Excellent lighting design and advanced lighting equipment allow us to enjoy the comfort and convenience brought by lighting. But the glare problem has always been a very headache.
What is glare? Why is glare? How to reduce or even eliminate glare?
Glare: A visual phenomenon that causes an uncomfortable feeling or a reduced ability to observe a detail or target due to an unsuitable brightness distribution or range of brightness in the field of view, or an extreme contrast. [5]
Glare is a bad visual phenomenon that can make people feel irritated, causing sore eyes, tearing, decreased vision, and may even temporarily lose sight.
The mechanism by which glare reduces visual function can be understood as follows: Light emitted by a glare source scatters in the direction of the retina to form a bright light curtain superimposed on a clear scene image. This light curtain has an equivalent veiling luminance, which is equivalent to an increase in background brightness, a decrease in contrast, a glare, and difficulty in seeing the target.
Figure 1 Human eye and glare
2 Interpretation of terms related to glare
(1) Visual field:
The eye looks at a target, and objects in a certain space outside the gaze point can also be seen. This spatial extent is called the visual field. The field of view is divided into a horizontal field of view and a vertical field of view, reflecting the viewing range of the human eye in the horizontal and vertical directions.
Under static conditions, the horizontal field of view of both eyes can reach 160 degrees, and the vertical field of view can reach 50 degrees. In a moving vehicle, the driver's visual field size is related to the speed of the vehicle. The faster the speed, the more the point is extended, the smaller the field of view, and the surrounding scenery is difficult to see.
Traffic psychology research shows that the vehicle speed is 40km/h, the horizontal moving field is 100 degrees, the vehicle speed is 70km/h, the horizontal moving field is 65 degrees, the vehicle speed is 100km/h, and the horizontal moving field is 40 degrees. The vertical moving field, that is, the viewing space of the driver's line of sight in the vertical direction, is usually 30 degrees.
Affected by human visual parallax and poor lighting conditions, the ability of the human eye to correctly distinguish colors in a dark environment is very poor. Therefore, the ability to visually resolve depends on the difference in brightness between the object and its background. An object can only be seen when it has a certain brightness contrast.
(2) Brightness contrast:
C=⊿L/Lb=∣Lo-Lb∣/Lb
Where ⊿L——identifies the difference between the brightness of the object and the background brightness;
Lo - identify the object brightness (cd / m2);
Lb - background brightness (cd/m2).
The brightness contrast has the distinction of "positive contrast" and "negative contrast". Positive contrast when the brightness of the object is higher than the background brightness; negative contrast when the brightness of the object is lower than the background brightness.
(3) Critical (threshold) contrast Ct (threshold contrast):
Ct = ⊿Lt /Lb
In the formula, ⊿Lt——the difference in critical brightness, the brightness difference between the target and the background when the human eye can recognize the target.
(4) Visibility Level VL (Visibility Level):
The ratio of the actual brightness contrast C of the target to the background and the critical contrast Ct.
VL=C/Ct=⊿L /⊿Lt
Glare is divided into discomfort glare and disability glare according to the degree of influence.
(5) Discomfort glare:
It refers to glare that makes people's eyes feel uncomfortable in the field of vision, but does not necessarily reduce the visibility of visual objects. This glare is also known as mental glare.
(6) Disability glare:
It is the glare that reduces people's visual function in the field of vision. It is a glare that reduces the visibility of a visual object, but does not necessarily create an uncomfortable feeling.
According to its formation mechanism, glare is divided into direct glare, interference glare, reflected glare, and contrast glare.
(7) Direct glare:
The glare produced by the illuminant present in the field of view, especially in the direction of the line of sight.
(8) Disturbance glare:
Glare caused by the illuminant when there is an illuminant in the direction of the non-viewing object.
(9) Glare by reflection:
The glare caused by the reflection in the field of view, especially the glare generated by the reflected image near the line of sight.
(10) Comparing glare (comparison glare):
In the light environment, there is excessive glare formed by contrast of brightness.
3 Evaluation method of glare
So far, for the indoor environment, stadiums, motor vehicles and other lighting occasions, people have summarized the following four main glare evaluation expressions:
3.1 Unified glare value UGR
Unified glare rating UGR calculation: [5]
Where Lb is the background brightness (cd/m2);
La——the brightness of each luminaire in the direction of the observer (cd/m2);
Ω—the solid angle (sr) formed by the illuminating portion of each luminaire to the observer's eyes;
P - the position index of each individual luminaire.
The above parameters should be determined according to the following formulas and regulations:
1) Lb=Ei /Ï€
Where Ei is the indirect illuminance (Lx) of the observer's eye direction.
2) La = Ia/A·cosα
Where, Ia is the luminous intensity (cd) of the luminaire in the direction of the observer's eye;
A·cosα—the projected area of ​​the luminaire in the direction of the observer's eye (m2);
Α—the angle (°) between the normal of the surface of the fixture and the direction of the observer's eye.
3) ω=AP /r2
Where AP is the apparent area (m2) of the illuminating component of the luminaire in the direction of the observer's eye;
r —— the distance (m) between the center of the illuminating part of the luminaire and the observer's eye.
4) The ratio of the Gaussian position index P to H/R and T/R generated according to Fig. 2 is determined by the position index table.
Figure 2 Position index coordinates with the observer position as the origin
The system (R, T, H) generates a ratio of H/R to T/R for the center of the fixture.
Uniform glare value UGR application conditions:
(1) UGR is suitable for general lighting design in simple cubic rooms, not for rooms with indirect lighting and illuminated ceilings;
(2) Applicable to the case where the solid angle formed by the illuminating part of the luminaire is 0.1sr>ω>0.0003sr;
(3) The same type of lamps are arranged at even intervals;
(4) The luminaire is a double symmetrical light distribution;
(5) The height of the sitting observer's eyes is usually 1.2m, and the height of the standing observer's eyes is usually 1.5m;
(6) The observation position is generally at the midpoint of the longitudinal and lateral walls, and the line of sight is observed horizontally;
(7) The surface of the room is approximately 0.75 m above the ground, the mounting surface of the luminaire and the wall between the two surfaces.
3.2 Glare value GR
Calculation of the glare rating GR of outdoor sports venues: [5]
Where, Lv1 - the brightness of the light curtain (cd/m2) produced by the light emitted by the luminaire directly toward the eye;
Lve - The brightness of the light curtain (cd/m2) produced by the environment that causes light directly incident on the eye.
The parameters in the formula should be determined according to the following formula:
1)
Where Eeyei is the illuminance on the observer's eye, which is the illuminance (Lx) produced by the i light sources on the vertical plane of the line of sight;
Θi - the angle formed by the observer's line of sight and the direction in which the i sources are incident on the eye (°);
n - the total number of light sources.
2) Lve =0.035Lav
Where Lav is the average brightness (cd/m2) of the horizontally illuminated field that can be seen.
Lav = Ehorav · Ï/πΩ0
Where Ehorav is the average illuminance (Lx) of the illuminated field;
Ρ——the reflectance of the area during diffuse reflection;
Ω0 - 1 unit solid angle (sr).
Application conditions for glare value GR:
(1) This calculation method is used for various lighting patterns of outdoor sports venues that meet the uniformity of illumination under common conditions;
(2) For the line of sight lower than the eye height;
(3) The background seen is the illuminated site;
(4) The observer position for glare value calculation may be used to calculate the grid position for illuminance, or to use a standard observer position;
(5) A certain number of viewing directions can be selected by rotating at a certain number of angular intervals (5°...45°).
Correspondence between GR value and degree of uncomfortable glare:
3.3 Threshold increment TI for road lighting glare evaluation
Threshold increment is a measure of disability glare. Expressed as the percentage of brightness that needs to be increased between the object and its background for the purpose of seeing the same object when there is a glare source. [6]
The threshold increment is the average road surface brightness as the background brightness. When the background luminance range is 0.05cd/m2< Lb <5cd/m2, the calculation formula of TI is approximately:
TI=65Lv/Lav0.8(%)
Where TI is the threshold increment (%);
Lv—equivalent light curtain brightness (cd/m2), assuming that the observer always looks at the front side parallel to the road axis at an angle of 1° to the horizontal line (ie, always looks at a point on the road surface about 86 meters ahead);
Lav—The average brightness of the road surface (cd/m2).
The formula for calculating the equivalent light curtain brightness Lv:
Lv=10∑(Eeyei/θi2)
Where, Eeyi—the vertical illumination (Lx) of the illuminator in the observer's eye;
Θi—the angle between the illuminating center of the illuminator and the observer's line of sight (°). (1.5° < θ <60°)
The threshold increment TI can also be measured directly using a dedicated imaging luminance meter.
Table 1 Relationship between threshold increment TI and Lv and Lav Table: %
The average brightness Lav of the high-grade road surface is usually 1.0 to 2.0 cd/m2, and the threshold increment TI is required to be 10% or less. Therefore, when the Lav is 1.0 cd/m2, the equivalent light curtain brightness Lv should not be greater than 0.15 cd/m2. When the Lav is 1.5 cd/m2, the equivalent light curtain luminance Lv should not be greater than 0.20 cd/m2. When the Lav is 2.0 cd/m2, the equivalent light curtain luminance Lv should not be larger than 0.25 cd/m2.
Lv is the summation of 10Eeye/θ2 for all fixtures that are 1.5 to 60 degrees from the viewing target. In the case of the lamp arrangement, that is, all angles θ are fixed, the Eeye is as small as possible.
3.4 Glare Control Level G in Road Lighting
In road lighting, the discomfort glare felt by the driver can be measured by the glare control level G. The value of G is related to the nature and arrangement of the luminaire itself.
Factors affecting glare control level G include:
(1) Lamps and light sources:
1) In the C-γ system, the absolute light intensity of C=0, γ=80°, that is, in the vertical plane parallel to the road axis, the light intensity in the 80° direction from the lowest point of the lamp, ie I80.
2) The ratio of the absolute intensity of C=0, γ=80° direction to the absolute intensity of C=0, γ=88°, ie I80/I88.
3) The luminous area of ​​the luminaire as seen from the direction of 76° from the vertical direction of the luminaire, ie F.
4) The color coefficient of the light source used, denoted by C. When the light source is a low pressure sodium lamp, C=0.4; for a high pressure sodium lamp, C=0.1; for a high pressure mercury lamp, C=-0.1; for other light sources, C=0.
(2) Facility layout:
1) Average road surface brightness, ie Lav.
2) Horizontal line of sight (1.5m) from the height of the luminaire, ie h'.
3) The number of lamps per kilometer, ie p.
These parameters have the following relationship with the glare control level G: [4]
G=13.84-3.31logI80+1.3(log(I80/I88))1/2-0.08log(I80/I88)+1.29logF
+0.97logLav+4.4logh'-1.46logp+C
It should be noted that the parameters in the above formula are only suitable for the following ranges:
50cd≤I80≤7000cd;
1≤I80/I88≤50;
0.007m2 ≤ F ≤ 0.4m2;
0.3cd/m2≤Lav≤7cd/m2;
5m ≤ h' ≤ 20m;
20 ≤ p ≤ 100.
The relationship between glare control level G and subjective evaluation:
4 glare control strategy
From the above glare evaluation formulas, the direction of the glare control strategy can be summarized:
(1) Improve the background brightness in the field of view because the glare value is generally inversely proportional to the background brightness. But it is achieved by improving the lighting efficiency of the system, rather than simply increasing the power consumption.
(2) reducing the vertical illumination produced by the luminaire in the observer's eye, thereby reducing the equivalent light curtain brightness produced by the glare source in the observer's eye.
(3) Increase the angle between the viewing target and the illuminating surface of the luminaire as much as possible. This involves the rational placement of the luminaire.
(4) Reduce the brightness of the light-emitting surface of the lamp.
(5) Set up a scientific cut-off facility.
5 Summary
In most cases, glare can have a negative impact on the lighting effect, and even directly lead to the failure of the lighting design. Our goal is to find solutions that can both properly suppress glare and make efficient use of source light flux and improve energy efficiency. Therefore, scientific lighting design is crucial.
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