Full Width at Half Maximum (FWHM)

Monochromatic light

The degree of resolution of a light source directly affects our ability to detect details in an object. An image with high resolution is generally clear and crisp while an image with a low resolution may appear fuzzy or pixelated. We can influence optical resolution by manipulating various inputs, like optical lenses and display components, to achieve the desired resolution.

While there are several ways to adjust resolution, what we are really trying to do in such cases is change the light source’s bandwidth, and bandwidth is the measurement taken at what is knows as the light wave’s full width half the maximum (FWHM).

In this article we will discuss why FWHM is important in the world of photonics, especially as it applies to light emitting diodes (LEDs) and other specialty light sources.

Why FWHM Matters

The "full width at half maximum" (FWHM) is a measure of the width of an intensity distribution at the point where the intensity is at its highest. It is often employed to determine the width of a spectral line or the range of wavelengths in monochromatic light.

FWHM is a good measure of an image’s sharpness, or resolution. As conditions get fuzzy for seeing, focus, or guiding quality, the width spreads out in relation. The lower an FWHM value, the better, essentially sharper, the image. If two peaks have overlapping FWHMs, they are unresolvable, i.e., they will look like one peak.

Bandwidth in Optical Frequencies

A light source can have some optical bandwidth (or linewidth), meaning the width of the optical spectrum of the output. For narrow-linewidth lasers, the bandwidth can be extremely small—below 1 Hz, which is many orders of magnitude less than the mean optical frequency. On the other hand, ultrashort pulses with few-femtosecond pulse durations can have very large bandwidth—easily tens of terahertz.

Optical bandwidth refers to the frequency range that an optical element or photonic device can handle. It encompasses various aspects, such as the reflection bandwidth of a mirror, the optical transmission bandwidth of an optical fiber, the gain bandwidth of an optical amplifier, or the phase-matching bandwidth of a nonlinear optical device.

The optical bandwidth of a light source is strongly related to the temporal coherence, characterized with the coherence time.

Note that in optical fiber communications, the term “bandwidth” is often misused to describe the data rate achieved in the system. It is more appropriate to use the term data rate or data transmission capacity to avoid any confusion with optical bandwidth. Also note that there is no direct correlation between data transmission capacity and optical bandwidth.

Calculating FWHM

When referring to optical bandwidth, it is possible to express it either in frequency or wavelength units. However, it's worth noting that the conversion between nanometers and gigahertz is not fixed and instead depends on the center wavelength or frequency. For converting small wavelength intervals into frequency intervals, you can utilize the following equation, where 1 nm is worth more gigahertz if the center wavelength is shorter:


The FWHM represents the range of wavelengths over which the intensity of light emitted by the LED is at least half of its maximum intensity. The following outlines several applications of FWHM in light-emitting diodes:

  • Optical Communications: In optical communication systems, such as fiber optics, LEDs are used as light sources for data transmission. The FWHM of the LED affects the bandwidth of the signal that can be transmitted. LEDs with a narrower FWHM enable higher data rates and improved signal quality in optical communication systems.
  • Medical and Biotechnological Applications: LEDs are utilized in various medical and biotechnological applications, such as phototherapy, microscopy, and fluorescence imaging. The FWHM of LEDs used in these applications affects the accuracy and specificity of the light used for excitation, imaging, or therapeutic purposes.
  • Sensing and Detection: LEDs are employed in sensors and detectors for various purposes, including proximity sensing, object detection, and spectroscopy. The FWHM of the LED determines the resolution and sensitivity of these devices. Narrower FWHM LEDs are often preferred for applications requiring high precision and selectivity.
  • Automotive Lighting: LEDs have gained popularity in automotive lighting due to their energy efficiency and durability. The FWHM of LEDs used in automotive lighting affects the visibility, color rendering, and light distribution. Different FWHM values are selected based on the specific requirements of headlights, taillights, turn signals, and interior lighting.

LEDs, unlike laser lights, have a broader bandwidth that is advantageous in stimulating numerous fluorescent probes. Compared to arc lamps that produce extreme heat and an unceasing spectrum, LEDs are also cooler, more compact, and offer a more convenient approach to cycle the source on and off. Additionally, they allow for a quick selection of specific wavelengths. During the past four decades, LEDs have advanced at a pace that rivals microprocessors.

Learn More About FWHM

Dayy Photonics can advise on how to apply FWHM principles to your next project and offer solutions to solve for your optical requirements.

To learn more about FWHM, contact DAYY Photonics to talk about the specialized needs of your application.