Introduction

In the field of lighting design, illumination extends far beyond merely achieving specific illuminance levels; ensuring visual comfort is equally paramount. Sports lighting design encompasses multiple parameters including horizontal illuminance, vertical illuminance, uniformity, glare, and uniformity gradient. Among these, horizontal illuminance serves as the fundamental metric frequently referenced, whilst vertical illuminance is primarily employed for venues requiring high-definition live broadcasts or television coverage. As for uniformity, glare, and uniformity gradient, these directly impact visual comfort. While lighting uniformity (What’s light uniformity? and How to improve it?) is a familiar concept to many practitioners, uniformity gradient remains a relatively niche parameter. Although referenced in standards such as EN12193 and RP-6-15, detailed explanations are often lacking. This article will focus on explaining what lighting uniformity and uniformity gradient are, along with their calculation formulas. Simultaneously, we shall use ZGSM’s sports lighting design as an example to analyze the derivation of these two calculation results. Let us now proceed to the main text to delve deeper into these key parameters mainly including uniformity gradient that define quality sports lighting.

What’s lighting uniformity?

In sports lighting, illuminance uniformity serves as the core metric for assessing the consistency of light distribution across an entire playing surface. It encompasses multiple evaluation indicators, such as U1 and U2. Based on vertical and horizontal illuminance uniformity, it can be further subdivided into U1hor, U1vert, U2hor, and U2vert. Taking the most typical U2hor from the EN12193 standard as an example, it represents the ratio of the minimum illuminance to the average illuminance within the playing area. This metric indicates whether illuminance is uniform across the field; ideally, U2hor=1. A lower value often indicates the presence of excessively bright or dark areas within the playing area. Within EN 12193, most pitches require U2hor > 0.7 for Lighting Class I, though requirements are appropriately reduced for lower lighting classes. Achieving high uniformity standards relies heavily on precise optical design and simulation calculations. Lighting simulations enable optimization of luminaire layout, encompassing mast height selection, beam angles, and optimization of luminaire wattage and light distribution. Furthermore, employing asymmetric lenses (light distribution) enables uniform illumination across all corners of the pitch while reducing glare. Naturally, this relies heavily on excellent product design (primarily light distribution). As LED manufacturers, delivering uniformity that meets the standards of top international competitions (such as FIFA and FIBA) necessitates an indispensable combination of product quality, light distribution (What’s light distribution?), and lighting simulation.

What’s uniformity gradient?

Uniformity Gradient (UG) is likewise employed to measure uniformity in sports lighting applications. However, unlike lighting uniformity, UG calculates the difference in illuminance between the measured point / calculation point and its four adjacent points. This metric quantifies the rate of decline in values between measurement points. In high-speed sports, the true velocity of the visual target (ball) must be discernible to enable athletes to react appropriately. Should sudden variations occur in the illumination levels across a sports field, rapidly moving targets transitioning between light levels may appear to accelerate or decelerate, thereby impairing sports judgement. Specifically, uniformity gradient measures the rate of change in values between adjacent measurement points (horizontal, vertical, or diagonal). The uniformity gradient value in the report indicates the highest ratio or greatest rate of change. Many practitioners note that the rate of change (UG value) varies with the distance between measurement points. Consequently, the table below showing UG limits for different sports speeds demonstrates that greater distances permit higher permissible UG value thresholds. ZGSM’s practical application of ZGSM has also observed that larger grid distances typically yield higher calculated UG values. The UG value is expressed as a numerical ratio; within Dialux reports (What’s Dialux and its application in street light and sports lighting?), it denotes the highest ratio or greatest rate of change.

How are the light uniformity and uniformity gradient calculated?

Calculation grid

To measure/calculate illuminance or luminance (More about illuminance vs luminance) on the reference surfaces under consideration, we require a scientifically planned measurement grid and measurement points, which is also called calculation grid or points. EN 12193 specifies particular measurement grids for sports facilities (such as running tracks). Accordingly, it defines reference surfaces and the number of grid points in two directions relative to these surfaces for various sports disciplines. This standard establishes principles for dividing the measuring grid; for instance, when the length-to-width ratio (D:B) of the field must not exceed 2:1, we can determine the point spacing P via the formula P=0.2*5*logD, thereby confirming the grid density. Taking a 36m × 18m tennis court as an example, calculating P = 2.447m yields M ≥ D/P = 14.7, thus confirming M = 15. Similarly, calculating N as N ≥ 18/2.447 = 7.36, the nearest odd number being 9, results in 15 × 9 measurement points. For different court dimensions, EN 12193 also recommends corresponding measurement points. For instance, a 105×68-metre football field would yield 21×15 points via the above formula, yet EN 12193 specifies 21×13 measurement points – a slight deviation from the standard calculation method.

Calculation formula for lighting uniformity

Illuminance uniformity is measured by two metrics: U1 (minimum/maximum) and U2 (minimum/average). Furthermore, illuminance is categorized into horizontal and vertical components, hence U1 is subdivided into U1 hor and U1 vert, while U2 is divided into U2 hor and U2 vert. The following four formulae are provided, along with their respective guiding principles. Equation 1: U1hor = Emin hor / Emax hor. U2hor aims to simultaneously eliminate excessively dark shadows and overly bright spots, achieving ultimate light smoothness. Only when the pitch contains neither excessively bright nor excessively dark areas can Emin/Emax approach the ideal value of 1. Equation 2: U2hor = Emin hor / Eave hor. U1hor prioritizes overall brightness consistency to prevent underexposed zones, aiming for Emin to approach Eave. Equation 3: U1vert = Emin vert / Emax ver, and equation 4: U2vert = Emin vert / Eave vert. For pitches requiring live broadcasts, vertical illuminance (Vertical illuminance and why it’s important in sports lighting?) must meet specifications. Compliance with U1vert and U2vert prevents underexposed or overexposed areas on players’ faces and kits.

Calculation formula for uniformity gradient

By calculating the difference in illuminance between each point within the measurement grid and its adjacent neighbouring points (typically in four directions), the smoothness of light distribution is assessed. The formula for this is: Uniformity gradient = Max(| Ep – Eneighbour |/Ep). Therefore, we must first define the measuring grid. The Uniformity Gradient (UG) is calculated for each point Ep within the grid by determining the rate of change in its value relative to the four adjacent points above, below, left, and right. Through calculation, we obtain the uniformity gradient value for each point, thereby verifying whether the overall UG value of the court meets standard requirements. For instance, for a standard tennis court, our lighting simulation (ZGSM lighting simulation service) yields illuminance values for each calculated point as shown in the diagram. To compute the UG value for the red-marked point, we derive: UGleft = 9.2%, UGright = 22.8%, UGup = 1.2%, UGdown = 2.4%. Thus, UGmax = 22.8%. Dialux also provides corresponding results, indicating UG=22%, consistent with the calculated outcome (the slight discrepancy arises from rounding).

How to improve UG (uniformity gradient) in sports lighting?

Is there a connection between lighting uniformity and uniformity gradient? Does higher lighting uniformity necessarily imply a higher uniformity gradient? In fact, through the calculation formula and process, we understand that one focuses on the overall effect while the other emphasizes localized areas. In most cases, better lighting uniformity results in a smaller uniformity gradient value, but the relationship between the two is not linear. Furthermore, the prevailing view holds that fewer, higher-wattage floodlights produce more unnatural light transitions, akin to stage lighting. Consequently, employing a greater number of LED floodlights (ZGSM LED floodlights) can enhance the uniformity gradient. We shall now examine two scenarios to illustrate this point.

Better uniformity to get better UG

The following is a tennis court lighting simulation. The right-hand image shows a symmetrical light distribution, where we can observe poor illumination uniformity. In Dialux, we simultaneously calculated the uniformity gradient (UG) value. The results show that many points now exceed 20%, with some calculation points surpassing 60%. To achieve better illumination uniformity, ZGSM recommends that such tennis courts adopt an asymmetrical light distribution. As shown in the left image below, the illumination uniformity reaches 0.7, while the UG value only exceeds 20% in a small portion of the area, with a maximum value of 32% – significantly lower than the other lighting scheme. In summary, employing asymmetric light distribution in such courts achieves ideal illumination uniformity while significantly reducing the UG value, thereby minimizing abrupt changes in light intensity. Furthermore, asymmetric distribution also lowers glare values, enhancing visual comfort for court users. Contact ZGSM for further information on reducing glare in sports lighting.

Avoid using small number of high-power sports lights

Why can we achieve a lower UG value by not employing a small number of high-power sports lights? The reason is straightforward: using more lights enables us to attain superior lighting uniformity. Based on our aforementioned analysis, we can thus obtain better uniformity gradient results. But if uniformity remains the same, will the UG results also be identical? The following lighting simulation analyses two scenarios. The first employs 24 x 1500W Glomax high mast lights (ZGSM Glomax high mast lights), yielding uniformity values of U1=0.47 and U2=0.71. The second utilizes 48 x 750W Glomax high mast lights, achieving U1=0.46 and U2=0.71. Both demonstrate comparable uniformity, with illuminance levels around 500 lux. However, examining the uniformity gradient table reveals substantial differences: the former achieves UGmax=50%, while the latter only reaches 36%. Furthermore, the latter shows only a handful of calculated points exceeding 20%.

ZGSM LED sports lighting design

Summary

In sports lighting design, light uniformity and uniformity gradient are two core yet frequently confused concepts. Lighting uniformity measures the overall consistency of light distribution across the entire venue, commonly evaluated using the ratio of minimum to average illuminance (U2). Uniformity gradient, however, focuses on local details, calculating the rate of change in illuminance between adjacent measurement points to reflect the smoothness of light transitions. Both calculations rely on measurement grids specified by standards such as EN 12193 (More about EN12193) . Uniformity assessment is relatively macro-level, whereas uniformity gradient analysis is more granular, requiring individual examination of luminance changes between each grid point and its neighbour points. In the main text, we note that sports fields with high light uniformity typically exhibit higher uniformity gradients. However, exceptions exist. For instance, sports fields illuminated with fewer high-wattage luminaires may exhibit a larger uniformity gradient despite achieving equivalent illuminance uniformity that meets requirements. In summary, high-quality sports lighting design ( how to avoid costly mistakes in sports court lighting ) necessitates a holistic consideration of both light uniformity and gradient uniformity. Only through careful selection of light distribution, simulation calculations, and judicious luminaire choice and layout can we achieve the ideal lighting scenario: high light uniformity coupled with a low uniformity gradient.

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