I. Definition of Illuminance Uniformity
Illuminance Uniformity (Uniformity Ratio of Illuminance) is a technical indicator that measures the uniformity of illuminance distribution on a specified surface. It reflects the degree of dispersion of illuminance values at various points within a lit area and is one of the core parameters for evaluating lighting quality.
According to the definition in the Chinese national standard Standard for Lighting Design of Buildings (GB 50034-2013), illuminance uniformity is typically calculated in the following two ways:
- U₀ (Illuminance Uniformity): The ratio of the minimum illuminance (E_min) to the average illuminance (E_av) on the specified surface.
- U₁: The ratio of the minimum illuminance (E_min) to the maximum illuminance (E_max) on the specified surface.
Among these, U₀ is the most commonly used evaluation index in domestic and international lighting standards. Its value ranges from 0 to 1, with values closer to 1 indicating a more uniform illuminance distribution.
II. Illuminance Uniformity Standards for Different Venues
Standard values for illuminance uniformity vary according to venue function, the fineness of visual tasks, and requirements such as television broadcasting. They are primarily based on Standard for Lighting Design of Buildings (GB 50034-2013), General Code for Building Environment (GB 55016-2021), and Standard for Lighting Design of Urban Roads (CJJ 45-2015).
Venue Type | Evaluation Index | Standard Requirement (Minimum) | Primary Reference |
Areas for prolonged visual tasks (offices, classrooms) | U₀ | 0.6 | GB 50034 / GB 55016 |
Areas with high visual demands (precision work) | U₀ | 0.7 | GB 50034 |
General areas (corridors, storage rooms) | U₀ | 0.4 | GB 50034 |
Sports venues with TV broadcasting (horizontal illuminance) | U₁ / U₀ | 0.5 / 0.7 | GB 50034 |
Sports venues with TV broadcasting (vertical illuminance for main camera) | U₁ / U₀ | 0.4 / 0.6 | GB 50034 |
Professional competition without TV broadcast (horizontal illuminance) | U₁ / U₀ | 0.5 / 0.7 | GB 50034 |
Amateur competition without TV broadcast (horizontal illuminance) | U₁ / U₀ | 0.4 / 0.6 | GB 50034 |
Classroom blackboards | U₀ | 0.8 | GB 55016 |
Operating rooms | U₀ | 0.7 | GB 55016 |
Vehicle lanes (overall luminance uniformity) | U₀ | 0.4 | CJJ 45 |
Road intersections (illuminance uniformity) | U₀ | 0.4 | CJJ 45 |
General lighting in exhibition halls | U₀ | 0.7 | Lighting Design Handbook |
Lighting for flat exhibits | U₀ | 0.8 | Lighting Design Handbook |
The Lighting Design Handbook also specifies that the illuminance uniformity U₀ for operational areas in industrial settings should not be less than 0.7, with the adjacent surrounding area not less than 0.5.
III. Calculation Methods and Procedures for Illuminance Uniformity
Illuminance uniformity needs to be determined through illuminance calculation or on-site measurement. During the engineering design phase, the point-by-point calculation method or professional lighting simulation software is often used to obtain illuminance values at various points for subsequent calculation.
Calculation Steps:
- Define the calculation area and grid: Based on code requirements, divide the lighting area into a number of rectangular or square grids of equal area. The grid should cover the entire work plane or reference plane.
- Obtain illuminance values at each point:
- Design Phase: Use the utilization factor method to calculate average illuminance, and combine it with the luminaire’s photometric distribution curve and installation parameters to calculate illuminance at each grid point via the point-by-point method, or use software simulation directly.
- Acceptance Phase: Use a calibrated, qualified illuminance meter to perform on-site measurements at pre-defined grid points. Measurement methods include the “center method” (measurement point at the grid center) and the “four-point method” (measurement points at the four corners of the grid).
- Calculate key parameters:
- Average Illuminance (E_av): Arithmetic mean of illuminance values from all measurement points.
where is the illuminance value at the i-th measurement point, and N is the total number of points.
- Minimum Illuminance (E_min): The smallest illuminance value among all measurement points.
- Maximum Illuminance (E_max): The largest illuminance value among all measurement points (used for calculating U₁).
- Calculate Illuminance Uniformity:
- Comparative Evaluation: Compare the calculated U₀ (or U₁) value with the requirements of the current national standard for the specific venue. If it meets the requirement, the design passes; if not, the luminaire layout must be adjusted (e.g., reducing spacing, changing to luminaires with wider distribution, increasing mounting height, etc.) and recalculated.
IV. Engineering Factors Affecting Illuminance Uniformity
In design practice, the following factors directly influence the final illuminance uniformity:
- Luminaire Photometric Distribution: The distribution curve of the luminaire determines how luminous flux is distributed in space. Luminaires with wide distributions are generally more conducive to achieving uniform illuminance than those with narrow distributions.
- Luminaire Spacing-to-Height Ratio (L/H): The ratio of the center-to-center spacing (L) of adjacent luminaires to the mounting height (H) of the luminaire above the work plane. Each luminaire has its own “maximum allowable spacing-to-height ratio.” When the actual ratio exceeds this limit, illuminance uniformity drops significantly, leading to noticeable “dark spots.” The design must ensure the spacing-to-height ratio in both directions does not exceed the luminaire’s maximum allowable ratio.
- Luminaire Mounting Height: While ensuring glare control, appropriately increasing the mounting height of luminaires can help expand the coverage area of a single luminaire, thereby improving uniformity, though this may reduce illuminance levels.
- Surface Reflectance: The reflectance of indoor surfaces such as ceilings, walls, and floors affects the secondary distribution of luminous flux. Increasing the reflectance of these surfaces (e.g., using light-colored paints) can effectively improve overall illuminance uniformity.
- Luminaire Layout: Different layout schemes (e.g., uniform ceiling grid, wall washing, grouped luminaires) have varying impacts on uniformity. For areas requiring uniform illumination, symmetrical and uniform layouts should be prioritized.
V. Measurement and Acceptance of Illuminance Uniformity
After project completion, on-site measurements are required to verify whether illuminance uniformity meets design requirements. Measurements should follow these principles:
- Measuring Instrument: A calibrated and certified illuminance meter with a precision level of at least Grade 1 should be used.
- Measurement Conditions: Measurements should be conducted after the luminaires have been seasoned (typically 100 hours) and room surfaces are clean. The supply voltage should be monitored and recorded during measurement.
- Measurement Method: Following the provisions of GB/T 5700 Methods of Photometry, a measurement grid is laid out over the area. An illuminance meter is used to measure the horizontal (or vertical) illuminance at each grid point. The measurement height should correspond to the design reference plane (typically floor level or 0.75m work plane).
- Data Processing: Calculate the measured average illuminance and illuminance uniformity. According to the International Commission on Illumination (CIE), the relative error between the measured average illuminance and the design calculated value should not exceed ±10%.
- Result Judgment: Compare the measured illuminance uniformity with the requirements of the design standards and applicable codes. A formal lighting test report should be issued.