Glossary of Display Optical Measurement Terms
A
Ambient Contrast Ratio
A basic reflection test. It is a measure of the display contrast ratio under diffuse illumination conditions and is meant to emulate typically office lighting environment of 500 lux. The test involves setting up an hemispherical (or equivalent) diffuse surround in front of the display surface. A reference luminance measurement is made on a diffuse reflection standard in place of the display. The standard is then removed, replaced by the DUT, and measurements are repeated on the actual DUT surface. Westar offers the necessary apparatus to perform this test.
C
Chromaticity
A measure of color hue and saturation typically plotted on CIE chromaticity graphs. Popular CIE charts include the 1931 CIE Chromaticity Diagram (x, y) and the updated 1976 Uniform Chromaticity Scale (UCS) Chromaticity Diagram (u’, v’).
Chromaticity Uniformity
A measure of the constancy of the Color value across the active area of a display. Typical uniformity arrangements include 5-, 9-, and 13-point uniformity measurements.
Classroom Training
Classroom & laboratory training are given by a Westar engineer when the system is delivered and set-up. Classroom training includes printed training materials and may be attended by up to 8 customer employees. Laboratory training lasts 1~2 days and covers all aspects of system alignment, ViewPoint™ software operation, writing TestStand™ Test Sequences, and basic maintenance.
Color Gamut
A measure of the producible color range of a display. Typically calculated from the area enclosed by a triangle on the CIE Diagram formed by the three primary colors of the display.
Color Gamut %NTSC
A measure of how much of the NTSC color gamut area is covered (reproducible) by a display device. Described as a percentage of the NTSC color gamut area that is covered by the display color gamut, and therefore is calculated on the 1976 (USC) CIE diagram.
Contrast Ratio
A simple ratio of bright-state luminance to dark-state luminance.
Contrast Transfer Function / Modulation Transfer Function
Contrast Transfer Function / Modulation Transfer Function. Two closely related terms that describe an optical system’s ability to pass / render / display image detail. CTF / MTF are typically expressed in a graphical plot of contrast (or modulation factor) versus spatial frequency, e.g., line pairs per mm. The curve typically rolls off (diminishes) with increasing spatial frequency. Measurement methods vary, but CTF / MTF are commonly derived from a DUT image taken with a CCD camera. Westar offers CCD camera-based solutions for measuring the CTF / MTF of flat panel displays.
D
Dark Room
A dedicated room suitable for low light measurements. Open table FPM Systems (FPM H Series systems) require such a room to control ambient light. Stray ambient light from adjacent work areas, equipment, or windows must be eliminated when making low light measurements. Direct or indirect (reflected) light from these sources can cause serious errors in light measurements resulting in artificially high readings or erroneous colorimetric values. Fully enclosed systems such as the QuickTest, Paneltest, and FPM V series systems do not require a dark room and may be used in well lit ambient environments where other work is going on.
Defect Detection
A general class of display measurements aimed at identifying malfunctioning pixels (single pixels, rows, columns) or other area-type problems (blemishes, blotches, scratches, mura, delaminations) with the display performance. Measurement of these types of defects always involves high end CCD cameras or video photometers and sophisticated image processing software. Westar offers a wide variety of defect detection problem solving capability, but solving a specific problem is almost always a custom endeavor requiring sample defects and an “algorithm tuning” exercise.
Diffuse Reflection
A measure of how much a display surface scatters light; i.e., the degree a display surface acts like a perfectly Lambertian surface. Traditionally, simplified reflection models only addressed the specular and diffuse components of the display surface. In most applications, it is adequate to measure specular or diffuse reflection factors only, but this ignores a potentially significant intermediate reflection component called “haze.”
The test involves setting up a large area diffuse luminance source at an off-normal angle. The LMD is arranged to make measurements along the DUT normal axis. A reference luminance measurement is made on a diffuse reflection standard in place of the display. The standard is then removed, replaced by the DUT, and measurements are repeated on the actual DUT surface. Westar offers the necessary apparatus to perform this test.
DUT Drive
Display Under Test Drive. DUT Drive refers to all forms of electrical stimulus supplied to the display under test. This includes the video test pattern generator, power supplies, discrete I/O, serial data buses, etc. Check out our LCD Testers for more information.
DUT Drive Supplies
Display Under Test Supplies. The power supplies needed to power the panel, module, display head assembly, or finished product DUT.
DUT Drive Test Pattern Generator
Refers to a simple test pattern generator used to drive the analog or digital video signals needed by the display being tested.
DUT Fixtures
Clamps or holding brackets designed to immobilize the DUT during test. DUT fixture design is usually a custom endeavor due to the wide variety of DUT shapes and sizes. Westar offers some standard “universal” fixtures suitable for thin module mounting.
DUT Max Diagonal
The maximum size DUT supported by a optical measurement system may be limited by the goniometer size (a purely physical limitation) or by the length of the X and Y axes (limited movement range of the LMD). In the FPM System specification, DUT Max. Diagonal is conservatively given based on the X axis travel assuming a 4:3 aspect ratio DUT, or the goniometer size, whichever is less.
E
Electrical Meters
Digital Multi-Meters (DMMs) or SourceMeters used to measure or excite / measure the DUT.
F
Flicker
A key figure of merit for displays that describes the level of time variance (instability) of the luminance. The negative effects of flicker on the display user are well documented. Flicker arises from various sources such as the display frame rate (in conjunction with the e-o persistence) or backlight beating against the frame rate (AMLCD). The measurement involves a light detector with an analog output detector, such as the Westar TRD-100 and TRD-200 Temporal Response Detectors, a data acquisition function, and software. Flicker is typically derived by performing a real-time Fast Fourier Transform on the luminance signal and extracting the peak AC components from the frequency magnitude plot. Numerically, flicker is defined in several standards such as the VESA FPDM or ISO 13406-2.
Full Luminance Black
Center screen measurement of luminance along the normal axis with the display driven to a full screen, black pattern.
Full Luminance White
Center screen measurement of luminance along the normal axis with the display driven to a full screen, full white pattern.
G
Gamma
A numerical factor which defines the shape of the luminance vs. gray scale curve. The term dates to the CRT era and originally defined the luminance vs. cathode drive voltage characteristic. In flat panel display technology, the gamma factor is derived from the luminance vs. gray level curve by taking the slope (linear regression) of the curve when plotted in log-log format.
Goniometer
A two-axis rotational stage used to rotate the DUT for viewing angle measurements. Two types of goniometers are available for the FPM-500 series systems: The Polar Goniometer rotates the display about a vertical (inclination) axis and a normal (azimuthal) axis. The Orthogonal Goniometer rotates the display about a vertical (turn) axis and a horizontal (tip) axis. Each goniometer type is capable of achieving a full ±90° viewing angle both vertically and horizontally. Finally, regardless of the mechanical arrangement chosen, the ViewPoint™ software supports both polar- and orthogonal-viewing angle coordinates and automatically performs the required transformations to achieve the desired angle.
Grayscale vs. Luminance
A series of luminance measurements made at regular gray level intervals. The resulting luminance vs. gray level curve defines the fundamental electro-optical transfer function of the display. The gamma of the display is derived from this curve by taking the slope (linear regression) of the curve when plotted in log-log format.
I
ISO Standard
International Standards Organization’s Flat Panel Display measurement standard. Formerly titled “Ergonomic requirements for work with visual displays based on flat panels – Part 2: Ergonomic requirements for flat panel displays”
L
Light Leakage
A measure of the luminance uniformity near the edges of the display active area. This test typically involves a series of tightly spaced luminance measurements (dotted line format) near, and perpendicular to the edge of the display. Small spot size is important to avoid overlap of the individual measurements.
LMD
Light Measurement Devices. The term LMD is used generically to describe an optical instrument used to make photometric, colorimetric, or spectral measurements of light. LMDs can also include video photometers, CCD cameras, and temporal measurement sensors. Our Display Optical Measurement Systems work with more LMDs than any other display measurement system. Standard photometers and spectroradiometers from Photo Research, Topcon, Optronic Laboratories, and Minolta are all supported. This broad range of available instrument interfaces allows you to pick the best instrument for the job.
LMD Interface
FPM Systems are compatible with the widest range of commercial photometers and spectroradiometers. An LMD interface is defined by remote control software including a Graphical User Interface in the ViewPoint™ Manual Control Interface and a corresponding Active X Test Object for use with the ViewPoint™ Automated Test Sequencer.
LMD Optics
Refers to the method used to gather light. Traditional LMDs use an image forming objective lens that focuses a spot on the detector surface. Fiber optically coupled optical probes are used when the LMD is remotely located. In this case, the light is gathered by a remote objective lens and relayed down a fiber optic cable and coupled back into the LMD.
Luminance
A basic measurement of display light level as seen by the eye. Formally defined as the measure of luminous flux (radiated power) per unit solid angle and per unit projected area. Units are fL (foot-Lamberts) or Cd/m2 (candela per meter squared). Luminance measurements consider the spectrally-weighted photopic response of the human eye. Light Measurement Devices (Photometers) are designed to measure luminance.
Luminance Uniformity
A measure of the constancy of the Luminance level across the active area of a display. Typical uniformity arrangements include 5-, 9-, and 13-point uniformity measurements.
Luminance vs. Blue Level
A series of luminance measurements made at regular blue level intervals (red and green set to zero) along the display normal axis.
Luminance vs. Gray (Off Axis)
A series of luminance measurements made at regular gray level intervals along an off-normal display axis (at specified angle).
Luminance vs. Gray (On Axis)
A series of luminance measurements made at regular gray level intervals along the display normal axis. The resulting luminance vs. gray level curve defines the fundamental “on-axis” electro-optical transfer function of the display.
Luminance vs. Green Level
A series of luminance measurements made at regular green level intervals (red and blue set to zero) along the display normal axis.
Luminance vs. Red Level
A series of luminance measurements made at regular red level intervals (green and blue set to zero) along the display normal axis.
M
MIL-L Standard
Military Specification that defines Night Vision Imaging System (NVIS) compatible interior aircraft lighting. Includes measurement methods for spectral emission compliance and sunlight reflection testing.
O
Off Axis Gamma
Gamma derived from the Off-Axis Luminance vs. Gray Level curve.
On Axis Gamma
Gamma derived from the On-Axis Luminance vs. Gray Level curve.
P
Power Consumption
A measure of the electrical power consumption of the DUT. DUT power is typically measured by the programmable DC power supplies. Indirect power measurement through a DMM is also possible. Westar offers this capability through fully integrated AC/DC power supplies and DMMs. Our ViewPoint™ software allows the power supply and DMM test assets to be automatically programmed and read with power calculations derived from measured voltage and current.
Power Efficiency
A measure of the DUT efficiency at producing luminance. Typically computed from full bright state luminance reading divided by the electrical power consumption. This factor will vary by display technology (OLED vs. AMLCD vs. Plasma, etc.) and may involve the entire display or only certain display components (such as the backlight).
R
Reflection Tests
Specialized display tests involving ambient light sources pointed at the display surface. Typically reference measurements are made on a reflection standard (specular or diffuse) and then mathematically compared to actual measurements of the display surface. Reflection tests are defined in at least three popular display measurement standards: VESA FPDM, ISO 13406-2, and MIL-L-85762A. Another specialize reflection test, Front Frame Gloss, is defined in TCO ’99 / ’03.
Residual Image
Also known as image retention, this test is a measure of the effects of long term static images on the display. A high contrast image is displayed for a relatively long period of time (burn-in time) and then post burn-in luminance measurements are made on full-white and full-black images. The phenomenon is more familiar with CRT and Plasma technologies (phosphor based) than with AMLCD, but can occur due to liquid crystal drive imbalances.
Response Time
A key figure of merit for displays that describes how fast the display switches from its dark state to bright state and vice-a-versa. The measurement is important for displays which must show changing information (images) at fast rates such as motion video. The test involves a light detector with an analog output detector, such as the Westar TRD-100 or TRD-200 Temporal Response Detector, a data acquisition function, and software to extract the 10%~90% and 90%~10% transition times of the luminance signal when a black-white-black blinking test pattern is shown.
Response Time – Gray Level
A measure of the luminance transition time when the display is switched between specified gray levels.
S
Shadowing
A measure of the degree of interference (cross-talk) between adjacent areas of the display active area. Simple shadowing measurements involve a full-white (followed by full-black) square on a mid gray background and measuring the effect on the adjacent gray area. A more sophisticated shadowing test is defined in VESA FPDM 303-4 “Shadowing – Gray Scale Artifacts” and involves first determining the worst case combination of box- and background-gray levels.
Specular Reflection
A measure of how “mirror like” a display surface is; i.e., the degree a distinct image of an object is reflected by the display surface. Traditionally, simplified reflection models only addressed the specular and diffuse components of the display surface. In most applications, it is adequate to measure specular or diffuse reflection factors only, but this ignores a potentially significant intermediate reflection component called “haze.”
The test involves setting up a small area luminance source at an off-normal, complementary angle to the LMD. A reference luminance measurement is made on a mirror standard in place of the display. The standard is then removed, replaced by the DUT, and measurements are repeated on the actual DUT surface. Westar offers the necessary apparatus to perform this test.
Sunlight Contrast Ratio
A measure of display contrast in the presence of high ambient light (simulated sunlight). The test is formally defined in MIL-L-85762A, paragraph 4.8.16, “Daylight legibility and readability inspection” and is intended to evaluate cockpit display readability in the bright sunlight. Westar offers the required lighting apparatus, fully integrated with the FPM System, and supporting test sequences and written procedures.
T
TCO Angular Dependence
A viewing angle luminance test that considers the display user’s vantage point (eye point) as it sees the left and right sides of a computer monitor. The display angle and working distance of the LMD are specified and require some degree of precise motion control to achieve. The FPM 5-axis Systems automatically perform this test allowing the user to specify the display size, which strongly affects the measurement geometry.
TCO Standard
TCO (The Swedish Confederation of Professional Employees) Development’s display measurement standards. These standards are used for quality certification and environmental labeling primarily for Flat Panel Display computer monitors.
Temperature Testing
An environmental test that involves heating or cooling the display during optical measurements. FPM “–TC” Systems include a full size industrial thermal chamber (with the motion base built inside). The chambers are automatically commanded to the desired temperature (-35°C ~ +135°C). Temperature tests are useful for engineering development, design verification testing, or compliance testing for displays intended for harsh environments such as automotive, aerospace, and industrial applications.
Thermal Chamber
A fully integrated industrial-grade thermal chamber capable of cooling and heating the DUT between –40°C ~ +85°C. The system motion base and DUT are located inside the chamber. Optical measurements are made through a fiber-optically coupled probe so that the LMDs remain outside the chamber in an ambient environment. Check out our FPM-505R Reflective Display Measurement System for more information.
V
VESA Standard
Video Electronics Standards Association Flat Panel Display Measurement Standard tests. Now published as “FPDM 2.0 – Flat Panel Display Measurements”
Viewing Angle (Full Cone)
A measure of luminance (or chromaticity) across multiple viewing angles forming a solid angle (cone) usually centered on the display normal axis. The FPM Systems perform this test automatically allowing the user to specify the cone subtense and angular resolution (step size) for each measurement. Data is automatically logged to an Excel spreadsheet where 3-D viewing angle plots are given. Goniometric solutions to the viewing angle test allow direct measurements at each discrete angle using a spot photometer or spectroradiometer.
Viewing Angle Horizontal
A measure of how the luminance (or chromaticity) varies with left-to-right viewing angle. This measurement is especially important for LCD technologies which exhibit relatively high viewing angle dependence due to the use of polarizers. Near Lambertian emitters, including display technologies such as CRT, OLED, Plasma, and FED, are not characterized by problematic viewing angle dependence. FPM Systems automatically measure horizontal viewing angle and employ our unique “Spot Tracking” algorithm to maintain working distance and instrument focus during all measurements.
Viewing Angle Vertical
A measure of how the luminance (or chromaticity) varies with up-down viewing angle. This measurement is especially important for LCD technologies which exhibit relatively high viewing angle dependence due to the use of polarizers. Lambertian emitters, including display technologies such as CRT, OLED, Plasma, and FED, are not characterized by problematic viewing angle dependence. FPM Systems automatically measure vertical viewing angle and employ our unique “Spot Tracking” algorithm to maintain working distance and instrument focus during all measurements.
W
Warm Up Time
A measure of how much time the display requires to reach a predefined, stable, luminance level following power on. Typically, the display must be left off and allowed to reach ambient temperature before beginning the test. Luminance measurements are made at regular time intervals (30 ~ 60 seconds apart) as the display is powered on an warms up.
Working Distance
Working Distance. W.D. is the distance between the LMD and the display surface when measurements are made. It is typically measured from the front of the LMD lens to the display. The working distance, in conjunction with the LMD measurement aperture, defines the measurement spot size. W.D. is fully programmable in the ViewPoint software, but is limited to a range based on the length of the Z axis and the spacing between the goniometer and the x-y-z stage. During viewing angle measurements, the ViewPoint software adjusts the Z axis position to maintain the desired working distance. In this way, the nominal measurement spot size and instrument focus remain constant resulting in more accurate measurements.
White Point Temperature
The Correlated Color Temperature (CCT) of display full screen, full white pattern.
1,2,3,..
13 Point Color Uniformity
A series of chromaticity measurements made at 13 predetermined points. The measurement points are typically arranged to include 8 points evenly distributed near the edges of the display (10% of display height or width away from the edge), a center point, and 4 additional points surrounding the center point. The measurements may be made with a full screen white, red, green, or blue test pattern.
13 Point Luminance Uniformity
A series of luminance measurements made at 13 predetermined points. The measurement points are typically arranged to include 8 points evenly distributed near the edges of the display (10% of display height or width away from the edge), a center point, and 4 additional points surrounding the center point. The measurements may be made with a full screen white or black test pattern.