The most common form of lighting assessment is an illuminance check to see whether there is enough light on the desktop. It is a fair concern, and it is a very simple assessment that should be encouraged. On the other hand, it is only a small factor in achieving visual comfort, as shown in the number of other issues raised in this report. One of them is uniformity, which is often associated with illuminance. Not only is it important to achieve a particular level of illuminance, it is also important to have a certain degree of uniformity of both illuminance and luminance across the space to ensure visual comfort.

Recommended Illuminance

A particular illuminance level is generally recommended for specific tasks so that the task can be performed adequately; typically general office work requires 500 lux. There are numerous guidelines available that provide the suggested level of illuminance including IESNA, CIBSE and standards of individual countries. These will generally provide a minimum level of illuminance. In a modern office this should be achieved by the electric lighting alone. Assuming this is the case, any daylight the office receives will supplement the level of illuminance (or if the electric lighting uses daylight responsive controls, the electric light output may be reduced). In most cases, it is accepted that the higher the level of illuminance, the greater the visual comfort, although Velds has illustrated that the risk of glare increases significantly if there is very high illuminance.

.1 Daylight Factor

Any surface of the office which can ‘see’ the sky will receive significantly more than 500 lux and any part which receives very little daylight will be ‘topped up’ by the artificial light. As this report is only concerned with the daylighting performance of offices it seems pointless to consider a pure lux / footcandle measure of illuminance. A better measure is the daylight factor, which expresses the amount internal illuminance (from daylight) as a percentage of the external daylight available. This then allows analysis of the distribution of daylight in the space and determines how of the daylight available is utilised. In most situations a higher daylight factor is preferred. Typical offices would receive approximately 5-10% right next to the windows and could drop to virtually nothing further back in the space.

Recommended Uniformity

There is some debate as to how much uniformity is recommended. If the lighting is not uniform enough the office will be uncomfortable, but if it is ‘too’ uniform the space becomes boring and sterile. A balance between the two needs to be struck, however, where that balance is, is open for debate.

.1 Illuminance

As with many of the other sections in this report, there are extensive recommendations for the preferred levels of uniformity of illuminance for artificial lighting situations. The research conducted for daylighting uniformity is significantly less, probably due to daylight’s variability.

Slater and Boyce conducted a study on the limits of acceptable uniformity. First, they investigated recommended uniformity ratios around the world, which is summarised in the following table.

Illuminance Uniformity Recommendations

CIBSE Code for Interior Lighting '84 0.8 min/avg

British Standard BS 8206: Part 1 0.7 min/max
Code of Practice for Artificial Lighting ‘92 0.8 min/avg

German Standard DIN 5035 '79 0.67 min/avg

Australian Standard AS 1680 '76 0.67 min/avg

Netherlands Standard '81 0.7 min/max

CIE Guide on Interior Lighting '86 0.8 min/avg

There are clearly significant differences between these recommendations, which raises questions as to what the appropriate level is. The pair then went on to conduct an experiment, where subjects completed a series of tasks under artificial light. The subjects rated the lighting conditions themselves, whilst their performance was also measured. The uniformity of the light was varied for different tasks. The study concluded that a uniformity ratio of 0.7 (min/max) was acceptable for all tasks and 0.5 was adequate for a task in the centre of the desk. This would suggest that the recommendations are far too tight, even for non-daylit offices.

Daylight, by its nature is directional when it enters a building. Most offices are side-lit and thus the amount of daylight near the windows will far exceed that at the back of the room. This ‘flow’ of light will create a very non-uniform lighting condition, which would not come close to meeting the recommendations above. It is unrealistic to expect daylit environments to meet the requirements for artificially lit environments, where it is far easier to achieve uniformity using regular spaced ceiling-mounted luminaires. Furthermore, people expect an uneven distribution of light from a window and are generally more tolerant of daylight.

Littlefair has proposed some methods of assessing the uniformity of daylit spaces based largely on daylight factors.

The first is simply an assessment of the room proportions. It is more suited as a guide during the design stage, but it could potentially be used for post occupancy assessment. If a daylit room has windows in one wall only, the depth of the room L should not exceed the limiting value given by:
,
which can be rearranged to find L:

where W is the room width
H is the window head height above floor level
Rb is the average reflectance of the surfaces in the rear half of the room (away from the window)

If L exceeds the limiting value the rear half of the room will appear gloomy.

This is a very simple assessment method. It is very quick and easy to gather the required information and perform the calculation. However, it is a very ‘crude’ formula which may not give a true indication of the daylighting performance.

The second criterion suggested is a measure of the general balance of illumination in the room, originally put forward by Lynes :

“The average daylight factor, or average illuminance, in the front half of the room should not exceed three times the average daylight factor (or illuminance) in the back half.”

This criterion is more detailed and is likely to provide a more useful analysis of the lighting performance. However it is still limited to situations with only one window and only separates the room into two zones, making distribution analysis difficult.

The last suggestion Littlefair makes relies on minimum daylight factor or illuminance levels:

“The minimum daylight factor – or, for sunny locations, illuminance at the worst lit point – should exceed 1% or 100 lux. The space as a whole will then have no particularly dark or gloomy patches.”

This method of uniformity assessment will ensure that there is an adequate amount of illuminance in the office. But it does not really consider uniformity. As it is only concerned with the worst lit point only one measurement is needed.

.2 Luminance

Essentially, the need for uniform luminance has been detailed in previous sections. If the luminance is too non-uniform it could create an uncomfortable luminance ratio as discussed above. Generally offices with windows in more than one wall have a more even luminance (and illuminance) throughout the space. This is because different orientations of the sky can be seen that can contribute to illumination from different directions. Furthermore, the wall surrounding the window has a greater luminance thus reducing the contrast with the window and decreasing the glare, although veiling reflections may become a problem.

Possible Measurements Needed in Assessment Procedure

To measure the uniformity of the luminance no additional measurements beyond what is required to assess the glare and luminance ratios will be necessary.

It appears that to get an accurate impression of the illuminance level and uniformity required for visual comfort the daylight factors in the office will have to be determined. The daylight factor can either be measured manually or calculated theoretically. A manual measurement involves taking simultaneous indoor and outdoor illuminance measurements during a perfectly overcast day (or as close to it as possible), until a grid of daylight factors is determined for the space. Alternatively, the following equation can be used to calculate average daylight factor:

D =

Where: D = average daylight factor
T = transmittance
Aw = window area
θ = visible sky angle (at glazing)
A = surface area of all surfaces
R = average reflectance of walls
And,
R =

Where: L = luminance of surface
E = illuminance at surface

The theoretical calculation method cannot be used to assess the uniformity as it simply provides an average daylight factor for the entire room. On the other hand, it could potentially be used for the second criterion suggested above, where the room is divided into a front half and a back half. One would simply have to assume a pane of glass (acting as a window) with 100% transmittance sliced down between the two halves of the room. Using the theoretical method saves a lot of time and disturbance of the occupants. This technique provides sufficient analysis of the uniformity of illuminance.

A simple spot measurement in the worst lit location to ensure the light level (from daylight) is above 100 lux or 1% daylight factor will be sufficient to check illuminance.

Future Research Needed

The research regarding what levels of uniformity from daylight are acceptable is still very limited in comparison to that for electric lighting. However, one might also argue that it is pointless even investigating uniformity and illuminance from daylight alone. Very few offices rely solely on daylight, most of the time there will be a component of artificial light as well. None of the assessment measures or recommendations above are designed for environments with both daylight and artificial light. Admittedly, designing an assessment measure that considers both is a far more difficult task. But it is needed as this is the lighting condition for most offices.