In order to estimate directional variation of gap frequency over a over covered maize canopy（LAI=5）

an experiment using a narrow FOV thermal infrared camera mounted on a crane boom was conducted at night on July 26

1999.A geometric optical and radiative transfer model was proposed to simulate hemispherical gap frequency.Observations showed most gaps appeared between the adjacent hedgerows

which lead the canopy still to keep row feature in thermal infrared images.The measured gap frequency confirmed the results mentioned in many papers that very little azimuthal variation appeared except the observations parallel to the rows.Based on already obtained geometrical parameters

the developed model captured the main features of the measured gap frequency

the simulated gap frequency showed a fairly good agreement with observed gap frequency.