each of which can be selectively tilted between one of at least two separate positions, wherein in one of the positions of each micromirror, light from the light source incident upon the micromirror is deflected away from an optical axis and in a second of the at least two positions of the micromirror, light from the light source is reflected along the optical axis; characterised in that the image former comprises: projection optics (60, 61, 63) for receiving the light reflected from the micromirrors along the optical axis and imaging the pattern of the micromirrors onto the active surface of the substrate, wherein the projection optics include two doublet focusing lenses and an adjustable aperture, the first doublet lens configured to pass light through the adjustable aperture located at the back focal plane of the first doublet lens and the second doublet lens configured to receive the light passed through the aperture and to focus that light onto the active surface of the substrate. a light source for providing a light beam; and a micromirror device for receiving the light beam from the source, the micromirror device formed of an array of electronically addressable micromirrors, each of which can be selectively tilted between one of at least two separate positions, wherein in one of the positions of each micromirror, light from the light source incident upon the micromirror is deflected away from an optical axis and in a second of the at least two positions of the micromirror, light from the light source is reflected along the optical axis; projection optics (60, 61, 63) for receiving the light reflected from the micromirrors along the optical axis and imaging the pattern of the micromirrors onto the active surface of the substrate, wherein the projection optics include two doublet focusing lenses and an adjustable aperture, the first doublet lens configured to pass light through the adjustable aperture located at the back focal plane of the first doublet lens and the second doublet lens configured to receive the light passed through the aperture and to focus that light onto the active surface of the substrate.