Distributed Photometer: Principle and Applications

Distributed photometers are essential measurement devices used to test the photometric performance of lighting fixtures. They are categorized into vertical and horizontal types and are capable of measuring parameters such as spatial light intensity distribution, total luminous flux, and luminaire efficiency. The system comprises precision turntables and control systems, spectral analysis systems, standard lamps, alignment systems, and computer processing systems.

To ensure accurate measurements, distributed photometers require coordination with large darkrooms with low reflectance materials to prevent reflected light from entering the probe. Given the measurement system’s sensitivity to environmental temperature, maintaining the light source’s temperature stability is crucial. Distributed photometers read measurements angle by angle, which may be time-consuming. However, if only luminaire luminous flux parameters are needed, there are no strict requirements for measurement distance.

There are two measurement methods for measuring luminous flux based on the arrangement of the measurement optical path: illuminance integration and luminous intensity integration.

• Illuminance Integration: This method is flexible and accurate, not limited by the measurement distance, suitable for smaller measurement spaces. Using a compact distributed photometer, it measures the light source’s illuminance distribution in space and integrates across the entire space to obtain total luminous flux. Since it is insensitive to measurement distance and light source installation position and avoids the use of reflectors, it can achieve high-precision measurements and is the method recommended by the International Commission on Illumination (CIE) for achieving the basic unit of luminous flux.

• Luminous Intensity Integration: This method measures the light source’s light intensity distribution in space and integrates across the entire space to obtain total luminous flux. Sufficient distance is required for measuring light intensity distribution, approximating the test object as a point source and utilizing the inverse square law for light intensity measurement.

In the field of lighting engineering, distributed photometers are vital tools for assessing light source luminous intensity distribution, aiding in the design and optimization of lighting systems. Distributed photometers are mainly classified into several types: luminaire rotating, detector rotating, and luminaire fixed, each with specific application scenarios and measurement methods.

Principle:

Distributed photometers measure luminous intensity by rotating the lighting device at a fixed distance to conduct illuminance measurements, capturing luminous intensity. With sufficient angle steps and range, the luminous flux of the lighting device can be calculated. A goniometer is a commonly used tool for measuring luminous flux and the luminous intensity distribution of lighting devices or light sources. Based on the luminous intensity distribution, attributes of lighting applications such as horizontal/vertical illuminance curves or photometric diagrams can be inferred. Depending on the lighting device’s rotation method and photometric data system during the measurement process, goniometers are typically divided into three groups: Class A, Class B, and Class C, distinguished as follows:

• Class A: Suitable for representing relatively limited beam automotive lighting. Class A goniometers have fixed horizontal and vertical axes, rotating the light source around the horizontal axis for measurement.
• Class B: Suitable for displays and floodlights. Class B goniometers have a fixed vertical axis and a movable horizontal axis, rotating the light source around the vertical axis for measurement.
• Class C: Suitable for general lighting systems. Class C goniometers are highly specialized types with fixed vertical axes and movable horizontal axes, conducting measurements on the C plane or conical surface. Class C goniometers are similar to Class B but with the light source rotating 90°.

Photometric curve, also known as luminous intensity distribution curve, describes the spatial light emission distribution of a lighting fixture or light source. It is a persistent curve that records the light intensity in various directions and contains multiple pieces of information about the luminaire, such as luminous flux, power, size, manufacturer, etc.

Classification based on symmetry:

• Axial Symmetry: Also known as rotational symmetry, where the photometric curve in all directions is essentially symmetrical. Common cylindrical lamps and industrial lamps belong to this category.
• Symmetrical: When the photometric curves of the lamp’s c0° and c180° profiles are symmetrical, and the c90° and c270° profiles are also symmetrical, it is called symmetrical photometry.
• Asymmetrical: When either the c0°-180° or c90°-270° profile photometric distributions are asymmetrical.

Classification based on beam angle:

• Narrow beam (< 20°)
• Medium beam (20° – 40°)
• Wide beam (> 40°)

Photometric curve testing usually employs two representation methods:

• Polar Coordinate Representation: Suitable for describing the light distribution of indoor and road lamps. It represents the luminaire’s light center at the polar coordinate origin, uses vectors to represent light intensity, and uses angles to represent the angle between the light intensity vector and the light axis.
• Rectangular Coordinate Representation: Suitable for describing the light distribution of floodlights and narrow-beam lighting fixtures and light sources. It represents the light center at the rectangular coordinate origin, uses the horizontal coordinate to represent the direction angle, and uses the vertical coordinate to represent light intensity.

Shanghai Lisun LSG-6000 rotating probe vertical distributed photometer (full-space distributed photometer) fully meets the requirements of LM-79-19, EN13032-1 clause 6.1.1.3 distributed photometer type 4, CIE S025, SASO2902, IS16106, and GB for light parameter testing. The LSG-6000 is the latest upgraded product in response to the requirements of clause 7.3.1 of LM-79-19 standard, capable of automatically testing 3D light intensity distribution curves, with the testing distance designed according to customer requirements. It can meet the testing requirements of various light sources such as LED light sources, plant lighting lamps, HID light sources, indoor and outdoor lighting, street lamps, and floodlights.

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