Edwin H. Land

Lightness and Retinex Theory

Sensations of color show a strong correlation with reflectance, even though the amount of visible light reaching the eye depends on the product of reflectance and illumination. The visual system must achieve this remarkable result by a scheme that does not measure flux. Such a scheme is described as the basis of retinex theory. This theory assumes that there are three independent cone systems, each starting with a set of receptors peaking, respectively, in the long-, middle-, and short-wavelength regions of the visible spectrum. Each system forms a separate image of the world in terms of lightness that shows a strong correlation with reflectance within its particular band of wavelengths. These images are not mixed, but rather are compared to generate color sensations. The problem then becomes how the lightness of areas in these separate images can be independent of flux. This article describes the mathematics of a lightness scheme that generates lightness numbers, the biologic correlate of reflectance, independent of the flux from objects

Some Aspects of the Development of Sheet Polarizers

This paper reviews the 25 years of activity, by the author and his co-workers, in the development of synthetic sheet polarizers. The early work during the nineteenth century is described briefly, and then the various stages of the modern development in the author’s laboratory are chronicled. A description is given of the nature and the optical properties of the currently-available sheet polarizers of the Polaroid J, H, K, and L types, and of the quantitative methods used in characterizing them. The reasons are given for developing special polarizers for each of a variety of applications such as optical instruments, vectographs, and headlights.

A technique for comparing human visual responses with a mathematical model for lightness.

ABSTRACT This paper reports a technique for comparing the human visual responses with lightness predictions made by a mathematical model. The human visual responses are determined by having a number of observers compare the lightnesses in a Test Display with those in a Standard Display. The mathematical models predictions are made by processing numbers that are identical to the luminances in the Test Display. These predictions are then scaled relative to the same Standard Lightness Display used by human observers. Methods of analyzing the results are discussed, as well as a variety of situations that can be used to establish whether a particular model can be considered a general model for lightness.

A Comparative Survey of Some Possible Systems of Polarized Headlights

In a polarizing headlight system polarizers are placed on automobile headlights and in a viewer before the eyes of automobile drivers. These polarizers are so oriented that the viewer is crossed with the headlights of an approaching car. This blocks most of the light from approaching headlights. A polarizing system has other features which make it different from a non-polarizing system. The amount of the polarized component of the reflected light from an object illuminated by a car’s headlights which is transmitted back through the viewer in the same car depends on the type and orientation of the polarizers. Thus objects have different brightness depending on the polarizing system being used. Similarly, the contrast between objects and their background will be different for different polarizing systems. In this paper some basic matrices of an optical calculus are derived. This calculus is then used to show the orientation of the headlights and viewer polarizers for all possible polarizing systems. For each of these systems the relative brightness of a specularly reflecting object is determined. Finally, equations are derived for the contrast of various types of objects with their background and a few special cases are calculated.

A New One-Step Photographic Process

This paper describes a process which produces finished poisitive pictures, directly from the camera, in about one minute after the exposure. The camera is described as containing a strip of paper in addition to the negative material and as feeding this strip of paper, in contact with the exposed negative, through simple pressure rollers and thence out of the camera box. The process to be used with the camera is described as having but one step, instead of the many steps of conventional photography; and it is shown that this can be achieved by associating with the strips a reagent which, when spread between the two strips by the pressure of the rollers, is capable of developing the silver halide of the negative and forming the positive image at one and the same time. Several classes of processes are discussed, their characteristics explored and certain principles established for obtaining satisfactory picture quality, stability, speed of operation, etc. In one of these processes, the reagent spread between the negative and the positive strip consists of a small amount of viscous liquid containing, in rather high concentration, the necessary constituents for developing the negative image, for forming at the same time a silver complex with the unexposed grains in the negative, for transferring the soluble complex to the positive sheet, and there creating and stabilizing the positive silver image. This process runs to completion in about one minute. When the two strips are peeled apart, both are essentially dry. One strip is the finished positive picture. The process operates at temperatures from less than 30°F to over 100°F. The paper discusses the control of the rates of reduction, silver ion complex formation, and ion diffusion in the several reaction fronts; principles of stabilization of the positive picture; control of the relative rates of growth of density in negative and positive; conversion of the silver ions to particles of silver of adequate size and the control of the color of the image as a function of particle size; and various characteristic curves which have been obtained with different negative materials. These factors are then interpreted with relation to the photographic usefulness of the process.

Smitty Stevens’ Test of Retinex Theory

This paper describes an experiment that Smitty performed while I was giving the William James lectures at Harvard. It was a modification of Craik’s and Cornsweet’s experiments using mixtures of red and blue instead of black and white. Smitty’s experiment tested whether the colors produced were the colors one would expect from the predictions of Retinex theory. This paper describes the experiment, the predictions, and the results.

Detection of Tracks and Stars in Nuclear Track Plates

Tracks and stars in nuclear track plates are usually found by ordinary bright-field microscope procedures affording limited field of view and requiring continual focusing up and down. Standard dark-field illumination schemes fail due to background scattering by fog grains. A rotating-azimuth, planar, dark-field, illumination scheme has been developed which provides sufficient contrast so that low magnification (50 to 100 times) may be used. The field of view is thereby increased to 2 to 4 mm and the depth of field becomes comparable to the total thickness of the emulsion.

Role Of The Retinex

The retinex is the name given to each of four independent systems. Each of these uses a liaison between the whole area of the retina, the pathway to the cortex, and the cortex to generate what we call objects in the outer world. Each of the systems is rigorously independent of the others although the band of wavelengths used by each overlaps rather broadly the bands used by the others. The colors of the objects generated are determined by the comparison, presumably cortical, of the four different constructs produced by the four retinexes. Each of the retinexes associated with the left eye has a sister retinex associated with the right eye, and these pairs impose a geometric rigidity on the space created by the retinal-cortical system.

Six Eyes Of Man

The human visual system generates many dimensions of orthogonality and utilizes the output of eight orthogonal systems to generate hypersensitivity, color and depth. Each system can be regarded as a sense in itself and the outputs of all or any can be placed in apposition to give a multiply populated class of conjoint information. Each system has two kinds of independence: (1) each acts alone in the acquisition, utilization and rendition of that array of information for which its own mechanisms are suigeneris; (2) each has an independence manifested only as a consequence of having its informational output placed in apposition to the output of one or many of the other systems.