Product Code Database
photo with incremental contrast levels, clockwise from bottom left]] Contrast is the difference in luminance or color that makes an object (or its representation in an image or Display device) visible against a background of different luminance or color. The human visual system is more sensitive to contrast than to absolute luminance; thus, we can perceive the world similarly despite significant changes in illumination throughout the day or across different locations.
The maximum contrast of an image is termed the contrast ratio or dynamic range. In images where the contrast ratio approaches the maximum possible for the medium, there is a conservation of contrast. In such cases, increasing contrast in certain parts of the image will necessarily result in a decrease in contrast elsewhere. Brightening an image increases contrast in darker areas but decreases it in brighter areas; conversely, darkening the image will have the opposite effect. Bleach bypass reduces contrast in the darkest and brightest parts of an image while enhancing luminance contrast in areas of intermediate brightness.
The high-frequency cut-off represents the optics limitations of the visual system's ability to resolve detail and is typically about 60 cpd. The high-frequency cut-off is also related to the packing density of the photoreceptor cells: a finer matrix can resolve finer gratings.
The low frequency drop-off is due to lateral inhibition within the retinal ganglion cells. A typical retinal ganglion cell's receptive field comprises a central region in which light either excites or inhibits the cell, and a surround region in which light has the opposite effects.
One experimental phenomenon is the inhibition of blue in the periphery if blue light is displayed against a white background, leading to a yellow surrounding. The yellow is derived from the inhibition of blue on the surroundings by the center. Since white minus blue is red and green, this mixes to become yellow."eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
For example, Display contrast, contrast depends on the properties of the picture source or file and the properties of the computer display, including its variable settings. For some screens Viewing cone between the screen surface and the observer's line of sight is also important.
Various definitions of contrast are used in different situations. Here, luminance contrast is used as an example, but the formulas can also be applied to other physical quantities. In many cases, the definitions of contrast represent a ratio of the type
The rationale behind this is that a small difference is negligible if the average luminance is high, while the same small difference matters if the average luminance is low (see Weber–Fechner law). Below, some common definitions are given.
with and representing the luminance of the features and the background, respectively. The measure is also referred to as Weber fraction, since it is the term that is constant in Weber's Law. Weber contrast is commonly used in cases where small features are present on a large uniform background, i.e., where the average luminance is approximately equal to the background luminance.
with and representing the highest and lowest luminance. The denominator represents twice the average of the maximum and minimum luminances.
This form of contrast is an effective way to quantify contrast for periodic functions and is also known as the modulation of a periodic signal . Modulation quantifies the relative amount by which the amplitude (or difference) of stands out from the average value (or background) .
In general, refers to the contrast of the periodic signal relative to its average value. If , then has no contrast. If two periodic functions and have the same average value, then has more contrast than if .Prince, Jerry L., Links, Jonathan M. Medical Imaging Signals and Systems, (2006). pg 65 Ch 3 Image Quality, 3.2 Contrast, 3.2.1 Modulation.
where intensities are the -th -th element of the two-dimensional image of size by . is the average intensity of all pixel values in the image. The image is assumed to have its pixel intensities normalized in the range .
As mentioned above, contrast sensitivity describes the ability of the visual system to distinguish bright and dim components of a static image. Visual acuity can be defined as the angle with which one can resolve two points as being separate since the image is shown with 100% contrast and is projected onto the fovea of the retina.Sadun, A. A. Optics lecture on 03/06/2013. University of Southern California. Thus, when an optometrist or ophthalmologist assesses a patient's visual acuity using a Snellen chart or some other Landolt C, the target image is displayed at high contrast, e.g., black letters of decreasing size on a white background. A subsequent contrast sensitivity exam may demonstrate difficulty with decreased contrast (using, e.g., the Pelli–Robson chart, which consists of uniform-sized but increasingly pale grey letters on a white background).
To assess a patient's contrast sensitivity, one of several diagnostic exams may be used. Most charts in an ophthalmologist's or optometrist'
/ref> Contrast threshold can be defined as the minimum contrast that can be resolved by the patient. Contrast sensitivity is typically expressed as the reciprocal of the threshold contrast for detection of a given pattern (i.e., 1 ÷ contrast threshold).
Using the results of a contrast sensitivity exam, a contrast sensitivity curve can be plotted, with spatial frequency on the horizontal, and contrast threshold on the vertical axis. Also known as contrast sensitivity function (CSF), the plot demonstrates the normal range of contrast sensitivity, and will indicate diminished contrast sensitivity in patients who fall below the normal curve. Some graphs contain "contrast sensitivity acuity equivalents", with lower acuity values falling in the area under the curve. In patients with normal visual acuity and concomitant reduced contrast sensitivity, the area under the curve serves as a graphical representation of the visual deficit. It can be because of this impairment in contrast sensitivity that patients have difficulty driving at night, climbing stairs and other activities of daily living in which contrast is reduced.Sia DI, Martin S, Wittert G, Casson RJ. "Age-related change in contrast sensitivity among Australian male adults: Florey Adult Male Ageing Study". Acta Ophthalmol. 2012 Mar 16.
on 03/05/2019. In an intermediate spatial frequency, the peak (brighter bars) of the pattern is detected by the center of the receptive field, while the troughs (darker bars) are detected by the inhibitory periphery of the receptive field. For this reason, low- and high-spatial frequencies elicit excitatory and inhibitory impulses by overlapping frequency peaks and troughs in the center and periphery of the neuronal receptive field.Tsui JM, Pack CC. "Contrast sensitivity of MT receptive field centers and surrounds." J Neurophysiol. 2011 Oct;106(4):1888-900. Other environmental, physiological, and anatomical factors influence the neuronal transmission of sinusoidal patterns, including adaptation.Cravo AM, Rohenkohl G, Wyart V, Nobre AC. "Temporal expectation enhances contrast sensitivity by phase entrainment of low-frequency oscillations in visual cortex." J Neurosci. 2013 Feb 27;33(9):4002-10.
Decreased contrast sensitivity arises from multiple etiologies, including retinal disorders such as age-related macular degeneration (ARMD), amblyopia, lens abnormalities, such as cataract, and by higher-order neural dysfunction, including stroke and Alzheimer's disease.Risacher SL, Wudunn D, Pepin SM, MaGee TR, McDonald BC, Flashman LA, Wishart HA, Pixley HS, Rabin LA, Paré N, Englert JJ, Schwartz E, Curtain JR, West JD, O'Neill DP, Santulli RB, Newman RW, Saykin AJ. "Visual contrast sensitivity in Alzheimer's disease, mild cognitive impairment, and older adults with cognitive complaints." Neurobiol Aging. 2013 Apr;34(4):1133-44. In light of the multitude of etiologies leading to decreased contrast sensitivity, contrast sensitivity tests are useful in the characterization and monitoring of dysfunction, and less helpful in detection of disease.
A separate study by Knoll et alKnoll, H.A., Tousey, R., Hulburt, E.O., JOSA, 36, p480 (1946) investigated thresholds for point sources by requiring subjects to vary the brightness of the source to find the level at which it was just visible. A mathematical formula for the resulting threshold curve was proposed by Selig Hecht,Hecht, S., JOSA, 37, p59 (1947) with separate branches for scotopic and photopic vision. Hecht's formula was used by WeaverWeaver, H.F., PASP, 59, p232 (1947) to model the naked-eye visibility of stars. The same formula was used later by Bradley SchaeferSchaefer, B.E., PASP, 102, p212 to model stellar visibility through a telescope.
Andrew Crumey showed that Hecht's formula fitted the data very poorly at low light levels, so was not really suitable for modelling stellar visibility. Crumey instead constructed a more accurate and general model applicable to both the Blackwell and Knoll et al data. Crumey's model covers all light levels, from zero background luminance to daylight levels, and instead of parameter-tuning is based on an underlying linearity related to Ricco's law. Crumey used it to model astronomical visibility for targets of arbitrary size, and to study the effects of light pollution.
|
|
