In order to measure radiance factor of ground targets, a triple-band optical imaging system which contains bands of ultraviolet, visible and near infrared is developed. Radiance response characteristic of the triple-band optical imaging system is calibrated using an integrating sphere. The fitted equations between radiance and gray level of image, and between radiance and integration time of detector are got. A method of measurement of radiance factor of ground targets using two diffuser plates with different radiance factors as references to estoblish radiance equations is put forward, by which there would not be any atmospheric parameter in the expression of radiance factor of ground targets. An outdoor measurement of radiance factor of green grass using the triple-band optical imaging system is made. During the measurement, four diffuser plates with the radiance factors of 0.99, 0.50, 0.20 and 0.10 are used. From the measurement, the radiance factors of green grass in the bands of ultraviolet, visible and near infrared are got, which are 0.07, 0.184 and 0.429 respectively.