Large ultrasensitive detector arrays are needed for present and future observatories for far infrared, submillimeter wave (THz), and millimeter wave astronomy. With increasing array size, it is increasingly important to control stray radiation inside the detector chips themselves, the surface wave. We demonstrate this effect with focal plane arrays of 880 lens-antenna coupled microwave kinetic inductance detectors (MKIDs). Presented here are near field measurements of the MKID optical response versus the position on the array of a reimaged optical source. We demonstrate that the optical response of a detector in these arrays saturates off-pixel at the $sim-{text{30}}$-dB level compared to the peak pixel response. The result is that the power detected from a point source at the pixel position is at a similar level to the stray response integrated over the chip area. With such a contribution, it would be impossible to measure extended sources, while the point source sensitivity is degraded due to an increase of the stray loading. However, we show that by incorporating an on-chip stray light absorber, the surface wave contribution is reduced by a factor $>$10. With the on-chip stray light absorber, the point source response is close to simulations down to the $sim -text{35}$-dB level, the simulation based on an ideal Gaussian illumination of the optics. In addition, as a crosscheck, we show that the extended source response of a single pixel in the array with the absorbing grid is in agreement with the integral of the point source measurements.
