A particular video spot detector is applied to video records of a sea surface. The detector provides an irradiance time series from an arbitrarily chosen pixel in the video frame. Because the technique picks out only one pixel, it allows for a real-time sampling of a long time series at a high sampling rate (50 Hz) without the need for a large storage capacity. Another advantage is the (statistical) homogenous (space-independent) light conditions within each pixel. The technique is analyzed theoretically by a pure sky light reflection model, i.e., irradiance distributions from beneath the surface are neglected. A remarkably linear correspondence is found between detected irradiance and the horizontal wave slope component parallel with the line-of-sight incidence plane when an overcast sky is assumed (cardioidal radiance model). The pure reflection model requires, however, small look angles with the horizontal that introduce a strong nonlinear geometrical (projection) effect. The great impact that this effect has on, for example, the statistical distribution of the detected irradiance values, is studied assuming a Gaussian ocean wave surface. The theoretical findings are verified by experimental evidence taken from the North Sea. This indicates that the effect of geometric projection is a strong one that should be considered properly by optical analysis of the sea surface.