How to Measure Light Intensity
Understand photometers that measure lux and foot-candles., Know how to interpret illuminance units., Understand lumens and luminance., Measure beam and field angle.
Step-by-Step Guide
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Step 1: Understand photometers that measure lux and foot-candles.
These are units that describe the intensity of light on a surface, or illuminance.
Photometers that measure this are usually what people are looking for when they want to set up a photo shoot, or test whether a room is too bright or too dim.Some light meters are specialized for different types of lighting.
For instance, one may produce more accurate results when used to measure sodium lighting.You can even purchase a "light meter" in some mobile device's app stores.
Check the reviews first, as some of these apps are inaccurate.
Lux is the modern accepted standard, but some devices still measure in foot-candles.
Use this online calculator to convert between them. -
Step 2: Know how to interpret illuminance units.
Here are a few examples of typical illuminance measurements, to help you determine whether your lighting needs to change:
Most office work is comfortably done at 250 – 500 lux (23–46 foot-candles).
Supermarkets or work areas that involve drawing or other detail work are typically illuminated to 750 – 1,000 lux (70–93 foot-candles).
The upper end of this range is equivalent to an indoor area next to a window on a clear, sunny day. , If a light bulb or lamp label or advertisement mentions "lumens," it is describing how much total energy is emitted as visible light, a concept called luminance.
Here's what you need to know:
The "initial lumens" describes how much light will be given off once the light is stabilized.
This takes about 100 hours of use for fluorescent and HID lights.The "mean lumens" or "rated lumens" tells you the estimated average luminance over the life span of the device.
The actual luminance will be brighter than this early on, and become dimmer than this near the end of the light source's recommended lifespan.
To figure out how many lumens you need, use the steps above to determine how many foot candles of illuminance you want in a room, and multiply by the square footage of the room.Aim higher than the result for rooms with dark walls, and aim lower for rooms with other major light sources. , Flashlights and other devices that emit light in a certain direction can be described using these two additional terms.You can find these yourself using a photometer that measures lux or foot-candles, and with a straightedge and protractor:
Hold the photometer directly in the path of the brightest beam.
Move it around until you find the spot with the maximum intensity (illuminance).
Staying the same distance from the light source, move the photometer in one direction, until the light intensity drops to 50% of the maximum level.
Use a taut string or other straightedge to mark the line from the light source to this point.
Walk in the other direction until you find the spot on the opposite side of the beam with 50% maximum illumination.
Mark a new line from this spot.
Use a protractor to measure the angle between your two lines.
This is the "beam angle," and describes the angle illuminated brightly by the light source.
To find the field angle, repeat these steps, but mark the two spots where the beam intensity reaches 10% of the maximum level. -
Step 3: Understand lumens and luminance.
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Step 4: Measure beam and field angle.
Detailed Guide
These are units that describe the intensity of light on a surface, or illuminance.
Photometers that measure this are usually what people are looking for when they want to set up a photo shoot, or test whether a room is too bright or too dim.Some light meters are specialized for different types of lighting.
For instance, one may produce more accurate results when used to measure sodium lighting.You can even purchase a "light meter" in some mobile device's app stores.
Check the reviews first, as some of these apps are inaccurate.
Lux is the modern accepted standard, but some devices still measure in foot-candles.
Use this online calculator to convert between them.
Here are a few examples of typical illuminance measurements, to help you determine whether your lighting needs to change:
Most office work is comfortably done at 250 – 500 lux (23–46 foot-candles).
Supermarkets or work areas that involve drawing or other detail work are typically illuminated to 750 – 1,000 lux (70–93 foot-candles).
The upper end of this range is equivalent to an indoor area next to a window on a clear, sunny day. , If a light bulb or lamp label or advertisement mentions "lumens," it is describing how much total energy is emitted as visible light, a concept called luminance.
Here's what you need to know:
The "initial lumens" describes how much light will be given off once the light is stabilized.
This takes about 100 hours of use for fluorescent and HID lights.The "mean lumens" or "rated lumens" tells you the estimated average luminance over the life span of the device.
The actual luminance will be brighter than this early on, and become dimmer than this near the end of the light source's recommended lifespan.
To figure out how many lumens you need, use the steps above to determine how many foot candles of illuminance you want in a room, and multiply by the square footage of the room.Aim higher than the result for rooms with dark walls, and aim lower for rooms with other major light sources. , Flashlights and other devices that emit light in a certain direction can be described using these two additional terms.You can find these yourself using a photometer that measures lux or foot-candles, and with a straightedge and protractor:
Hold the photometer directly in the path of the brightest beam.
Move it around until you find the spot with the maximum intensity (illuminance).
Staying the same distance from the light source, move the photometer in one direction, until the light intensity drops to 50% of the maximum level.
Use a taut string or other straightedge to mark the line from the light source to this point.
Walk in the other direction until you find the spot on the opposite side of the beam with 50% maximum illumination.
Mark a new line from this spot.
Use a protractor to measure the angle between your two lines.
This is the "beam angle," and describes the angle illuminated brightly by the light source.
To find the field angle, repeat these steps, but mark the two spots where the beam intensity reaches 10% of the maximum level.
About the Author
Donna Jordan
Professional writer focused on creating easy-to-follow practical skills tutorials.
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