Red light is the longest-wavelength portion of visible light. Depending on the convention used, visible red is commonly placed at roughly 620–750 nanometers (nm). In LED engineering, however, practical red LEDs often occupy a narrower range; a U.S. Department of Energy reference lists common red LED wavelengths around 630–660 nm.

Red Light Wavelength Range

The exact boundaries between colors are not absolute because human color perception changes gradually across the visible spectrum.

What Does 630 nm Red Light Mean?

A 630 nm LED produces visible red light and is a common wavelength for red LED applications.

Typical uses include decorative lighting, indicators, displays, signaling, architectural lighting and some specialized lighting systems.

For SMD LED products, common red specifications include 620–625 nm, 625–630 nm, and 630–635 nm.

What Is 660 nm Deep Red?

660 nm is usually described as deep red. It appears darker and deeper than a typical 620–630 nm red LED.

660 nm LEDs are particularly important in horticultural lighting because deep-red photons are efficiently used in photosynthesis.

A typical full-spectrum grow-light design may therefore combine:

White LED + 450 nm Blue + 660 nm Deep Red + optional 730 nm Far Red

Red vs. Deep Red vs. Far Red

Although these wavelengths are relatively close numerically, they have different applications.

620–630 nm Red provides a bright visible red appearance and is widely used for general colored lighting.

660 nm Deep Red produces a deeper visible red and is widely used in horticultural applications.

730 nm Far Red lies beyond or at the extreme edge of human visible sensitivity and is commonly used in horticulture to influence plant photomorphogenic responses.

Red LED Wavelengths

Unlike white LEDs, monochromatic red LEDs emit light over a relatively narrow spectral band. The semiconductor composition determines the emission wavelength.

Common red and amber LED technologies include AlGaInP-based semiconductor systems. DOE notes that red LEDs in the 650–660 nm region can achieve high efficiency, while performance changes as emission shifts toward shorter wavelengths.

For professional LED sourcing, specifications may include:

Dominant wavelength (λd): the wavelength associated with the perceived color.

Peak wavelength (λp): wavelength where spectral output reaches its maximum.

FWHM: the spectral bandwidth measured at half the maximum intensity.

Therefore, specifying simply "red LED" is less precise than specifying something like λd = 625–630 nm.

Red Light vs. Infrared Light

Red and infrared should not be confused.

Visible red light is detectable by the human eye, while infrared wavelengths extend beyond the visible red region.

For example:

630 nm → Red

660 nm → Deep Red

730 nm → Far Red / edge of visibility

850 nm → Near Infrared

940 nm → Near Infrared

An 850 nm or 940 nm IR LED can therefore emit substantial optical radiation even though the output is not visible like a conventional red LED.

Common Applications

Different red wavelengths are selected according to the application.

620–630 nm: indicators, displays, decorative lighting and signaling.

630–660 nm: architectural, specialty and general red LED applications.

660 nm: horticulture and plant-growth lighting.

730 nm: horticultural far-red supplementation.

850/940 nm: security cameras, sensors, remote controls and infrared illumination.

Which Red LED Wavelength Should You Choose?

For a bright conventional red appearance, 620–630 nm is a common choice.

For a deeper red appearance, consider 630–650 nm.

For horticultural lighting, 660 nm Deep Red is one of the most important wavelengths.

For far-red plant-lighting applications, 730 nm is commonly used.

For invisible or nearly invisible IR applications, move beyond visible red to wavelengths such as 850 nm or 940 nm.

The wavelength of visible red light is approximately 620–750 nm, although the exact boundary depends on the convention being used. In LED products, common red emitters are concentrated in narrower ranges, with DOE documentation listing 630–660 nm as a representative red LED range.

For LED applications, the most useful distinction is often:

620–630 nm = Red → 660 nm = Deep Red → 730 nm = Far Red → 850/940 nm = Infrared

When selecting 2835, 3030, 3535, or 5050 SMD LEDs, specifying the required dominant or peak wavelength is more accurate than simply requesting a "red LED."