Visual Detection of Noisy Patterns

(1) In a previous paper(1) a statistical theory of visual thresholds was reported which accounts for the influence both of quantum noise and of visual noise added to the test patterns. This theory finds particular application in the pre­ diction of thresholds of vision through image intensifying systems which superimpose system noise on the observed scenes. In a subsequent paper(2) an additional factor was introduced to allow for alternative mechanisms of noise integration within the summation areas. Here we report the results of continued experimental work designed to validate and to extend these theories of vision with noisy displays. In particular, an unequivocal experi­ mental demonstration of the predicted double-threshold response is reported. An experimental measurement of the background luminance, for which the stimulus-threshold contrast has a mini­ mum value, allows the calculation of two important parameters of vision. These factors are :-(i) The quantum efficiency (S o) of the eye, defined as the ratio of the number of quanta effectively absorbed at the retina to the number of quanta entering the eye. Thus, S O is a purely physi­ cal factor representative of sensi­ tivity. (ii) The factor of certainty (k). A detection threshold in the theory is considered to be the result of a decision with a predetermined prob­ ability of freedom from error, k is a measure of this probability; it is a subjective factor which can be varied by changing the instructions given to observers. In normal visual threshold determinations, these factors are not readily separable; it is the addition of the extra quantum-like noise to the test patterns that permits their separation. In our experiments, these factors were determined for a range of observers. 2. The experimental system The added visual noise used in the experi­ ment was quantum noise of the type obtained in the output from an image intensifier. The noise arises in intensifying systems as a result of the detection of individual photons at the input photocathode. The form of the added noise is similar to that of retinal quantum noise, except that the 'quantum' presented to the retina by the intensifier is much larger. To avoid confusion, the added noise will be referred to as 'scintillation noise 1. The experimental realisation of such a noisy display consisted of cinematographically projected loops of film of electronically produced random dot patterns. The presence of a stimulus was signalled by an increase in the …