When choosing the right lighting for an office, room or other interior/exterior, it’s important to understand the way the light will work in the space.

This includes aesthetic factors — like the color, strength and feel of the light — but also efficiency factors, such as the power supply available, energy efficiency and, in enclosed spaces, thermal management.

In this guide we’ll be focusing on the brightness of the light, to dispel some myths around luminosity — what makes one bulb brighter than another, after all?

If you’ve purchased incandescent light bulbs in the past, you’d be forgiven for thinking that the higher the wattage, the brighter the light will appear.

Whilst this was true in the past, there are now options available which provide a brighter bulb with less energy consumption. Not only is this approach better value for money in terms of energy used, it also extends the lifespan of your light bulbs.

So next time you’re choosing a light bulb, instead of assessing the watts in each option, look for ‘luminous efficacy’.

What is a Lumen?

According to the Collins English Dictionary, the definition is:

The derived SI unit of luminous flux; the flux emitted in a solid angle of 1 steradian by a point source having a uniform intensity of 1 candela

More simply put:

A lumen is the amount of visible light given off by a light source over a period of time. The higher the lumens, the brighter a light source will appear.

What is ‘efficacy’ and how is it different from ‘efficiency’?

Efficacy and efficiency are commonly confused, so let’s break it down:

  • Efficacy is the ability of something to produce the desired result. For example, in the context of lighting, it is the ability of a light source to actually produce light.
  • Efficiency is the ability to do something without waste. For example, an LED light bulb is over 10 times more efficient than in incandescent, because it doesn’t waste energy on heat.

What is luminous efficacy?

Put a ‘Lumen’ and ‘efficacy’ together, and you get ‘luminous efficacy’.

Whilst this sounds like a complicated term, its meaning is quite straightforward. ‘luminous efficacy’ measures how a light source converts power into visible light, measured in Lumens.

For electricity, this is usually expressed as Lumens per Watt or lm/W.

What does lumen per watt mean, and how does this differ across types of bulbs?

Lumens per watt and luminous efficacy are both terms used to explain the same concept — how well a light source (i.e. bulb) produces visible light. And different types of light bulbs have different luminous efficacy or lm/W rates.

Consider a traditional incandescent light bulb. This produces light by heating a filament until it is so hot that it glows, producing a lot of heat in the process. This wastes energy and gives a luminous efficacy range of around 5 to 15 lumens per watt.

Compare this with a fluorescent, which uses electricity to charge electrons in a vapor, to then emit ultraviolet light. Humans can't see ultraviolet light, so a coating on the glass converts it to visible light. This conversion wastes energy so fluorescents have a luminous efficacy of 60 to 100 lumens per watt.

To save energy (and therefore money), we want to do two things. First, reduce the waste heat produced. Secondly, produce only light which is visible by humans.

Thankfully, this technology exists — the light-emitting diode (LED).

LEDs work by electroluminescence and to explain why this provides better Luminous Efficiency compared to other bulbs, we need to dip into the science quickly.

In an LED, light is generated by passing electrons across two sides of a diode, known as a p-n junction. On the n-type side, there are extra electrons. On the p-type side, there is a hole. Between the two, there is an area with no electrons or holes. With a jump across, the electron becomes a complete atom and a burst of energy is released in the form of light.

This burst of energy is then filtered to produce visible light. There is very little heat, just an efficient light generation. This produces a luminous efficacy of 100 to 170 lumens per watt.

Luminous efficacy examples

Type of Light Minimum (lm/W) Maximum (lm/W) Typical (lm/W)
Candle - - 0.3
Incandescent 10 20 15
Compact Fluorescent 40 65 50
Linear Fluorescent 50 100 75
Halogen 15 20 17.5
Mercury Vapor 30 60 50
Metal Halide 50 90 80
High-pressure Sodium 85 150 120
Low-pressure Sodium 100 200 150
Induction 75 95 80
LED 60 100+ 90

How do you calculate luminous efficacy?

If you want to calculate the luminous efficacy of your own bulbs, you can do so in three easy steps:

1. Measure the lumens produced by the light source.

2. Measure the energy consumption of the light source in watts.

3. Divide the lumens by the energy consumption.

But, generally speaking, you should be able to find a bulb’s lumens per watt labelled on its packaging.

Why is luminous efficacy important?

Cost savings and energy independence

The US Department of Energy believes that huge energy savings can be made by switching to LEDs, cutting business and homeowner electricity bills by $50 billion a year.

lighting luminous efficacy graph lm/w

With improved efficacy of lighting, the US is actually projected save 490 terawatt-hours of energy by 2035. This is nearly twice the amount of energy that the Energy Information Administration predicts will be produced by solar power in 2035.

With such a large saving of energy, the need to import oil, coal and gas is greatly reduced, improving energy independence and security.

What’s more, as these improved bulbs provide more light, fewer bulbs are required. For example, in street lighting applications you might only need 4 street lamps rather than 8 to provide the same level of visibility.

Technological impact

With improved lighting efficacy, additional technology can be added without additional power consumption and within the bulb. For example, Internet of Things (IoT) technology can be added to light bulbs to automatically control them and turn them off when not required, saving further energy.

Low heat LEDs with high luminous efficacy can also be used in farming. Plants grow best and produce their best crops when they receive high-quality lighting, so improved luminous efficacy could improve crop yields and reduce food prices.

Researchers also suggest that improved lighting could lead to urban agriculture becoming more commonplace, bringing locally produced fresh food to cities without the need for transport emissions.

Longer lasting bulbs

Poor luminous efficacy can also reduce the lifespan of light sources, as heat causes damage to components within the lightbulb. A longer-lasting light bulb means less manufacturing is required, reducing carbon emissions and costs.

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