Measuring Light (Leaving Cert Construction Studies): Revision Notes

Measuring light

Light measurement is essential for ensuring buildings provide adequate illumination for daily activities. Understanding how to quantify and assess light levels helps create comfortable and functional spaces.

Key units for measuring light

Three fundamental units are used to measure different aspects of light, each serving a specific purpose in lighting design and assessment.

Watt

Watt represents the standard unit for measuring power consumption. It quantifies how much electrical energy a light source uses per second. This measurement tells you about the energy cost of running a lamp, not how bright it appears. For example, traditional incandescent bulbs consume more watts than modern LED bulbs whilst producing similar brightness levels.

Lumen

Lumen measures the total amount of visible light emitted by a source. This unit indicates the actual brightness or light output you can expect from a bulb or lamp. Higher lumen values mean brighter light sources. A useful comparison shows that both a 40-watt tungsten bulb and a 7-watt fluorescent lamp can produce 400-500 lumens, demonstrating that wattage and brightness are different measurements.

Lux

Lux quantifies how much light actually reaches and illuminates a surface area. It represents lumens spread over each square metre of space. This measurement is crucial because it shows whether a room has sufficient lighting for its intended activities. Unlike lumens which measure light at the source, lux measures light where people actually use it.

Tools for measuring illumination

Lux metre

A lux metre is a handheld device that measures illumination levels at specific locations. This instrument enables precise mapping of light distribution across any space. By taking readings at multiple points, you can identify areas that may need additional lighting or have excessive brightness.

The device works by detecting light levels and displaying the measurement in lux units. This allows for systematic assessment of lighting quality and compliance with recommended standards.

Practical light assessment

Creating a light map

To properly assess lighting in a room, you can conduct a systematic measurement process:

1. Divide the space into one-metre grid squares
2. Create a floor plan showing these grid divisions
3. Take lux readings at each grid intersection point
4. Record measurements directly on your map
5. Connect points with similar readings to create lighting contours
6. Repeat the process under different lighting conditions (daylight only, artificial lights, mixed lighting)

This systematic approach reveals how light distributes throughout a space and identifies areas requiring improvement.

Degree of efficiency method

This calculation method determines appropriate window sizes to achieve desired natural light levels in buildings. The process considers multiple factors that affect how much outdoor light actually reaches interior spaces.

The formula

The degree of efficiency method uses this calculation:

$L_i = L_o \times WF \times E \times \left(\frac{WA}{FA}\right)$

Understanding the variables

• $L_i$ = Required light level inside the room (lux)
• $L_o$ = Available outdoor light (standard overcast sky = 5000 lux)
• $WF$ = Window factor accounting for vertical orientation (typically 0.5)
• $E$ = Efficiency coefficient for light loss through glazing and obstructions (typically 0.4)
• $WA$ = Window area in square metres
• $FA$ = Floor area in square metres

Light standards and benchmarks

The Commission Internationale de l'Eclairage (CIE) establishes international lighting standards used for comparing different lighting conditions. These standards provide reference points for adequate illumination levels in various situations.

A typical overcast sky serves as the baseline measurement, assigned a value of 5000 lux. This standardisation allows consistent calculations and comparisons across different projects and locations.