Like all detectors, the relationship between input photons and output counts for WHIRCAM is not exactly linear. But as can be seen in Figure 5, it is almost linear over most of the dynamic range, up to about 80% of well capacity. While it is possible to correct for the deviations from exact linearity over this range, via a linearity correction table, this is currently not available. Hence if you require very precise photometry, you would be advised to create a linearity map, by exposing the detector to a reasonably flat, but constant, light source, over a range of exposure times, to accurately measure the non-linearities of every pixel.
Figure 5: Accumulation of counts in odd and even columns, showing offset for odd columns in the reset level, the degree of linearity, and the onset of saturation.