Visual modulation transfer function as a predictor of acuity

The use of Fourier and modulation transfer techniques implies the ability to characterize an optical system and predict the output resulting from various inputs. In this study, the optical system is defined as a projection system, rear projection screen, and the human eye. A determination of modulation thresholds of visibility for a series of sinusoidal (in transmittance) test objects results after some manipulation in the modulation transfer function of the complete system. This result is then used to predict the subject\u27s ability to resolve conventional acuity challenges. These conventional test objects are then manufactured for projection in the system. Correlation between predicted acuity and observed acuity as tested may now be determined. The result indicates the degree to which optical designers may rely on modulation transfer and fourier techniques to predict the visual performance of a system. Presentation of targets was made using a modified slide projector. Changes in modulation were achieved by addition of a veiling glare projector. Subject response consisted of a forced choice, yes or no, as to whether modulation was perceived in the field of view. Thresholds of perception were defined as that modulation correctly observed 50% of the time. Two-bar targets were produced, and characterized by microdensitometer traces. Using the discrete fourier transform, these targets were convolved with the spread function defined by the M.T.F. Resulting absorption profiles permitted an estimate of image degra dation. When essential details of the object were lost it was predicted that visual resolution would fail. The accuracy of this prediction was tested by presentation of the two bar targets to the subject. Results indicate that while modulation transfer and fourier techniques permit rapid assessment of an optical system, that many factors affect subjective visual response, In testing it was found that while a high degree of correla tion exists between predicted resolution and actual resolution, variability inherent in threshold measurement precludes precision in prediction