When you dim a tungsten light, the color temperature shifts lower, causing the light to become more reddish or “warmer” in appearance. This effect occurs because as a tungsten filament is dimmed, it not only produces less overall light output, but the spectrum of light emission changes. Specifically, the filament produces proportionally more longer wavelength red light compared to shorter wavelength blue light as it is dimmed. This results in a lowering of the correlated color temperature.
How Tungsten Lights Work
In order to understand why dimming a tungsten light source causes a color shift, it’s helpful to first understand some basics about how tungsten lights operate.
Tungsten lights utilize a thin tungsten filament enclosed in a glass bulb containing an inert gas. When an electric current is passed through the filament, it heats up to incandescence and emits visible light. The hotter the filament, the whiter or bluer the light emission. As the filament temperature drops, the light shifts more to the red end of the spectrum.
The color temperature of a tungsten light source is correlated to the temperature of the filament during operation. For example, a common tungsten bulb calibrated to produce a “warm” 2,700K light has a filament temperature around 2700° Kelvin when powered on.
Dimming Effects on Tungsten Filament Temperature
When a tungsten light is dimmed through lowering the voltage, the electric current heating the filament is reduced. This causes the filament to drop in temperature.
For example, an incandescent bulb operated at its full 120V supply might heat its filament to 2700°K. When dimmed to 50% brightness, the filament may only reach 1300°K. This drastic drop in filament temperature shifts the spectrum of light output towards longer red wavelengths, causing the large color temperature change that makes the light appear more orange or red.
graphs to show the effect
| Dimming Level | Filament Temperature | Correlated Color Temperature |
|---|---|---|
| 100% (full power) | 2700K | 2700K (warm white) |
| 75% | 2100K | 2100K (warm orange) |
| 50% | 1500K | 1500K (deep orange) |
| 25% | 1200K | 1200K (deep red) |
This table shows example values for how dimming a 2700K tungsten light to lower brightness levels results in lower filament temperatures and correlated color temperatures. At full power the light remains at 2700K. But at 25% power, the color temperature could drop as low as 1200K, causing a very red/orange output.
Spectrum Comparison at Different Dimming Levels
We can also visualize how the spectrum of light emission changes at different dimming levels:
100% – Full 2700K spectrum |
50% – More red, less blue |
These simulated spectra show how at full power, the tungsten light emits a balance of wavelengths that correlates to 2700K. When dimmed to only 50% output, the spectrum shifts to favor longer red wavelengths over blue – causing the lower color temperature.
Why Light Spectrum Shifts when Dimmed
The reason the light spectrum from a tungsten filament shifts as the filament is dimmed relates to the physics of blackbody radiation and the Stefan-Boltzmann law.
In simple terms, the amount of energy radiated at different wavelengths by a blackbody (an idealized emitter) is proportional to the 4th power of its absolute temperature. What this means for a tungsten filament is that as it drops in temperature when dimmed, the short wavelength blue light output decreases much faster than the longer wavelength red output. This shifts the spectrum toward red, lowering the correlated color temperature substantially.
Dimming Other Light Sources
The strong color shift described occurs specifically for incandescent-type lights relying on a heated filament. Other light source technologies behave differently when dimmed:
LED Lights
LED lights mix different color LEDs to produce white light. When dimmed, the output of all LEDs drops relatively evenly, so the color temperature generally remains more stable compared to incandescents. However, at very low dim levels, the blue LED output can sometimes decrease disproportionately, causing a slight warming shift.
Fluorescent Lights
Fluorescents operate by exciting mercury vapor which emits ultraviolet light, causing a phosphor coating to glow and produce visible light. Dimmable fluorescent ballasts control brightness by regulating power to the mercury arc, which affects all visible wavelengths relatively evenly. So fluorescent color temperature is also fairly stable when dimmed.
Sodium/Metal Halide Lights
Gas discharge lights like high/low pressure sodium, or metal halides operate similarly to fluorescents. Dimming causes a fairly even drop in all wavelengths, so the large color shifts seen in incandescents do not occur. However, at very low power, sodium lamps can shift slightly orange.
Color Temperature vs Brightness
In summary, the correlated color temperature of most light sources remains fairly stable as they are dimmed – except for incandescent lights, where temperature drops substantially with brightness due to the unique physics of their heated filaments. This warm, reddening shift is essentially a side effect, not an intentional design feature of dimming a tungsten light.
Applications and Considerations
The ability to control both brightness and color temperature by dimming tungsten lamps is useful in some applications like theater/photography, where a “warmer” ambiance may be desired at lower illumination levels. However, the drastic drop in color temperature can also make dimmed tungsten lighting seem unnatural.
LED and other technology allows for much better control over dimming and tuning light color temperature independently of brightness. So the limitations of incandescent lamps in this area are becoming less of an issue with modern lighting.
Conclusion
Dimming a tungsten incandescent lamp causes a significant drop in the filament operating temperature. This shifts the emission spectrum toward longer red wavelengths, lowering the correlated color temperature substantially and giving the light a more orange/red cast at lower brightness levels. This effect is uniquely strong in tungsten lights due to the blackbody emission characteristics of the heated filament. Other light source technologies are less prone to color shifts when dimmed, allowing for more stable and customizable control over brightness and color temperature.
