Brooke Martin
A foot-candle is a measurement of light intensity, defined as the illuminance on a one-square-foot surface from a source of light. It is a non-metric measurement of energy, which is equivalent to approximately 10 lux or 10.57 lux. Foot-candles are used to ensure that areas requiring certain light levels are receiving them. This is especially important in electrical construction, horticulture, film, and other industries. To measure foot-candles, a light meter is positioned at the point where light levels are to be assessed, with the sensor facing the light source directly. Foot-candles can be converted to lumens, the metric measurement of light, by multiplying the number of foot-candles by 10.76.
| Characteristics | Values |
|---|---|
| Definition | A foot-candle is a measurement of light intensity, defined as the illuminance on a one-square-foot surface from a source of light. |
| Formula for conversion to lumens | Foot-candles x 10.76 = lumens |
| Formula for conversion from lumens | Lumens x 0.001496 = foot-candles |
| Foot-candle measurement for parking garages | 10 |
| Foot-candle measurement for industrial lighting | 20 |
| Foot-candle measurement for direct light indoors | 2,000-5,000 |
| Foot-candle measurement for bright indirect light | 1,000-2,000 |
| Foot-candle measurement for medium light | 250-1,000 |
| Foot-candle measurement for low light | 50-250 |
| Foot-candle measurement for ATMs (outdoor) | 5 foot-candles at 5 feet |
| Foot-candle measurement for ATMs (indoor) | 2 foot-candles in all unobstructed directions from a facility at 30 feet |
| Industries that use foot-candles | Architecture, lighting design, film, horticulture, museums, security |
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What You'll Learn
Using a light meter
To measure foot-candles using a light meter, position the meter at the point where you want to assess the light level, with its sensor facing the illumination source directly. For example, to measure the light falling on a work desk, place the device on the desk's surface. The sensor should have a clear, unobstructed view of the light source. When the meter is exposed to multiple light sources, such as daylight from a window and interior artificial lighting, it records the cumulative light intensity, which could distort the readings. In such cases, it is recommended to block off or turn off additional light sources and repeat the measurement. Additionally, light intensity changes throughout the day, so it is advisable to take multiple measurements at different times and calculate an average.
Foot-candles are a unit of measurement commonly used in places that follow the United States customary units, while "lux" is used in other parts of the world. Lux is similar to foot-candles but measures light over a one-meter square instead of a one-foot square. One foot-candle is equal to approximately 10 lux, or more specifically, 10.76 lux. To convert foot-candles to lux, divide the foot-candles value by 10.76.
Light meters are essential tools for cinematographers and photographers, helping them understand the lighting conditions and make necessary adjustments. They are also used in various industries, such as horticulture, where precise foot-candle readings are required to ensure that plants receive the appropriate amount and type of light.
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Understanding lux and lumens
Lux and lumens are two units of measurement that are used to quantify light. Both are related to brightness, but they measure different things.
Lumens are a measure of luminous flux, or the total amount of light emitted in all directions by a single light source. Lumens do not take into account the surface that the light illuminates, nor the distance of the light source from that surface. Lumens are important for comparing the total amount of light emitted by a bulb. Lumens are also important for measuring luminous efficacy, i.e. the amount of light a light source produces in relation to the power it consumes (lm/W).
Lux, on the other hand, is a measure of how much light falls on a particular surface, or the illuminance of that surface. Lux takes into account the area over which the luminous flux (lumens) is spread. Lux is important for knowing how bright a surface will appear, such as a tabletop, reading material, or photography subjects. Without sufficient lux levels, it can be difficult to read or take good photographs.
The difference between lux and lumens can be understood by the following example: a 500-lumen light source will illuminate 1 square meter with 500 lux. The same light source will only illuminate 10 square meters with 50 lux.
Lux is a standardised unit of measurement of light level intensity, commonly referred to as "illuminance" or "illumination". The lux symbol: lx is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). One lux is equal to one lumen per square meter.
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Foot-candles in horticulture
Horticulture requires very accurate foot-candle readings to ensure that all plants receive the appropriate amount and type of light. While the human eye automatically compensates for brightness, making our ability to judge light levels deceptive, foot-candles allow us to accurately determine the amount of light required for various environments.
A foot-candle is a measurement of light intensity, defined as the amount of light received by a one-square-foot surface that is one foot away from a light source, equivalent to one candle. In other words, it measures the brightness of the light falling on a surface. The unit is commonly used in architecture, lighting design, and the film industry, but it is becoming increasingly popular in the plant industry.
To measure foot-candles, a light meter is positioned at the point where you want to assess the light level, with its sensor facing the light source directly. For example, to measure the light falling on a work desk, the meter would be placed on the desk's surface. It is important to ensure that the sensor is not shaded or blocked and has a clear, unobstructed view of the light source. As light intensity can fluctuate throughout the day, it is recommended to take multiple measurements at different times and calculate an average.
In the context of horticulture, the number of foot-candles required will depend on the light intensity needs of the plants. Low-light plants generally require between 50 and 250 foot-candles, while medium-light plants prefer 250 to 1,000 foot-candles. Plants requiring high light intensity typically need at least 1,000 foot-candles and may benefit from higher intensities for optimal growth and flowering.
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Foot-candles in film
Foot-candles are a unit of measurement for light intensity, commonly used in film and photography to measure the amount of light that falls on a surface. Cinematographers and photographers use foot-candles to determine the amount of light required for various environments accurately.
In film, foot-candles are used to gauge light and create the desired exposure. Cinematographers use light meters to measure foot-candles and calculate the lighting requirements for a shot. They consider the number of light fixtures, their mounting heights, and positions within a space to achieve the desired light intensity and distribution.
To measure foot-candles, the light meter is positioned at the point where light levels need to be assessed, with its sensor facing the light source directly. It is important to ensure that the sensor is not shaded or blocked and that multiple measurements are taken at different times of the day to account for fluctuations in natural light intensity.
Foot-candles are particularly useful in film as they help cinematographers understand how light falls on a surface. This understanding of light and its interaction with surfaces is crucial for capturing the desired images. By manipulating light and exposure, filmmakers can create artistic and technical effects that bring their creative visions to life.
Additionally, foot-candles are essential for ensuring the safety and security of film sets. By measuring foot-candles, filmmakers can maintain the necessary light levels to prevent accidents and ensure the functionality of security equipment, such as cameras and lighting.
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Foot-candles in architecture
Foot-candles are a unit of measurement for light intensity, defined as the amount of light that saturates a one-foot square with one lumen. Foot-candles are used to ensure that areas requiring certain light levels are receiving them. This is especially important in electrical construction and architecture, where different activities and spaces require different light levels. For instance, detailed work such as reading or surgery requires higher foot-candle levels, while hallways and lobbies need lower levels.
In architecture, foot-candles are crucial for achieving the desired lighting levels for various tasks and ambiances. Lighting design software allows architects to simulate different lighting scenarios and calculate foot-candles accurately, taking into account factors such as fixture types, room geometry, and surface properties. Autodesk Revit, for example, is a building information modelling (BIM) software that includes lighting analysis tools.
Foot-candle measurements are also essential for creating lighting plans, which detail the light levels and distribution for a given location, indoors or outdoors. These plans can be created using software that simulates the space and allows for the addition of lighting fixtures, mounting heights, and fixture locations. The software then calculates the light levels and balance, producing a report with average, maximum, and minimum foot-candle values.
To measure foot-candles, a light meter is positioned at the point where light levels are to be assessed, with its sensor facing the light source directly. It is important to ensure that the sensor is not shaded or blocked and that multiple measurements are taken at different times of the day to account for fluctuations in light intensity.
Foot-candles are commonly used in the United States and in industries such as architecture, lighting design, film, and horticulture. They are essential for determining the amount of light required for various environments and ensuring that specific lighting requirements are met.
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Frequently asked questions
Foot-candles are a measurement of light intensity, defined as the illuminance on a one-square-foot surface from a source of light.
You can use a light meter to measure foot-candles. Position the light meter at the point where you want to assess the light level, ensuring its sensor faces the light source directly.
You can increase the foot-candles in a room by adding more light sources.
One foot-candle equals 10.76 lumens. To convert foot-candles to lumens, multiply the amount of foot-candles measured by 10.76.
The number of foot-candles you need depends on the location and what the space is used for. The Illuminating Engineering Society of North America (IES) has set guidelines to help determine the amount of lumens needed for different rooms.
