How Much Far-Red and Deep-Red Light Does a Good Full Spectrum Bulb Include?

Over the years of working with supplemental plant lighting, one of the most frequent questions we receive is: "You claim your bulbs are 'full-spectrum'-but exactly how much deep-red and far-red light do they contain?" Many manufacturers simply label their products as "full-spectrum" and provide a generic spectral graph, leaving the actual proportions vague.

This article cuts through the fluff to address one core question: What is the appropriate range for the proportions of deep-red and far-red light in a high-quality full-spectrum bulb, and why?

These terms are often used interchangeably, but they actually refer to two distinct spectral bands:

Deep-red light: Typically spans the 650–670 nm wavelength range, with a common peak at 660 nm. This is one of the most efficient ranges for chlorophyll absorption, serving as a primary driver for photosynthesis.

Far-red light: Typically spans the 700–750 nm range, with a common peak at 730 nm. It is nearly invisible to the human eye; while it does not directly drive most photosynthetic reactions, it plays a crucial role in regulating plant morphology and flowering signals.

In short, deep-red light does the "heavy lifting" for photosynthesis, while far-red light acts as a "signaling agent." Because they serve entirely different functions, their respective proportions within a bulb naturally differ.

In full-spectrum bulbs-which typically consist of white LEDs supplemented with additional red light-a common industry standard is as follows:

The red light band (600–700 nm) accounts for approximately 25%–35% of the total photon flux.

Within this, deep-red light (centered at 660 nm) typically makes up about 10%–20% of the total photon flux.

If the proportion falls below 10%, there is insufficient red light, which can lead to stunted growth and poor flowering or fruiting. Conversely, if it exceeds 25%, the light takes on a distinct purplish-red hue-which is visually unappealing and generally unsuitable for home environments.

Furthermore, such an imbalance often means there is a deficiency in the blue and green light components, which may not necessarily result in better leaf development.

The proportion of far-red light should be kept much more modest. In most well-designed full-spectrum lamps, far-red light accounts for 3%–10% of the total photon flux, with 5% being the most common figure.

In the industry, the ratio of red light to far-red light (R:FR) is also a key metric. Natural sunlight has an R:FR ratio of approximately 1.0–1.2; if a lamp's spectrum maintains a ratio close to this range, plants will generally develop normal morphology. If the ratio is too low, plants may "perceive" themselves as being shaded and will consequently stretch their stems in an attempt to reach more light.

Furthermore, when far-red light and red light are applied simultaneously, photosynthetic efficiency is higher than when either is used alone-a phenomenon known as the Emerson effect. This is the primary reason why high-quality full-spectrum lamps include a certain amount of far-red light.

This is the most common misconception we encounter-the belief that "higher parameter values equate to greater professional quality." In reality:

Too much deep-red light: Energy consumption increases, but the relative proportions of blue and green light decrease, leading to thinner leaves and a less robust plant structure.

Too much far-red light: This causes "stretching" (etiolation), elongated internodes, and narrower leaves-which is particularly undesirable for leafy greens, succulents, and potted plants where a compact growth habit is preferred.

Luminous efficacy and cost: The wall-plug efficiency (electrical-to-optical conversion efficiency) of far-red LEDs is generally lower than that of white or blue LED chips. Increasing their proportion negatively impacts both overall luminous efficacy and production costs.

Therefore, a truly "good" spectrum is defined by its overall balance, rather than by how impressive a single specific parameter value might appear.

The following are general guidelines we use when designing light spectra; however, the optimal settings will depend on the specific crop and the distance between the light source and the plants:

For seedlings, leafy greens, and ornamental foliage plants: Keep far-red light at 3%–5% to ensure a compact growth habit, and set deep-red light at 10%–15%. For flowering and fruit-bearing plants: Far-red light can be increased to 5%–10%, and deep-red light to 15%–20%, which helps improve flowering performance.

For indoor ornamental plants: Prioritize visual aesthetics; it is recommended to keep deep-red light levels moderate and far-red light at around 3% to avoid a purplish or overly dim appearance.

Deep-red light (660nm): Should account for approximately 10%–20% of the total photon flux, with total red light (including deep-red) making up about 25%–35%.

Far-red light (730nm): Should account for about 3%–10%-with 5% being the most common-to ensure an R:FR (red to far-red) ratio that closely mimics natural light.

Higher is not necessarily better; achieving the right balance is more important than the absolute values.
Lower levels are suitable for seedling cultivation and foliage plants, while levels can be increased for flowering and fruit-bearing stages.
When purchasing, look for spectral charts, PPF (Photosynthetic Photon Flux) data, and test reports.

Need a custom solution or more information?

We offer full-spectrum bulbs with various color temperatures and specific ratios of deep-red and far-red light. We also provide spectral customization tailored to your target crops, usage scenarios, and branding requirements, complete with spectral charts and test data.

Feel free to leave a message or send us an email. Let us know your specific application (e.g., home use, seedling cultivation, or commercial growing), and we will provide a suitable spectral solution along with a price quote.

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Professional Full Spectrum E27 LED Grow Bulb Lighting