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Outdoor Light Lumens

By March 21st, 2024No Comments

To effectively choose the appropriate light source for illuminating any subject, it is essential to have a comprehensive understanding of the properties of light. Let’s explore some of the primary ways in which light is measured by humans.

LUMINOUS FLUX Luminous flux refers to the total perceived power of light from a specific source as perceived by the human eye. It represents what we actually see. Luminous flux is quantified using units called lumens (lm). You may wonder why we don’t simply use the term “brightness.” Well, luminous flux is measurable and objective, as it can be assigned a specific value. On the other hand, brightness is subjective, varying from person to person. The luminous flux emitted by a light source can be measured, and the greater the number of lumens produced, the brighter the light.

LUMINOUS INTENSITY Luminous intensity pertains to the amount of light radiated in a specific direction by a point source. It is expressed by the luminous flux leaving the source within a designated angle. While we often associate the total lumens with the brightness of a light source, this calculation describes light radiating in all directions. Luminous intensity, measured in units called candelas (cd), provides information about the intensity at a particular point. Imagine a flashlight with an adjustable beam. When you rotate the flashlight head to increase or decrease the beam’s width, the total lumens (luminous flux) remain the same, but the luminous intensity (candelas) changes.

ILLUMINANCE Illuminance refers to the amount of light that strikes a surface or object and is measured in units of foot-candles (fc) or lux (lx). Foot-candles measure the light produced by a source on a one-square-foot surface located precisely one foot away from the source. A foot-candle is defined as one lumen per square foot. In modern times, foot-candles are a critical property of light that contractors must calculate and comprehend. If a source produces an excessive amount of foot-candles, the lighting system will be overly bright. Conversely, if it produces an insufficient amount, the system will be too dim. Achieving the desired lighting effects for homeowners requires foot-candles to be adjusted appropriately. The metric unit of measurement for this property is lux (lx), with one lux equal to one lumen per square meter. Therefore, one foot-candle is equal to 10 lux.

There is a wide variety of lamps available, ranging from incandescent bulbs to LEDs. The brightness of a lamp is typically measured in watts (W), which represents its power output. Generally, lamps with higher wattages are brighter, within the same type of lamp.

The visual brightness of a lamp can be influenced significantly by its beam angle. For example, a 20 W lamp with a narrow beam angle will appear much brighter than a 50 W lamp with a wide beam spread, when both are shined on the same surface from the same distance.

Luminous efficacy is a measure of how effectively a light source produces visible light. It is calculated by dividing the luminous flux (the total amount of visible light emitted) by the power consumed, measured in lumens per watt.

In landscape lighting design, various factors such as luminous flux and beam angle work together to achieve the desired effects. It’s important to note that brightness is subjective, and understanding how different properties of light can enhance, alter, complement, or diminish a particular setting is a crucial skill

Photometrics charts represent the amount of light in terms of perceived brightness to the human eye, measured in foot-candles (or lux). These charts provide information about the brightness of a light source at different distances and beam angles. It is essential to refer to photometrics charts when designing landscape lighting to ensure optimal performance.

The relationship between foot-candles and brightness can be understood as follows:

  • 10,000 fc = Direct sunlight
  • 1,000 to 5,000 fc = Daylight
  • 100 fc = Overcast sky
  • 10 fc = Dusk
  • 1 fc = Twilight
  • 1/10 fc = Moonlight

For example, the NP fixture from FX Luminaire’s Designer Series is a popular uplight with 6 LEDs. At a distance of 25′ (7.6 m) from the light source, it produces an equivalent of 1.71 fc of light with a beam width of 10.8′ (3.3 m).

To apply this information, let’s consider a scenario where you want to evenly light a tree canopy that is approximately 20′ (6.1 m) in diameter and 25′ (7.6 m) from the ground. Since the tree is the focal point, it should be brighter than its surroundings. With a beam width of 10.8′ (3.3 m), you may need three to four fixtures to fully illuminate the canopy. If you prefer more light at the top of the canopy, you can add more fixtures and overlap their beam widths or consider upgrading to the 9 LED version of the NP uplight.

It’s important to use photometrics charts to determine the number of fixtures required to achieve the desired lighting effect accurately.

When it comes to lamp life, traditional lamps have an “Average Rated Life” based on manufacturer testing to a median time-to-failure value. It is recommended to replace conventional lamps when they reach their Average Rated Life to avoid disappointments and ensure the system’s performance. On the other hand, LEDs don’t abruptly “burn out” but gradually dim over time. LED life is defined as the duration it takes for the light output to fade to a certain percentage of its original strength. The standard measurement is represented by the letter “L” followed by a numerical value, such as L70, which indicates the time for an LED to reach 70% of its original light output. Factors like junction temperature, operating current, luminaire type, and manufacturing materials can affect an LED’s lifespan, making it challenging to determine its service life accurately.

It’s also worth noting that different lamps have different lifespans in the field, and dimming capabilities vary among light sources. While most incandescent lamps can be dimmed, many LEDs are also dimmable. When selecting light sources for an area that would benefit from dimming, ensure they are specifically rated for dimming, and choose a compatible dimmer.

George Gitau

Meet George, an advocate for traditional craftsmanship. I will provide you with educational content, techniques and ideas for your next garden and home improvement project. Together, let's create beautiful spaces that are not only beautiful but also functional

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