From Principle to Standard: A Comprehensive Analysis of Disability Glare Control in Road Lighting

1. What is Threshold Increment (TI)?

Threshold Increment (TI) is a metric used to quantitatively evaluate disability glare in road lighting.

Disability Glare: Refers to the reduction in a driver’s ability to see objects due to the presence of high-luminance glare sources (e.g., streetlights) in the field of view, which may not necessarily cause significant discomfort.
Definition of TI: It represents the percentage increase in the luminance contrast required between an object and its background to achieve the same level of visibility when a glare source is present.
Simplified understanding: Think of it as glare causing objects on the road to become “blurry.” TI measures how much more contrast (difference in brightness) needs to be added to make them clearly visible again.

2. Why Limit Threshold Increment (TI)?

Limiting Threshold Increment (TI) in vehicular roadway lighting is crucial for the following reasons:

Ensuring Driving Safety: Disability glare directly impairs a driver’s ability to observe road obstacles (such as pedestrians, debris, or potholes). Without proper control, it reduces the reliability of a driver’s situational judgment and increases the risk of accidents.
Enhancing Visual Comfort: Although TI primarily addresses disability glare, related studies indicate that if disability glare is restricted to a satisfactory level, the impact of discomfort glare (annoying glare) can typically be considered negligible.
Complying with National Standards: China’s Standard for Lighting Design of Urban Road Lighting (CJJ 45-2015) explicitly stipulates Threshold Increment (TI) as a mandatory evaluation metric for vehicular roadway lighting, setting strict maximum initial values for different road classes.

3. How is Threshold Increment (TI) Calculated?

According to the Standard for Lighting Design of Urban Road Lighting and the Lighting Design Handbook, when the average road surface luminance ( $L_{a v}$ ) is within the range of $0.05 cd/m^{2}$ to $5 cd/m^{2}$ , the formula for calculating Threshold Increment (TI) is:

$T I = L ^{a v 0.8} \cdot θ ^{2} K \cdot E ^{e} (%)$

The parameters in the formula are defined as follows:

$T I$ : Threshold Increment, unit is % (percentage).
$L_{a v}$ : Initial average luminance of the road surface, unit is $cd/m^{2}$ .
$E_{e}$ : The initial illuminance produced by a single luminaire within the observer’s field of view on a surface perpendicular to the line of sight at the observer’s eye level (1.5m above the road surface), unit is lx.
$θ$ : The angle between the line of sight (typically $1^{\circ}$ below horizontal) and the line connecting the observer’s eye to the photometric center of a luminaire, unit is degrees. Its effective range is generally $1. 5^{\circ} < θ < 6 0^{\circ}$ .
$K$ : A constant related to the observer’s age. Usually taken as 650 for a 23-year-old observer. The value for other ages is calculated by:

$K = 641 \times [1 + (66.4 A)^{4}]$ where $A$ is the observer’s age in years.

Key Points in Calculation:

The Threshold Increment (TI) must be summed for the contributions of all luminaires within a 500-meter range ahead of the observer.
Calculations should be based on the parameters at the initial operation of the lighting system after installation.

4. Application and Standard Values in Practical Scenarios

4.1 Vehicular Roadway Lighting

China’s Standard for Lighting Design of Urban Road Lighting (CJJ 45-2015) clearly specifies the maximum initial values for Threshold Increment (TI):

Class	Road Type	Maximum Initial Value for Glare Restriction TI (%)
I	Expressways, Arterial Roads	10
II	Sub-arterial Roads	10
III	Local Roads	15

Implication: For instance, on an arterial road, the initial threshold increment caused by the designed lighting system must not exceed 10%.

4.2 Interchange Area Lighting

In areas like intersections, ramps, and interchanges, due to non-standard luminaire arrangements and rapid changes in the driver’s viewpoint, TI cannot be accurately calculated. Therefore, these areas adopt an alternative method: restricting the luminous intensity in specific directions to control glare. The specific requirement is that the luminous intensity at an $8 0^{\circ}$ vertical angle should not exceed $30 cd/klm$ , and at a $9 0^{\circ}$ vertical angle should not exceed $10 cd/klm$ , measured from the driver’s viewing angle towards the luminaire.

4.3 Landscape Lighting

According to the Code for Urban Appearance and Environmental Sanitation Engineering Projects, landscape lighting installations should avoid causing glare for motor vehicle drivers. The requirement is that the Threshold Increment for glare caused to drivers should not exceed 15%.

4.4 Pedestrian Roadway Lighting

For pedestrians, the primary concern is discomfort glare. Its control method differs from the TI metric used for vehicular roads. It involves limiting the maximum luminous intensity at specific vertical angles (e.g., $7 0^{\circ}, 8 0^{\circ}, 9 0^{\circ}$ ).

5. Relationship Between TI and Revealing Power (RP)

Research referenced in the source materials shows that the magnitude of Threshold Increment (TI) directly affects a driver’s ability to detect obstacles, known as Revealing Power (RP).

Conditions: When the average road surface luminance is $L_{a v} = 2 cd/m^{2}$ and the overall luminance uniformity is $U_{0} = 0.4$ :
- If TI = 30%, the Revealing Power RP is approximately 65%.
- If TI = 7%, the Revealing Power RP can increase to 80%.
Conclusion: A smaller TI value indicates better glare control, resulting in less loss of visual function for the driver and more reliable detection of road obstacles.

From Principle to Standard: A Comprehensive Analysis of Disability Glare Control in Road Lighting – Threshold Increment