Glare is the visual dis-comfortability of human eyes due to improper level of luminance distribution of the luminaire to view an object. Glare can be classified into two types,
- Disability Glare
- Discomfort Glare
Disability glare makes human eyes disable to see any object for a little while. For an example, when we look at any bright source for a few seconds and then we look at any object with low brightness, we become unable to see this object properly, rather we see black spot for some times. This is one type of momentarily blindness.
Disability glare is measured at threshold increment value. With the aid of the luminous distribution of a luminaire and using the nomogram the threshold increment of a luminaire installation can be determined.
The nomogram is a graphical representation of the formula for the equivalent veiling luminance; threshold increment value is represented by TI and calculated in percentage.
It is defined as
Where, LV is veiling luminance and Lavg is the average luminance of the object or road surface. Where, LV is veiling luminance and Lavg is the average luminance of the object or road surface.
Discomfort glare is not the cause of momentarily blindness like disability glare, but it is responsible to affect the visibility of the human eyes to an extent for long time. This kind of glare depends on the luminaire installation. If the luminance is in higher value, the human eyes cannot observe the object with lower luminance properly beyond this higher luminance. Discomfort glare is calculated in logarithmic term. If we have the specific luminaire index (SLI) of a luminaire, we can easily calculate this discomfort glare of that luminaire. SLI is the luminaire light distribution characteristics.
It is denoted by glare control mark (G).
For road lighting, discomfort glare control mark is given by
SLI = Specific Luminaire Index,
Lavg = average road surface luminance (cd/m2)
h’ = reduced mounting height (m).
p = number of luminaires per kilometer.
SLI is calculated in terms of logarithmic value.
I80 and I88 are the luminous intensity (cd) along the downward vertical directions in the vertical parallel to the road axis correspond to angle 80 degree and 88 degree respectively.
F is the apparent light emitting area (m2) of the luminaire as seen at an angle ɣ = 76 degree to the downward vertical.
C is the color factor according to the SPD of the Lamp used. For low pressure sodium lamp C = 0.4 and C = 0 for all other white lamps.
When SLI 4, glare control is high.
Higher value of SLI means lower chance to create discomfort glare.