Luminous efficacy facts for kids

Quick facts for kids Luminous efficacy
Common symbols	K
SI unit	lm⋅W−1
In SI base units	cd⋅s3⋅kg−1⋅m−2
Dimension

Luminous efficacy is a way to measure how well a light source produces visible light. It compares the amount of light produced to the amount of energy used. In the International System of Units (SI), this is measured in lumens per watt (lm/W).

Think of it like the gas mileage of a car, but for light bulbs. If a light source has high luminous efficacy, it produces a lot of light for the energy it consumes. If it has low efficacy, it wastes a lot of energy, usually turning it into heat instead of light.

Understanding Light and Power

To understand luminous efficacy, it helps to know how light works. Not all energy that comes from a light source is visible to humans.

• Luminous flux: This is the measure of the total amount of visible light emitted by a source. It is measured in lumens.
• Power: This is the rate at which energy is used or produced, measured in watts.

Some light sources, like the sun or old-fashioned light bulbs, produce a lot of invisible radiation. This includes infrared (which we feel as heat) and ultraviolet light. Since our eyes cannot see this radiation, it does not count as "light" for luminous efficacy. Therefore, a light bulb that gets very hot is usually less efficient because it is turning electricity into heat rather than visible light.

How Our Eyes Detect Light

This graph shows how human eyes respond to different colors of light. We see green light (in the middle) much better than red or blue light at the edges.

Our eyes are not equally sensitive to all colors. We see green and yellow light much better than we see red or blue light. This means that a green light will look brighter to us than a red light, even if they both use the same amount of energy.

Day Vision vs Night Vision

Scientists use a standard curve called the luminous efficiency function to describe how an average human eye sees light.

• Photopic vision: This is how we see during the day when it is bright. Our eyes are most sensitive to yellowish-green light (specifically a wavelength of about 555 nanometers).
• Scotopic vision: This is how we see at night or in dark conditions. Our eyes adjust to be more sensitive to blue-green light, but we lose the ability to see colors clearly.

Because our eyes are so sensitive to green light, a pure green light source has the highest possible luminous efficacy. It can produce about 683 lumens for every watt of radiant energy.

Measuring Light Source Efficiency

When we talk about how good a light bulb is, we often look at the "overall luminous efficacy" or "wall-plug efficacy." This measures the total light coming out of the device compared to the electricity plugged into the wall.

Energy Loss

Real-world light sources are never 100% efficient. They lose energy in two main ways:

1. Heat: Much of the electricity is turned into heat. For example, an old incandescent bulb gets too hot to touch because it is wasting energy.
2. Invisible Light: Some energy is turned into infrared or ultraviolet light, which humans cannot see.

Efficacy vs Efficiency

Sometimes you might see the word "efficiency" used instead of "efficacy."

• Efficacy is the ratio of light to power (lumens per watt).
• Efficiency is often a percentage. It compares the efficacy of a light source to the maximum possible efficacy (683 lm/W). For example, if a light source produces 68.3 lm/W, it might be said to have 10% luminous efficiency.

Comparing Common Light Sources

Different types of lamps have very different efficacy ratings. Modern technology has allowed us to create lights that save a lot of electricity.

A graph showing the efficiency of a perfect "black body" radiator at different temperatures.

The energy from a hot object (like a star or a filament) is spread out. The grey lines show the small part that is visible light.

Incandescent Bulbs

These are the classic glass bulbs with a wire filament inside. They work by getting the wire very hot until it glows.

• How they work: Electricity heats a tungsten filament.
• Efficacy: Very low (about 15 lm/W).
• Why: Most of the energy turns into heat (infrared radiation) rather than visible light.

Fluorescent Lamps

These are the long tubes often seen in schools or offices, or the spiral-shaped compact fluorescent lamps (CFLs).

• How they work: Electricity excites a gas inside the tube, which makes a coating on the glass glow.
• Efficacy: Medium to High (about 60–100 lm/W).
• Why: They produce much less heat than incandescent bulbs.

LED Lights

Light Emitting Diodes (LEDs) are the modern standard for lighting.

• How they work: They use semiconductors to turn electricity directly into light.
• Efficacy: Very High (often over 100–200 lm/W).
• Why: They are designed to produce mostly visible light with very little waste heat.

Table of Light Sources

Here is a simplified list comparing how much light different sources produce for the energy they use.

SI photometry units

See also

• Photometry (The science of measuring light)
• Light pollution (Too much artificial light at night)
• Wall-plug efficiency (How efficient a device is overall)
• List of light sources