Recent studies have determined the efficiency of crop production by relating the rate of increase of dry matter in healthy growing crops to the interception of sunlight. In addition to knowledge of the incident light, such studies require measurement of light transmission in the crop, or timely information about leaf area for light interception to be estimated. Transmission measurements are necessarily confined to small areas while traditional methods of determining leaf area are laborious and often require destructive sampling of part of the crop. Remote sensing techniques offer a cheap, non-destructive system for sampling large areas. An airborne sensor is used to detect solar radiation reflected from crops in two spectral bands: the near infra-red band (780 - 940 nm) is strongly reflected by leaves due to the porous structure of the mesophyll; the red band (600 - 660 nm) is strongly absorbed by chlorophyll in the leaves. The ratio of red/infra-red reflected fluxes decreases with the percentage cover of healthy green leaf and is largely independent of the effects of varying solar irradiance. Measurements made over sugar beet showed that during the main period of growth, spectral ratios were linearly related to leaf cover and light interception. There was some evidence of hysteresis later in the season when the spectral ratios tended to increase in spite of constant leaf cover, and this may indicate senescence of the leaves and loss of chlorophyll. These relationships are consistent for a wide variety of crops and allow the light interception by the crop to be estimated by a single spectral measurement from above. This information may be used to predict future rates of growth and ultimately, crop yields.