With the human observer serving as the final detector in many imaging systems, the functioning of the visual system becomes an important design determinant. Here the contrast threshold is investigated in terms of underlying noise sources. Rose has shown that the threshold found at intermediate brightness levels can be accounted for on the basis of the quantum noise of the radiation absorbed by the retinal detectors. Here it is shown how reasonable and simple assumptions concerning other sources of noise would affect the threshold contrast over the full range of observable brightness levels. The resulting model may prove useful in predicting performance of observer-device systems. Results obtained with the proposed model are compared with ex-perimentally observed threshold data.