Importance of Display Contrast Measurement in Cockpit Instrumentation

Interior of a modern space capsule
Interior of a modern space capsule

Cockpit instrumentation in the aerospace industry has evolved over the past decades. In the past, control panels had backlit gauges, switches, and knobs, where now you are more likely to see an array of flat panel screens. There must be a consistent light and color emitting from these panels to reduce eyestrain, ease interpretation of data, and decrease distractions. In addition, they are comfortable to view equally in daylight and as well as night. Two key measurements are Luminance (a measure of brightness) and Display Contrast (the ratio between light and dark).

Luminance (Brightness) is a photometric measure of the luminous intensity per unit area of light traveling in a given direction. Brightness is defined as the luminance of the brightest component (white color) and is measured in candela per square meter (cd/m2 = nit) or foot-lamberts (1fL=3.426 nits). Typically, display luminance varies from 100 nits, found in most office monitors, up to 1000 nits front projection systems.

There are varieties of units used for luminance. In most countries, the most common unit for luminance is candela/square meter (cd/m2). However, in the U.S., the most common unit is the foot-lambert (fL); 1 foot-lambert (fL) equals 1/π candela/square foot, or 3.426 cd/m2. Professionals in the industry often use the term nit (nt). 1 nit unit is equivalent to 1 cd/m2.

To measure luminance you can use a luminance meter such as the LS-150 or LS-160.

Display Contrast is the ratio between the brightest colors (in most cases white) and the darkest color (in most cases black) that the monitor is capable of producing. Where there is no industry standard in measuring contrast the generally accepted process is to measure parts of a screen and either take the average or highest white and the average or lowest black and express it in ratio form bright:dark. As an example, if a screen has bright luminance of 150 nits and a dark luminance of 1 nit the contrast ratio would be shown as 150:1.

With newer high performance, OLEDs with darker blacks are now producing much wider ratios. Hence, if a monitor can output 7500 nits with a white screen and 0.010 nits with a black screen, it would have a contrast ratio of 750,000:1. A higher contrast produces more in-depth images with better screen quality, giving richer colors that make it easier to interpret images and data. A decent LCD screen might have a contrast ratio of 1,000:1. The contrast on an OLED display is far higher, at around 4000:1, with ultra-high-end units beginning to get close to 1,000,000:1. When an OLED screen shows black, its pixels produce almost no light whatsoever.

A display’s contrast ratio is one of the most important measurements of performance. In addition, it will be the most noticeable difference between two displays in a side-by-side comparison.

To measure contrast it is best to use a spectroradiometer such as the CS-2000 and CS-2000A. With high-end spectroradiometers, you can measure the darkest blacks down to super-low luminance of 0.003cd/m2 allowing measurements up to 1,000,000:1.

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