Use the quantity that matches the question
Luminance describes light leaving a surface in a specified direction and is measured in candela per square metre, cd/m². “Nit” is the common display-industry name for the same unit. Illuminance describes light arriving at a surface and is measured in lux. A lux meter placed at the screen and a luminance meter aimed at the screen answer different questions.
Record illuminance with the sensor plane and orientation stated. A reading on a horizontal desk cannot stand in for a reading at a vertical screen. For luminance, record the viewing direction, measurement field and screen area sampled. The measurement field must represent the intended pixels without unintentionally including the bezel or background.
Attach conditions to every luminance value
A useful specification identifies the image pattern, patch dimensions, surrounding image, elapsed time, display preset, brightness setting and environmental state. A small white patch and an all-white image can place different loads on the display. Automatic brightness control, thermal control and content-dependent processing may change the result over time.
| Field | Record |
|---|---|
| Device | Model, serial number, firmware and operating preset. |
| Signal | Resolution, frame rate, range, colour encoding and exact test image. |
| Pattern | Patch size, background values and position. |
| Geometry | Instrument distance, angle, aperture and measurement point. |
| Time and environment | Warm-up, measurement timing, ambient light and temperature. |
| Instrument | Model, calibration status, range and uncertainty information. |
A comparison example
Hypothetical example. One proposal lists 1,200 cd/m² for a short-duration small white window; another lists 800 cd/m² for sustained full-screen white. These values describe different operating conditions. Request both displays under one agreed pattern and timing before ranking their luminance. Neither number alone establishes readability in the installation.
Likewise, a control set to 50% is a setting, not a measured output. The relation between a control value and optical output can be nonlinear and can vary with mode. Save the setting for repeatability, and report the measured value separately.
Choose instruments and a repeatable sequence
Use a luminance-capable instrument for screen output and a suitable illuminance meter for the incoming light field. A camera can document the setup; quantitative work requires calibration, exposure control and a known measurement method. Check low-light capability before reporting black levels. If black is below the instrument's usable floor, record that limit rather than converting it into an unlimited contrast claim.
Warm the display in its intended mode, capture the configuration, measure the agreed points, repeat the sequence and retain the raw readings. Choose stability and repeatability criteria in the test plan. IDMS chapter 3 and sections 5.3–5.8 identify relevant setup and white/black measurement topics; use the complete procedure when specifying a formal IDMS measurement.
Sources and further reading
Calculations use the stated assumptions. Project procedures and acceptance limits must be defined for the supplied configuration.