Understading Luminous Efficacy (lumens per watt) and Its Impact on LED Performance (2024)

One of the most significant breakthroughs in the world of lighting technology is the development of LEDs (Light Emitting Diodes). Their energy efficiency, long lifespan, and versatility have made them a popular choice for a myriad of applications, from ambient home lighting to large-scale commercial installations. However, understanding LED performance involves more than just knowing their energy efficiency and lifespan. One critical aspect that can significantly impact LED performance is 'luminous efficacy.' This article aims to delve into the concept of luminous efficacy, its role in LED performance, and how it influences lighting design.

Defining Luminous Efficacy

Simply put, luminous efficacy measures how effectively a light source produces visible light. It's a ratio of the light output, or luminous flux, to the electric power consumed by the light source. The unit of luminous efficacy is lumens per watt (lm/w), which signifies the quantity of visible light produced for each watt of electrical power consumed.

For instance, consider a 1 Watt LED. If it's a high-efficacy LED, its light output can be extremely bright. Conversely, if it's a low-efficacy LED, the light output can be barely visible. The luminous efficacy, therefore, serves as a key performance indicator for LEDs.

Range of Luminous Efficacy in LEDs

The range of luminous efficacy in LEDs is quite broad, reflecting the wide spectrum of LED performance. At the top end, high-performance LEDs such as the Nichia 757G LEDs can reach up to 220 lumens per watt (lm/w), making them extremely efficient. Other high-performance LEDs from various manufacturers fall within the range of 150-200 lm/w. Conversely, low-performance LEDs exhibit an efficacy of less than 99 lumens per watt.

Top performance LED	High Performance LED	Good Performance LED	Low Performance LED
220 lm/W	150-200 lm/W	100-149 lm/W	50-99 lm/W

The other light sources on the market today have the following luminous efficacy:

Category	Type	Overall luminous efficacy (lm/W)
Incandescent	15, 40, 100W tungsten incandescent (230V)	8, 10, 13
Incandescent	5, 40, 100W tungsten incandescent (120V)	5, 12, 17
Halogen incandescent	100, 200, 500W tungsten halogen (230V)	16, 17, 19
	2.6W tungsten halogen (5.2V)	19
	Halogen-IR (120V)	17–24
	Tungsten quartz halogen (12–24V)	24
	Photographic and projection lamps	35
Arc lamp	Carbon arc lamp	2–7
	Xenon arc lamp	30–50
	Mercury-xenonarc lamp	50–55
	Ultra-high-pressure(UHP)mercury-vaporarc lamp, free mounted	58–78
Fluorescent	32W T12 tube with magnetic ballast	60
	9–32Wcompact fluorescent(with ballast)	46–75
	T8 tube with electronic ballast	80–100
	PL-S 11W U-tube, excluding ballast loss	82
	T5 tube	70–104
Gas discharge	1400Wsulfur lamp	100
	Metal halide lamp	65–115
	High-pressure sodium lamp	85–150
	Low-pressure sodium lamp	100–200

The variation in luminous efficacy among LEDs is due to differences in their design, material quality, and manufacturing processes. It's crucial to note that while higher luminous efficacy often indicates superior performance, it doesn't necessarily mean that the LED is the best fit for all applications. Other factors such as color quality, lifespan, and cost-effectiveness should also be considered.

Luminous Efficacy in Lighting Design

When planning a lighting installation, especially one that prioritizes energy efficiency, considering luminous efficacy alone isn't sufficient. How the light output is transmitted to the required area is equally critical. This concept is known as illuminance, which refers to the amount of light falling on a surface. Illuminance is contingent on the design of the lighting fixture and the viewing angle of the light source.

For instance, the same light source can create different levels of illuminance when placed in different fixtures. As illustrated in the image below, identical power consumption can yield various illumination results, from insufficient or uneven illumination to optimal lighting or even overly bright spots in certain areas.

This phenomenon brings us to the concept of 'useful light output,' or 'useful lumens.' Useful lumens represent the quantity of light from a source that contributes effectively to the illumination of a specific area.

Consider a non-directional 360° light-emitting source such as an incandescent bulb used to light a desk surface. Despite its high light output, its luminous efficacy will be relatively low because much of the light is emitted in non-useful directions. On the other hand, a light source with a narrower view angle, such as a 120° LED strip or a 90° LED spot, can achieve the same illuminance on the desk with lower power consumption and less total lumen output. This efficient use of light is what we refer to as 'useful lumens.'

**Why Useful Lumens Matter in Lighting Design**

This concept of useful lumens is a critical consideration in lighting design, particularly in scenarios where energy efficiency and targeted illumination are paramount. Efficiently directed light not only optimizes energy usage but also improves the overall quality of illumination, reducing glare and unwanted shadows. It's the reason why an LED light with a lower total lumen output but higher useful lumens can often outperform a higher total lumen light source in specific applications.

The Role of LEDs in Maximizing Useful Lumens

LEDs are particularly adept at maximizing useful lumens due to their inherent directional light output. Unlike traditional incandescent or fluorescent bulbs that emit light in all directions, LEDs emit light in a specific direction. This characteristic allows for more efficient use of light, minimizing wastage and maximizing illuminance in the intended area.

Consider the example of an LED strip with a 120° view angle. Such a design ensures that the light is directed exactly where it's needed, optimizing the illuminance on the target surface while minimizing light spill into unwanted areas. This ability to control the light output makes LEDs especially suitable for task lighting, spotlighting, or any application requiring precise light control.

LEDs with a narrower beam angle, such as LED spots with 90° or less, can provide even more focused illumination, enhancing the 'useful lumens' and making them ideal for spotlighting or accent lighting applications.

High-Efficacy LEDs at Lumistrips

At Lumistrips, we understand that efficient lighting is not just about maximizing energy efficiency but also about optimizing the useful light output. That's why we offer many LED strips and modules that maximize useful lumens by combining directed beam angles with high luminous efficacy LEDs.