This post provides an overview of key concepts in display and monitor calibration, with emphasis on general concepts and concise descriptions. Topics include photometric measurements (luminance, contrast ratio, gamma), color measurements (chromaticity, color temperature, gamut mapping, dominant wavelength and purity, white balance/gray balance) and multidimensional characteristics (uniformity, reflection, BRDF, MTF). For more detailed information on specific light measurement topics please refer to the following resources and guides from Gamma Scientific:
- Gamma Scientific Light Measurement Guides
- What You Need to Know About Display Calibration and Testing
- What is Photometry?
- How to Specify a Photometric System
- What is Colorimetry?
Luminance: A measurement of the brightness of an area on the display surface, as perceived by a standard human observer. The definition of the standard human observer is the cornerstone of the discipline of photometry.
Contrast Ratio: This metric compares the brightest to the darkest output produced by the display under specified conditions. The precise test conditions required may vary; light and dark readings may be averaged over several regions; ANSI Contrast compares the maximum display brightness to the minimum brightness achievable with the display powered-on. Other standards allow the dark measurement to be performed with the display powered off.
Gamma: A function which relates the brightness (luminance) of a display to the corresponding digital or electronic control parameters (grayscale). For color displays, gamma characterization is performed for each of the three color channels (red, green, and blue) independently.
Color: Though color perception is inherently subjective, it is possible to make objective measurements of the color of a source or object as perceived by a standard human observer. The three dimensions of color can be described in different ways; in perceptual terms, perhaps the most straightforward approach is to describe color in terms of brightness, hue, and purity, or saturation.
Gamut mapping (RGB plotting): All colors produced by a display are created by some combination of three primary colors: Red, Green, and Blue. In fact, each display color can be described in terms of the amount of R, G, and B primaries present.
Correlated Color Temperature (CCT): This metric is used to describe the color of a white light (such as a display backlight) by comparing its chromaticity to that of an idealized incandescent source, known as a black body.
White Balance, Gray Balance: After characterizing the chromaticity and gamma functions for each of a display’s three color channels (R, G, B), it is possible to calculate the amount of R, G, and B required to reproduce any color within the display’s color gamut. A particularly important feature is the location of the white point: The chromaticity of a specified white light, to which the observer is assumed to be adapted.
Dominant Wavelength & Purity: These values, taken together, represent an alternative description of chromaticity. Dominant wavelength corresponds to hue, while purity corresponds to saturation. The relationship between wavelength and hue can be understood in terms of the colors of the visible spectrum, as observed in the rainbow: Shorter wavelengths correspond to violet and blue hues; medium wavelengths to greens and yellow; longer wavelengths to orange and red hues.
An ideal display would render the same luminance and chromaticity for a given output, regardless of position on the screen surface and angle of view. In practice, all displays are non-uniform to some extent, with output varying as a function of position and angle. Such variation is characterized by mounting a photometric or colorimetric sensor in a positioning stage, which controls the position of the measurement spot and/or the angle of view.
Spatial Uniformity (Homogeneity): The spatial uniformity of a display is characterized by measuring spots at different positions on the display surface when the image displayed is nominally uniform (same R, G, B values for each pixel). The variation in actual luminance and/or chromaticity observed is reported and evaluated as a function of position (x,y) on the display screen.
Angular Uniformity: The angular uniformity of a display is characterized by measuring a single spot on the display surface from different viewing directions. The variation in actual luminance and/or chromaticity observed is reported and evaluated as a function of viewing direction. Viewing direction is specified in terms of two angles: The zenith angle (q) represents angular displacement from the normal or perpendicular direction; the azimuth angle (f) represents rotation about the normal axis. (If the display were a clock face, and f were defined as zero degrees at 12:00, then f would equal 30˚ at 1:00, 90˚ at 3:00, 180˚ at 6:00, etc.)
Veiling Glare: A secondary image due to reflection from the display surface that is superimposed upon the primary image rendered by the device. This secondary image will partially obscure or degrade the primary image. Such reflections can be specular (mirror-like), diffuse (matte), or something in-between: Hazy or blurred. Such glare can reduce the effective contrast produced by a display, or cause an undesired color shift. For this reason, it is important to characterize the reflectance properties of a display.
Specular Reflection: Reflection, as from a mirror or polished surface, which preserves the image of the source of the reflected light. Specular reflectance is a measure of the degree to which a surface will reflect light in this way. The specular reflectance of a display is measured by illuminating the display surface with a relatively small, well-defined source, and collecting light from the reflected image, which is confined to a specific direction.
Diffuse Reflectance: Reflection, as from a matte or rough surface, in which the reflected light is perfectly scattered, so that no image of the source of reflected light is produced. Instead, the reflected light renders the entire surface of the display itself visible as a “veil” through which the primary image is seen. Diffuse reflectance is a measure of the degree to which a surface will reflect light in this way. Diffuse reflectance of a display may be measured by illuminating the display surface as in the measurement of specular reflectance, and collecting the light reflected in all directions. The result may include both a specular and a diffuse component; in this case, it is necessary to subtract the contribution of specular reflectance to obtain the diffuse reflectance.
Haze: Reflection which produces a blurred image of the source of the reflected light. Since all real images are blurred to some extent, and few surfaces are perfectly diffuse, the practical definition of haze is somewhat arbitrary. Haze is defined as an intermediate case between specular and diffuse reflection, in which reflected light is scattered by more than some arbitrary small angle (qmin), and less than some arbitrary large angle (qmax) from the ideal specular path. Like diffuse reflectance, haze may be characterized by making measurements with sources and sensors of different sizes, then separating specular, diffuse, and haze components by analysis. A more rigorous approach is to use a goniometer to measure the BRDF of the display surface (see below).
More Complex Metrics
Bidirectional Reflectance Distribution Function (BRDF): A measurement of the reflection properties of a surface for given direction of illumination and direction of view. As in the measurement of angular uniformity, the viewing direction is specified in terms of two angles: The zenith angle (qv) and azimuth angle (fv). But here, the direction of illumination (qi, fi) must also be specified. BRDF is measured by mounting a light source and sensor on two positioning stages, and moving each independently with respect to the sample surface. A complete BRDF characterization of any surface is quite data-intensive; it can be both time-consuming to perform, and difficult to interpret. For this reason, such testing is often abridged, e.g. by limiting the number of directions of illumination.
Modulation Transfer Function (MTF): For various reasons, the image reproduced by a display may be somewhat blurred; sharp edges between dark and light areas may be softened, with a gray region of transition between them. If a pattern of fine lines is displayed, the contrast, or modulation, between dark and light areas may be reduced. Typically, the narrower and more closely-spaced the lines, the more pronounced this effect will be. The MTF of a display describes such a loss of contrast as a function of spatial frequency – that is, the number of lines in a given distance. Typically, MTF is measured by displaying an image with a sharp edge, and measuring the luminance of the display at several small, closely-spaced points on either side of that edge. The result is known as an “edge-spread function.” The MTF of the display can be calculated by analysis of this result, and used to predict the effect of the display upon any other type of image.