Abstract: Abstract: The current trend of using artificial light in horticulture applications is to use light emitting diodes (LEDs) instead of traditional light sources such as the High Pressure Sodium (HPS), Ceramic Metal Halide (CMH), or fluorescent lights. Compared with the traditional light sources, LED lights provide the following benefits: a narrow spectrum band, low operating temperature, low voltage, long lifetime, and physical robustness. There are two main basic measurement methods to evaluate the result of plant lighting: Photosynthetic Active Radiation watts (PAR watts) and Photosynthetic Photon Flux (PPF), which are both objective. Recognizing the lack of a systematic methodology in the optical design of LED horticulture light, this paper proposed a design method for LED horticulture lights with tunable light qualities. The mathematical model of efficient photosynthetic photon flux (PPF) distribution over the full light spectrum was established, which governs the quantum ratio of red and blue components to adjust in a specified range, such as from 4.0 to 9.0. To realize a full spectrum of visible light, we chose a mixed light of white and red LEDs, in which the light components were much richer than the blue and red combination commonly adopted in LED horticulture applications. For a given requirement of light quality, with a specific range of intensity and ratio of red and blue components best suitable for the photosynthesis of a specific plant, the optical design procedure was as follows First, we made a choice of the LED lamp type. Usually a white LED type and a red LED type were selected, in case of the error caused by effect of an unknown spectral component, which was a non-primary object of study in the experiment. Then, the light intensities over whole spectrum of the LEDs were measured with a spectrophotometer (Everfine Photo-E-INFO CO., LTD., Hangzhou) to get spectral concentration sheets, which could tell the PAR watts value in specific spectral range. From the light intensities recorded, the total PPF of red and blue components can be computed by the integral of the light intensity over the two wavelength range (i.e. 610nm-720nm for red light and 400nm-510nm for blue light). By constraining the total PPF of the illuminated area, our optical design model could be applied to calculate the total number of LED lamps of each type (as used in this paper, the white and red lamps), and their layout on the light panel. Experimental statistics were set by Matlab as fundamental data in an illuminant lighting system. If the optimization solution was not so ideal, we could change some elements of this mathematical model, such as the type of LED, the ratio of the LED numbers, or adding another type of LED. There are also other factors that should be considered, such as luminous efficiency, cost, electric power, etc. This paper used an example to illustrate the optical design procedure proposed, and the experimental result showed that the designed LED lighting panel could adjust its light quality with the ratio of red and blue light components from 4.0 to 9.0 (in terms of PPF, μmol/s) and at the same time, maintain the total PPF to a constant value.