Incandescent light is the warm glow produced when an electric current heats a thin wire (called a filament) inside a glass bulb until it’s hot enough to shine. It’s the oldest and simplest form of electric lighting, and for over a century it was the default light source in homes worldwide. The filament in a standard incandescent bulb reaches roughly 4,600°F (2,810 K), producing a soft, yellowish-white light that renders colors with perfect accuracy.
How an Incandescent Bulb Produces Light
The physics behind incandescent light is straightforward. When electricity flows through the bulb’s filament, the filament resists the current, and that resistance converts electrical energy into heat. As the filament gets hotter, the charged particles inside it vibrate faster and emit electromagnetic radiation, including visible light. This process is called thermal radiation, and it’s the same basic phenomenon that makes a campfire ember glow or a piece of metal turn red-hot in a forge.
The filament is made of tungsten, a metal with a melting point above 6,100°F, which allows it to glow white-hot without melting. Early bulbs used carbon filaments, but tungsten replaced carbon in 1904 because it could handle higher temperatures and last much longer. The glass bulb is filled with an inert gas (typically a mix of argon and nitrogen) to prevent the superheated tungsten from reacting with oxygen and burning up instantly. In 1913, physicist Irving Langmuir discovered that adding inert gas inside the bulb doubled its efficiency compared to vacuum-sealed designs.
Why Incandescent Light Looks So Good
One of the defining qualities of incandescent light is its color rendering. Color Rendering Index (CRI) measures how accurately a light source reveals the true colors of objects, on a scale from 0 to 100. Incandescent bulbs score a perfect 100. This is why skin tones, food, artwork, and fabrics look natural and rich under incandescent lighting, and why photographers, restaurants, and homeowners have long preferred it.
The color temperature of a standard incandescent bulb sits around 2,700 to 2,800 Kelvin, which places it firmly in the “warm white” range. That warmth comes from the fact that thermal radiation at these filament temperatures peaks in the infrared spectrum, with the visible portion skewing toward reds and yellows rather than blues.
The Efficiency Problem
For all their visual appeal, incandescent bulbs are remarkably inefficient. A standard bulb produces only 10 to 18 lumens per watt, meaning the vast majority of the electrical energy it consumes, roughly 90 to 95 percent, is released as heat rather than visible light. By comparison, modern LED bulbs produce 80 to 100 lumens per watt or more, delivering the same brightness for a fraction of the electricity.
Lifespan is the other major drawback. A typical incandescent bulb lasts 750 to 2,000 hours. An LED bulb lasts 40,000 to 50,000 hours. In practical terms, you’d burn through 20 to 60 incandescent bulbs in the time a single LED keeps working.
A Brief History
The first constant electric light was demonstrated in 1835, and for the next four decades inventors across the world experimented with different filament materials and bulb atmospheres. Thomas Edison patented his version in 1879 and 1880, commercializing a design that used a carbonized cotton thread filament lasting about 14.5 hours. His team later switched to a bamboo filament that extended bulb life to 1,200 hours, which became the standard for the next decade.
European inventors introduced the tungsten filament in 1904, a leap that made bulbs brighter and longer-lasting. Langmuir’s inert gas innovation followed in 1913. Over the next 40 years, incremental improvements reduced manufacturing costs and squeezed more light from each watt, cementing the incandescent bulb as the dominant light source through most of the 20th century.
Flicker and Dimming Compared to LEDs
Incandescent bulbs flicker at the frequency of the electrical supply, 50 or 60 times per second depending on your country. But the thermal mass of the filament means it can’t cool down and reheat that quickly, so the actual variation in brightness is very small. Researchers describe this as having a “low flicker index.” Fluorescent lights and many LED lamps, by contrast, have a high flicker index because they extinguish and reignite more abruptly with each electrical cycle. Some LED bulbs even control their perceived brightness by adjusting flicker frequency.
This difference matters for people who are sensitive to flicker, including some migraine sufferers. It also makes incandescent bulbs inherently compatible with simple dimmer switches. LEDs require specific dimmer-compatible circuitry, and cheap LED bulbs can buzz or flicker when dimmed.
The U.S. Sales Ban
Standard incandescent bulbs can no longer be sold in the United States for general household use. In 2022, the U.S. Department of Energy finalized a rule requiring all general service lamps to meet a minimum efficiency of 45 lumens per watt. Since incandescent bulbs top out around 18 lumens per watt, they fall far short of this threshold. The sales prohibition took effect on July 25, 2022.
The rule applies broadly to standard and halogen incandescent bulbs with medium screw bases producing between 310 and 2,600 lumens, which covers the range most people use at home (roughly 40-watt to 150-watt equivalent). However, many specialty bulbs remain exempt: appliance bulbs (like the one in your oven or refrigerator), plant grow lights, three-way bulbs, chandelier bulbs, infrared heat lamps, colored bulbs, bug lamps, and several other niche categories. If you still see incandescent bulbs for sale, they likely fall into one of these exempt categories.
Where Incandescent Bulbs Still Make Sense
Despite the ban on standard household bulbs, incandescent technology isn’t extinct. Heat lamps used in food service, bathroom ceiling heaters, and animal husbandry rely on the very property that makes incandescent bulbs “inefficient” as light sources: they’re excellent at producing heat. Easy-Bake Ovens famously used incandescent bulbs as their heat source for decades.
Appliance bulbs remain incandescent because they need to tolerate temperature extremes inside ovens, refrigerators, and dryers. Some decorative fixtures still use exempt incandescent styles for their warm aesthetic. And in very cold environments, the waste heat from an incandescent bulb can be a minor benefit, keeping enclosed fixtures or signage from icing over.
For general lighting, though, LEDs have caught up on the one quality that kept people loyal to incandescent bulbs for so long. Many LED bulbs now offer CRI scores of 90 or above at the same 2,700 K warm color temperature, closely mimicking the look of incandescent light at a fraction of the energy cost.