In this paper, we describe a visual experiment to measure the contrast detection threshold of both halftone image and continuous tone image. A continuous tone sinusoidal grating was halftoned with a classical 45 degree dot screen. A calibrated CRT monitor was used to display images. The observers were asked to make a forced choice that whether the displayed image contains a grating pattern. The contrast detection threshold was determined using Probit analysis. The threshold elevation, the ratio of contrast threshold for halftone grating to continuous tone grating, was calculated based on the measured contrast detection threshold. It was found that the threshold elevation strongly depends on halftone dot frequency. At a high halftone frequency, there is little difference in the measured contrast detection threshold between continuous tone grating and halftone grating, but at a lower halftone frequency, the detection threshold is significantly higher for halftone grating than that of the continuous tone grating. The threshold elevation is much higher for the gratings oriented at 45 degree where the peaks of the halftone frequency lies. A multiple channel vision model was implemented to predict the visual difference for both continuous tone and halftone image. The model correctly predicted the grating detection threshold of continuous tone grating, but it fails to predict the threshold elevation due to halftone.