Viewing and imaging systems are provided for use in screen printers for example. In one aspect, in a viewing apparatus for location between the screen and board in a screen printer, light from first and second light sources (23, 24), for illuminating the screen (2) and board (1) respectively, is introduced into the return light channel of the apparatus, and a light barrier (39) is provided to inhibit cross-talk between light from the two sources introduced into the channel. In another aspect, in a video probe for imaging regions of the screen and/or board in a screen printer, lighting means is controlled to generate pulses of light. The lighting means is controlled to generate light pulses repeatedly at a sufficient rate to produce a substantially continuous picture when the video signal is displayed. The pulses are preferably synchronised with the video signal and generated at the video frame rate. In a further aspect, imaging apparatus for imaging regions of a screen and/or board in a screen printer includes a plurality of lighting means which are controlled to generate respective light pulses of different durations, the temporal mid-points of pulses from different lighting means being coincident.