Let There be Light: Key Indicators to Describe and Design Visual Comfort
Architecture is an industry defined by evolution.
Architects are increasingly aware of our influence on the well-being and good health of the users of our projects. Natural lighting –and how it should be complemented with artificial lighting– is an essential factor to consider for the visual comfort of interior spaces. But, do we know how to handle it correctly?
The absence of discomfort at the time of seeing is not enough to measure the visual success of a space. Things like the rate of blinking, level of glare, or light blindness help to determine the environmental quality of a room. Other things that are vital to consider include representations of color, low reflection, and uniform distribution of light. People do well with clear views to the outside, so it’s also fundamental to perfect the quantity and location of the openings in the building envelope in order to control natural light intensity.
Both too little and too much light can cause visual discomfort. Important changes in light levels or sharp contrast (which is perceived as glare) can cause stress and fatigue as the human eye is permanently adapting to light levels. 
Thus, everything that enters through our eyes influences the health of our body and mind, affecting our biological clock (sleep and wakefulness), our heart rate, the functioning of our organs, and our state of mind. The variable and dynamic nature of natural lighting is an opportunity for architecture to positively contribute to occupants’ general well-being.
Aspects to Consider When Designing for Visual Comfort
Always prioritize natural light
Natural light will always be the most comfortable for human beings since it is the source of illumination to which our eyes adapt naturally. Not only does it have a proven impact on health and well-being –increasing awareness during daytime, improving sleep patterns, decreasing depression risks, among many others–, but also generates enormous energy savings, avoiding the reiterated use of artificial light.
When designing a new project, take full advantage of the orientation of the site and provide users with the best natural light possible through the correct design of openings. Depending on the specific use of each room, one should also consider a space’s variation of use at different moments or days.
Map the distribution of light, independent of the observer: Illuminance and Luminance
Illuminance, expressed in lux, is the luminous power that comes from all directions and reaches a given point, where a specific task will be performed.  When measuring it on a certain surface, for example on a desk in offices, ensure that the illuminance reaches 500 lux. Values very inferior or superior to that generate discomfort. This is valid for artificial lighting in offices, at workplace level, however, in order to take into account the natural variability of daylight, it is better to refer to the new European Standard on Daylighting, briefly explained below.
The Luminance, expressed in candela per square meter (cd/m2), corresponds to the different luminous intensities per unit area, emitted or reflected by the light sources and the surfaces that surround us.  It basically describes the brightness of light, from the point of view of visual perceptions and psychological sensations. By measuring it we can identify the contrasts of light and glare, and understand if the light is evenly distributed or if it comes from a specific source.
In both cases a Photometer must be used. To measure the Illuminance (lx) it’s called Lux Meter, and to measure Luminance (cd/m2) it’s known as Luminance Meter.
Evaluate the quantity and quality of light
To evaluate the quantity of light, the distribution of light in space and the Illuminance should be measured at specific and relevant points for the functions that will be carried out in the room.
In order to evaluate the quality of the light, the Useful Daylight Illuminance (UDI) must be modeled first, which integrates the evaluation of the daylight levels and the glare, setting as an acceptable range the values that move between 100 and 2000 lux.  The Daylight Autonomy (DA) must then be calculated, which is the percentage of annual daytime hours in which a specific point in a space is maintained above a certain level of illumination, set by the user. The new European Standard for Daylighting EN17037 states that the following criterias should be met (minimum requirements for Spatial Daylight Autonomy): illuminance of 300 lux over 50% of space during more than half of daylit hours and illuminance of 100 lux for 100% of space more than half of the daylit hours.
Daylight autonomy is determined by location, orientation, windows shade and position as well as Window-to-floor ratio and Visible Light Transmission of the glazing. The following aspects are fundamental to achieve an effective balance between all these variables:
Consider the relationship between openings and space: Window-to-Floor Ratio
The calculation of the relation between the area of the openings and the area of the space is called Window-to-Floor Ratio (WFR) and is obtained by dividing the total area of the openings by the total area of the space associated with them. This factor helps define the number of openings that will work effectively in each space of our project. It can also guide things like the openings’ size, location, and type of glazing. In some countries, like France, it is mandatory to have at least a WFR of 17% for all new residential buildings.
The WFR value must be multiplied by the value of the Visible Light Transmission of the chosen glass (VLT), explained below, to ensure that the design moves within the thresholds that ensure certain effective levels of visual comfort (usually a value above 0.15).
Decide the amount of light that must pass through the glass: Visible Light Transmission
As described above, the relationship between the openings and the space must be complemented with the Visible Light Transmission (VLT), which corresponds to the amount of visible light that passes through the glass. Glass with VLT of 50% allows 50 percent of light through and blocks the remaining 50 percent. With this, we can decide to incorporate large openings in our project and at the same time control the amount of light that passes through it, while also adding protection against UV rays and glare.
To achieve effective results, all of these analyses must be added to the calculation of other relevant factors, such as the solar heat gain coefficient and the U-Value. One must take local regulations into consideration, as well as the technical specifications of the types of windows chosen.
In addition, it’s essential to evaluate the views towards the outside, integrating qualitative factors such as the urban or natural landscape, or other elements that could be observed from each transparent surface. Consider the incorporation of solar control systems or other methods that influence visual comfort of the interiors of the architecture project.
 Indoor Environment And Well-Being. The Saint-Gobain Building Science Handbook.
 Useful Daylight Illuminance (UDI) / Daylighting Pattern Guide. <https://patternguide.advancedbuildings.net/using-this-guide/analysis-methods/useful-daylight-illuminance> (accessed February 2019).
 Daylight Autonomy (DA) / Daylighting Pattern Guide. <https://patternguide.advancedbuildings.net/using-this-guide/analysis-methods/daylight-autonomy> (accessed February 2019).