Reflected glare on horizontal visual tasks

Reflections of high luminance values on the visual task can reduce perceptibility of the visual task. Reflections of excessive luminance values on glossy surfaces, e.g. glossy paper, screens or measuring instruments in laboratories and control centres, can cause disturbance similar to direct glare. These processes named reflected glare or veiling reflection affect contrast levels, which are necessary for undisturbed vision. Reflected images in addition lead to fusion stimuli. These are caused by the eye trying to accommodate to (focus on) the visual information

  • at close range, e.g. the visual detail on the glossy paper or on-screen, and

  • at distant range, meaning the outline of the source of the mirrored image, e.g. luminaires,

and performing permanent accommodation tasks in the process. The consequences are fatigue and asthenopic discomfort which can manifest e.g. as eye irritation, dry or streaming eyes, increased light or flicker sensitivity, veiled vision, headaches as well as vertigo.

The contrast rendering factor (CRF) is used to describe disturbance caused by reflected glare on horizontal visual tasks. CRF is the relation of contrast of a defined and representative visual task with the actual lighting to be rated and the contrast in the same visual task with entirely diffuse reference lighting.

For literature on this subject, see e.g.

  • CIE publication no. 19.2 (TC-3.1) 1981 "An analytic model for describing the influence of lighting parameters upon visual performance" and

  • LiTG publication no. 13:1991 "The contrast rendering factor – a quality criterion for indoor lighting ", published by the photometric societies of Germany (LiTG), Austria (LTAG) and Switzerland (SLG).

Lighting with high CRFs helps to avoid visual disturbance on glossy horizontal reading presentations. According to lighting quality requirements, the levels in table are recommended for the CRF values.

CRF values can be predetermined using computer systems and measured with specialised measuring equipment (see also CRF values section in chapter 1.3.7 "Lighting measurement"). Corresponding computer graphics illustrate the planning results and contain e.g. curves of matching CRF values. On this basis, workstations with high contrast rendition requirements can be positioned in the room.

Reflected glare can be avoided or reduced using the following measures:

  • Luminaire and workstation arrangement: Luminaires and workstations must be paired in a way that prevents disturbing light reflections from occurring on the visual object in the main viewing direction as far as possible through suitable arrangement. For horizontal visual objects, this is provided with lateral incidence of light. However, excessive shadow formation must be avoided.

  • Surface design: Surfaces which may reflect light should be designed in a matt or non-reflecting way. This applies especially to workstation surfaces, paper, writing materials such as ink etc., keys on typewriters or EDV terminals etc. and computer monitors in particular.

  • Luminaire luminance limitation: Luminaires which can cause disturbing light reflections on the visual object, should feature low luminance values for the critical distribution directions. For luminaires which are across the sitting observer’s viewing direction as well as horizontal visual objects, this applies particularly in the distribution angle range of 20° ≤ γ ≤ 40° (see fig.).

  • Bright ceilings and bright walls: High ceiling luminance values can be created through high reflectance values for ceiling, walls, floor and furniture, preferably in connection with an indirect portion in the lighting. This way, disturbing reflections are reduced.

Table 1.10: Levels of contrast rendering factor CRF for different visual tasks

Figure 1.25: Relative frequency of viewing angles (above) for a typical sitting position at the workstation (below)

Figure 1.26: Reading presentations with high (left) and low reflected glare (right)