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In this study, a method based on the definition of illuminance was presented to design freeform lens for uniform illumination in direct-lit light emitting diode (LED) backlight with high distance-height ratio (DHR). The principle and algorithms of the design method were introduced and several design examples were presented and discussed. Compared with traditional hemispherical lens, the freeform lens designed by the present method can improve the illuminance uniformity greatly, especially when the DHR is large. Monte Carlo simulations show that when the DHR is 3, the uniformity of lighting performance of the optimized freeform lens could be improved as high as 0.8788 while that of the traditional hemispherical lens is only 0.1417.