Definition
A photometer is an optical instrument used to measure the luminous intensity of a light source or illuminated surface within a unit projected area in a specific direction. The measurement is measured in candelas per square meter (cd/m²) or nits (nits) and characterizes the level of brightness perceived by the human eye. In the field of optical metrology, photometer is one of the key equipment to achieve quantitative evaluation of optical radiation.
Principle
The core working principle of the photometer is based on optical imaging and photoelectric conversion. The instrument images the target to be measured on the photosensitive surface of a built-in photodetector (such as a silicon photodiode) through an optical lens, and the detector converts the received light signal into an electrical signal. After the signal is amplified and converted analog-to-digital, the brightness value is calculated by the instrument's internal processor based on a preset calibration factor. Its basic relationship can be expressed as:
L = k × I
where L is the brightness, I is the processed electrical signal, and k is the system constant obtained by the standard brightness source calibration.
Measurement method
In actual measurement, the corresponding method should be selected according to the target characteristics. For active luminaries (e.g., displays, indicators), the direct measurement method is usually used to align the photometer probe vertically at the area to be measured to ensure that the measured field of view covers the target area. For reflective or transmitted surfaces (e.g. road markings, optical films), the brightness at a specific geometric angle is measured under specified lighting conditions. When measuring, it is necessary to pay attention to the accurate focus of the instrument, avoid environmental stray light interference, and operate according to the measurement conditions specified by relevant standards (such as ISO/CIE, JIS, GB/T series standards).
Influencing factors
Measurement accuracy is influenced by various factors. The size of the field of view of the optical system determines the spatial resolution of the measurement area, and a smaller field of view is suitable for measuring local details. The spectral response of the detector needs to be matched with the CIE photovisual spectral optical efficiency function V(λ) through the filter, and the matching degree affects the accuracy of color measurement. The measured distance and angle will change the received luminous flux and must be consistent with the calibration state. Changes in ambient temperature may cause detector sensitivity drift, and high-precision measurements need to be performed in a temperature-controlled environment. In addition, the instrument's own nonlinear response, stray light suppression ability, and long-term stability are also technical parameters that need to be considered.
Applications
Luminance meters have a wide range of uses in several industries. In the field of display technology, it is used for the evaluation of brightness uniformity, contrast and viewing angle characteristics of liquid crystal panels and organic light-emitting diode displays. In the automotive industry, it is used in the optical performance testing of instrument panels, lights and road lighting systems. Airworthiness certification measurement for cockpit displays and signal indicators in the aerospace field. In architectural lighting projects, assist in assessing the quality of indoor and outdoor light environments. At the same time, it is also a basic tool for scientific research institutions to conduct visual perception research and optical material characterization analysis.
Key points of selection
Choosing a luminance meter should take into account the measurement needs and technical parameters. The measurement range should cover the expected brightness values, and common instruments can range from a few percent of a nit to millions of nits. Field of view selection depends on the target size and measurement distance, and is usually available in 1°, 0.2°, or adjustable field of view options. For color-sensitive applications, focus on the spectral matching level of the instrument. The use scenario determines the instrument form, and the laboratory environment can choose a desktop high-precision model, and portable equipment is suitable for on-site testing. Data interface capabilities, software analysis capabilities, and compliance with specific industry standards (e.g., SAE standards for the automotive industry, VESA standards for the display industry) are also aspects that need to be evaluated in the actual selection.