This paper presents theoretical predictions and experimental measurements of the enhanced illuminance available in high latitude greenhouses in winter by use of double-glazing comprising laser-cut panels and glass to deflect low elevation sunlight onto the crop. Provided the structure of the greenhouse is optimised for light enhancement, i.e, single slope south-facing roof with vertical north wall, the predicted daily time averaged enhancement, the ratio of horizontal illuminance with laser-cut panel double-glazing to that obtained with a single clear glazing, approaches 100% for most of winter in greenhouses at latitudes >50 degrees; much greater than the enhancement that can be achieved with other passive optical systems investigated. The predicted enhancements, which take account of diffuse sky and direct sunlight components, are strongly dependent on latitude and atmospheric turbidity and agree with experimental enhancements measured in scale model greenhouses.