Visual Discomfort

The most common aspects of lighting that cause visual discomfort as below:

1. Insufficient light 

For example, about the lighting in offices the recommended maintained illuminances on a horizontal working plane:

The following recommendations are applicable to quasi-domestic buildings occupied by young people.

2. Illuminance uniformity

The most preferred form of work surface lighting is one that provides a uniform lluminance over the area where the work is to be done (minimum/average illuminance ratio > 0.7) and lower illuminances outside that area.But for quasi-domestic buildings, a minimum illuminance uniformity of 0.8 is recommended.

      3. Glare

Discomfort glare is quantified by the Unified Glare Rating(UGR), derived from the equation:

where: UGR = Unified Glare Rating

Lb = background luminance (cd/m2), excluding the contribution of the glare sources.

This is numerically equal to the indirect illuminance on the plane of the observer’s eye, divided by

Ls = luminance of the luminaire (cd/m2)

= solid angle subtended at the observer’s eye by the luminaire (steradians)

p = Guth position index

UGR values typically range from 13 to 30 according to a scale of 3 units (i.e. 13,16,19,22,25,28,30) the lower the value, the less the discomfort. Where a luminous ceiling or uniform indirect lighting is used, discomfort glare is limited by setting a maximum average illuminance. Specifically, if a UGR value of 13 is desired then the average illuminance provided should not exceed 300 lx, for UGR = 16, the maximum average illuminance should not exceed 600 lx and for UGR = 19, the maximum average illuminance should not exceed 1,000 lx. The following are examples of maximum UGRL limits:

4.     Veiling reflections

Veiling reflections are luminous reflections from specular surfaces that physically change the contrast of the visual task and therefore change the stimulus presented to the visual system Although veiling reflections are usually considered a negative outcome of lighting that can cause discomfort, they can be used positively, but when they are, they are conventionally called highlights.

5.     Shadows

Although shadows can cause visual discomfort, it should be noted that they are also an essential element in revealing the form of three-dimensional objects.

6.     Flicker

Virtually all electric light sources that operate from an alternating-current supply produce regular fluctuations in the amount of light emitted. When these fluctuations become visible they are called flicker.