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What Are Luminous Efficacy & Efficiency? (Detailed Answers + Advice)

The measurement of how much light could be emitted by a light source and how much power it needs for that amount of light are the two most common measurements to represent the quality of the light fixture. It leads to an idea of how efficiently a lighting system can work.

Nowadays, various light sources are accessible with their unique specifications and sometimes come with such unfamiliar terms. Before adopting them to our requirements, we should watch the specification closely and grasp its fundamentals while considering the pros and cons.

Let's look at the lighting efficiency parameters—luminous efficacy and efficiency—and how each sort of light source is affected by them.

What Are Luminous Efficacy and Luminous Efficiency?

What are luminous efficacy and luminous efficiency?

Human eyes were unable to detect radiation outside of the visible wavelength (400-700 nm). Some wavelengths are more sensitive to the eyes than the others, even within this range. The standardized response of a typical human eye to light is shown in the curve below:

The Response of a Typical Human Eye to Different Wavelengths of Light
Chart: Wikipedia

Luminous Efficacy (Measured in lm/W)

Luminous Efficacy definition
The ratio between the lumens (lm) associated with a given optical power, weighted by the human eye response over wavelength and the electrical source power used to create the optical power (W).

Luminous efficacy represents the amount of emitted visible light (also known as luminous flux) in correlation to the input power used. In terms of visible wavelength, this term depicts:

  • The ability of the light source to generate radiation. If the light source emits more wavelengths outside the visible wavelength range, it will lower the luminous flux proportion to the radian flux. Hence, the luminous efficacy would also be reduced.
  • Power is consumed by a light source that provides a certain amount of radiation.

For example, a 90 W light bulb emits 900 lumens. The luminous efficacy of that light bulb would be 900 lumen / 90 W = 10 lm/W.

Luminous Efficiency (Measured in percent)

Luminous Efficiency definition
The ratio of the amount of light produced by the light source to the total amount of light coming out of the fixture. It’s measured in percent (%).

Alternatively, we can also measure the lighting efficiency by dividing the luminous efficacy with 683 lm/W. The 683 lm/W value represents the maximum value of luminous efficacy that a light fixture can generate.

The value of efficacy and efficiency are pretty much affected by:

  • Loss of the generated light (e.g., by absorption). It will have a lower value if the light fixture is located in an enclosed space by walls/surrounded by dark material. For better lighting and improved efficiency, applying white in buildings/rooms can help to minimize energy loss. White materials reflect or scatter most of the light instead of absorbing them.
  • Performance of the power supply, specifically in terms of efficiency. The high-frequency switching could drop the energy losses to only a few percent, requiring less power for the same illumination.

Comparing the efficacies of incandescent, CFL, and LED light bulbs

It's irrelevant to use Watt as a performance parameter of light bulbs since it's not all about the power. Besides, any light sources offer better efficiency to ensure the same level of performance while using less power. More technologies have been introduced to meet the demands of high-efficiency light sources, such as LED (Light-Emitting Diode) and CFL (Compact Fluorescent Lamp) bulbs which outstand the traditional incandescent bulbs in most ways.

LED Bulbs

LEDs are the best in energy efficiency and electricity costs. It offers a long lifespan (up to 15 years/25,000 hours) and low power consumption to produce a high brightness. Despite its higher price, these benefits will cut down the total operational costs.

CFL Bulbs

CFL bulbs are not as efficient as LEDs, requiring more energy and implying more operational costs. Another downside of the CFLs is the lower lifespan (8,000 hours), which means more replacement for expected utilization. Nevertheless, the overall performances are better than the incandescent bulb with a relatively lower price than LEDs.

Incandescent Bulbs

This traditional bulb got the worst performance. Even though it is the cheapest one, the bad efficiency would cost more and the short lifespan—typically only lasts 1,200 hours—might be a burden in case of frequent replacement along with utilization.

One of the performance aspects mainly considered is the brightness and its efficacy. To compare these parameters, we could simply find out the value of luminous flux (lumens) and power (Watt) for each bulb to achieve a specific level of brightness. For better understanding, let's use an example to compare bulbs from a marketplace with similar wattage specifications but different types:

Unit Price (USD)2.57.29455
Wattage (W)109999
Luminous Flux (lm)555005401,200800
Luminous Efficacy (lm/W)5.555.5660133.388.89
Luminous Efficiency (%)0.818.138.7819.5213.01

Based on the example, LED bulbs are the brightest among all bulb types. It is confirmed from the higher lumen value it has. To reach the same brightness levels, CFL and incandescent bulbs need greater energy, reducing their efficiency.

Helpful tips about choosing light sources

When choosing light sources, several factors must be considered: brightness requirement, power consumption, light source characteristic, and electrical compatibility.

Brightness Requirement

We should understand the lumen value on the unit packaging to get the correct brightness that suits our needs. With higher lumen, more visible light would be emitted, and the light output will be brighter.

Power Consumption

In general, power consumption would be equivalent to the lumen, but it depends on the efficiency of the light source. In this case, comparing the lumen output to the input power or checking the luminous efficacy can provide us an overview of device performance. We should strive for minimum energy usage to achieve the desired lumens.

Light Source Characteristic

Since there are many light source variations in the market, we should explore each feature to match its characteristic with the purpose, like as:

  • Color performance relative to the natural light is sometimes important for aesthetics use. This affects the wavelength range in which the light source should operate.
  • For a certain position with sufficient daylight, optimizing its use could reduce the luminous flux required from the lighting systems.
  • The illumination area also has direct impacts on numerous aspects of the specification and installation, including the beam/tilt angle and distance.

Electrical Compatibility

The light source must be compatible with the electricity source, particularly regarding voltage and frequency, to prevent any discrepancies that might be causing early failure or burnt bulbs.

Key Takeaways

  • Luminous efficacy describes the relationship between produced light and the power it consumes. Meanwhile, luminous efficiency states the ratio of the efficacy to its maximum value.
  • LED is the most superior bulb with many benefits, including high efficiency, long lifespan, and high potential cost saving due to its excellent reliability. It also outperforms CFL bulbs in every respect quite much. Incandescent bulbs are far away behind them, performance-wise.
  • By taking some considerations (brightness requirement, power consumption, light source characteristic, electrical compatibility, etc.) into account when choosing light sources, we can estimate how much light they would emit, how efficient the system is, and how to install them properly. This is important to ensure the light source would operate well and comply with the purpose.
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