What Is Light Irradiance and Why Is It Important?

Light irradiance is the power density of red light over a given area. It refers to the number of photons that are flowing through a certain area and are available for absorption into your cells. In simple terms, irradiance is the amount of light energy output from a source at the point when it connects with its target (your skin), rather than the energy level at its source.

The unit of measure for light irradiance is milliwatts per square centimeter (mW/cm²).

It is important to note that the wattage of a light-emitting diode (LED) bulb does not correlate in any way with its power output. Power consumption is unrelated to power density/irradiance. A device might consume a lot of energy but still have a low output, which is generally due to a poorer quality of components used in the construction of the panel. Just because a device has a high wattage number, that doesn’t mean all the power it draws is necessarily being converted to beneficial red light. Likewise, if a lot of heat is being generated from a red light device (which it shouldn’t), a lot of available power is wasted that way as well.

An efficient red light panel will have a lower power consumption level and a higher power density/irradiance.

While irradiance is often referred to as the power density of light, it is different from true light density. Light density refers to the intensity of light at the surface of its source. As previously mentioned, irradiance is the intensity of light when it hits its target, not at its source. The power density/irradiance of light decreases with distance from its source.

If power density/irradiance is not considered and only wavelength is factored into the equation of successful red light therapy, you could end up with a low-powered device that will not provide enough red light to be beneficial. No matter what the wavelength, if irradiance is too low, the device will not be as effective in cellular penetration.

A higher power density/irradiance output also makes for more efficient therapy. Greater therapeutic benefits are achieved in less time because the light you are absorbing is more intense.

Does It Matter How Many Bulbs a Red Light Panel Has?

No. The number of LED bulbs in a panel is irrelevant from an irradiance point of view, because they come in different classes, power levels, and beam angles. For example, a panel might have 50 lower or midrange power density bulbs or 25 high-powered bulbs and provide the same or similar level of therapy, as long as you're focusing on a small body part at a close distance of a few inches. The important factors are wavelength and power density/irradiance, not the number of LEDs.

Having said that, a panel with a larger number of bulbs would cover a larger area. Hence, for panels designed to cover the entire body or larger body parts, you will need to select one with a large number of bulbs or LEDs. For example, our medical grade full body panel with 480 LEDs can be a great choice.

What to Look for When Comparing the Irradiance Levels of Red Light Therapy Devices

When reviewing the irradiance readings of various devices for comparison, you need to know the distance at which the power output was measured. Power density/irradiance decreases the farther you get from the device, as controlled by the inverse square law.

The inverse square law dictates that a specified physical quantity is inversely proportional to the square of the distance from its source. In other words, the intensity of light at any given distance equals the inverse of the square of that distance. So, if the light source is 2 times as far away, the light is ¼ as intense as it was at its source. If it is 10 times farther away, the power density/irradiance is 100 times less.

This is why it is crucial to know the distance from which the irradiance readings were taken. Without that information, a reading is meaningless. If the minimum recommended usage distance is six inches (meaning you will receive therapy from six inches away from the surface of the light panel), then irradiance should be measured from a distance of six inches. A reading taken at the surface of a device is actually measuring light density, not irradiance/power density. A surface light density reading does not disclose the power density of the light your body will really absorb.

If the distance at which irradiance was measured is not stated by the manufacturer, you should assume that it was measured at the surface of the device. Be aware that the power density you will absorb is some unknown lesser number than the reading provided.

If a manufacturer doesn’t disclose any power density readings at all, be very wary. The irradiance level is most likely not being disclosed because the manufacturer doesn’t want you to know it, meaning it is probably a low number and an inferior device.

An honest and respectable manufacturer knows that irradiance is a key factor in the benefits of red light therapy and will disclose both the power density/irradiance measurements and the distance at which the measurements were taken.

What Is the Best Level of Irradiance?

In general, the higher the power density/irradiance numbers, the better the quality of the device. Higher irradiance also means less time spent in front of the light panel since more power density yields greater results in less time.

Some basic numbers to consider:

Most studies showing benefits of red LED and near-infrared (NIR) light therapy used light power outputs ranging from 20 to 200 mW/cm², depending upon therapeutic objectives.

For deeper cellular penetration (for example, fat loss , deep tissue healing, athletic performance enhancement , or other deeper mind related therapeutic needs ), a power density/irradiance reading of greater than 100 mW/cm² is recommended.

If a great depth of cellular penetration is not required (for example, for anti-aging, general skin health purposes, and other closer-to-the-surface treatment objectives), then an irradiance level of somewhere between 20 mW/cm² but less than 100 mW/cm² would be appropriate.

Remember that you can also self-adjust the level of irradiance by moving closer to or farther away from the device to suit different purposes. Having your own meter to measure irradiance might be helpful if you wish to utilize various irradiance levels for different purposes. Types of available measurement devices are discussed later in this article.

In the end, there is no “best” level of irradiance; there is only a level that is best for you . What you hope to get out of red light therapy, your budget, and other factors such as wavelength are considerations unique to you. Your individual treatment needs will determine your best options.

Devices for Measuring Light Irradiance

There is some controversy surrounding the best and most accurate tools for measuring light irradiance. The primary measurement devices are:

• Solar Power Meters (inaccurate) — Solar meters, also referred to as pyranometers, measure solar irradiance on a planar surface and are calibrated to the specific light wavelengths of the therapy device.
• Laser Power Meters (inaccurate) — Laser meters measure the power output of laser beams. Laser power meters are designed to analyze lasers within a particular range of wavelengths or intensities.
• Spectrometers (accurate) —  These devices can measure light intensity across a range of wavelengths, providing a spectral profile of the light source. They are highly versatile and can provide detailed information about light irradiance and its spectral composition. They can measure multiple parameters such as wavelength, flicker, and irradiance.

Any of these devices can be used to measure irradiance and will provide a power output reading.

Some manufacturers claim that solar power meters are the best and only reliable way to measure irradiance. Others claim that laser power meters are the only reliable source of measurement and that solar power meters measure falsely high (even double) levels of irradiance when compared to readings from laser power meters.

Laser meters are truly meant to measure laser light, which is much hotter and more intense than LED light. Laser meters are, in a sense, “overkill” when used to measure the irradiance of a red LED light panel.

Spectrometers are versatile, as in they can measure irradiances accurately even if the power output is very high. Not only that, they can also measure wavelengths being transmitted and show results on a light spectrum graph.

The TotalSpectrum MAX's irradiance measured at 6 inches using a $2599 Spectrometer (HP350FR) provides a very different rating from that provided by a solar power meter.

That being said, solar power meters are typically the industry standard for measuring upto 200mW/cm2 of irradiance, above which they may give inaccurate readings. Solar irradiance meters are much more cost-effective than laser meters or spectrometers, especially if you are going to purchase one yourself for at-home use. Multi-purpose spectrometers are more costly than simple solar meters, but they also provide more functionality and high accuracy.

The bottom line is that as long as the irradiance of different red light therapy devices is measured with the same type of measurement tool, the measurements will be comparable. If you’re not doing the testing yourself, try to determine which type of testing device was used by the manufacturer or their independent testing lab. This way you will know you are comparing “apples to apples”. If the type of measurement tool used is not provided, it was most likely a solar meter. You can also e-mail the manufacturer and ask them directly.

It is worth mentioning again that a respectable manufacturer like RLT Home will openly disclose irradiance/power density levels and specify the distance at which the readings were taken using different instruments. It is not advisable to buy a device without reviewing the manufacturer’s irradiance readings even if you have a meter with which to measure irradiance yourself.

Scientific Research and Additional Resources

Here are the links to a few studies that mention irradiance levels and their relationship to successful red light therapy for various purposes.

For those interested in the science behind light power density/irradiance, a scientific overview of the concept of irradiance and a more in-depth explanation of the inverse square law (which governs the correlation between light irradiance and distance from the light source) are also linked.

• liebertpub.com - Efficacy of RLT in Reduction of Fine Lines, Wrinkles, Skin Roughness (opens in a new tab)
• nih.gov - LLLT In Skin: Stimulating, Healing, Restoring (opens in a new tab)
• nih.gov - The Effect of RLT on Human Leukemic Cells (opens in a new tab)
• nih.gov - Photobiomodulation for Brain Disorders (opens in a new tab)
• rlthome.co - Red light therapy for your brain and mind (opens in a new tab)
• nih.gov - Wavelength and Beam Width on Penetration (opens in a new tab)
• sciencedirect.com - Irradiance – An Overview (opens in a new tab)
• sciencedots - Inverse Square Law: Graph (opens in a new tab)

In summary, when choosing a red light therapy device, it is essential to know the two most important aspects of the machine: the wavelength(s) being emitted and the power density/irradiance of light at a specific distance from the device’s surface.

Once you have done your research and selected a device with the optimal wavelengths and irradiance level to suit your needs, you can begin to realize the restorative and rejuvenating powers of red light therapy in the comfort of your own home.

How is the optical power of a red light therapy device represented in terms of joules / energy?

Energy (J) = Irradiance (W/cm²)×Area (cm²)×Time (s)

Let's calculate the energy in joules of our TotalSpectrum MAX panel which has 172 mW/cm² or 0.172 W/cm² irradiance at 8 inches.

Assuming the irradiance is applied over a 1 square centimeter area for 1 second, the energy in joules would be:

Energy=0.172W/cm²×1cm²×1s=0.172J So, 172 mW/cm² irradiance applied over 1 sq cm for 1 second results in an energy of 0.172 joules (at 8 inches distance).

What is the best frequency of red light?

Frequency=λc​ where: c is the speed of light ( ≈ 3 × 1 0 8 ≈3×108 meters per second), λ is the wavelength in meters.

For red light, the best wavelengths are 610nm 630nm 660nm 810nm 830nm & 850nm, which is 4.92 × 10^14 Hz, 4.76 × 10^14 Hz, 4.55 × 10^14 Hz, 3.70 × 10^14 Hz, 3.61 × 10^14 Hz, 3.53 × 10^14 Hz frequencies respectively.