Red Light Therapy and Heat! Where does it come from?
Does red light therapy create heat? Well, it depends on several factors and the context.
With red and near-infrared LED light devices, heat generation is an important consideration. Heat generation can be caused by 3 main factors:
1. Wavelengths used
2. Intensity of the wavelengths
3. The LED device heating up
Heat can affect the user who will feel warming or burning on their skin due to longer wavelengths or high intensity light. Or heat will be produced and contained within the device itself, sometimes needing a fan to keep the LEDs or Laser cool for longer term operation.
When we are strictly talking about Photobiomodulation, the researchers specifically defined it as a non-thermal process. This means they only want to study the photochemical biological effects of light.  This is important to separate photothermal effects from high-powered lasers and lamps as a separate science.
Also in terms of comfort and safety, we typically want to make sure the user is experiencing low heat effects during their dosage time of red light therapy. This is part of what makes Red Light Therapy so comfortable and intrinsically safe compared to heating methods.
So lets discuss the 3 causes of heating in some more detail.
With the optical window of wavelengths typically used in Red Light Therapy is usually between 600nm to 1000nm. It is generally agreed that longer wavelengths have more of a heating effect on the tissues. This is because at 980nm+ then water absorption dominates as the primary chromophore. This is fairly easy to lookup the actual water absorption spectrum we analyze below.
So we can see in the graph above that water absorption remains fairly low below 900nm with a peak at 980nm. It is only after 1100 that water absorption truly takes over. 
Often we hear some confusion about 850nm near-infrared being a heating wavelength compared to 660nm red. It is true that according to the data in these charts we see a very small raise in water absorption from 800nm to 900nm. Doing some math, we find that 850nm has over 11 times as much water absorption compared to 660nm! However, in comparison to the absorption peak at 980nm, this is 10 times more absorption compared to 850, and 120 times the absorption compared to 660nm. So based on the context and the scale, 850nm is hardly a heating wavelength, although it does have the potential to be relatively more heating than 660nm.The water absorption spectrum is why incandescent bulbs, halogens, near-infrared heat lamps, and infrared radiators will typically feel hot within a close proximity to them and are excluded from the science of photobiomodulation. They are emitting a large amount of their spectrum as longer near-infrared, mid-infrared, and far-infrared. The longer wavelengths of infrared will get absorbed mostly by the water in our skin and we biologically feel it as heat.
Even a high-powered red laser can induce thermal effects. This is why the industry started out as using Cold Lasers or "Low-level" laser therapy (LLLT). Higher powered lasers are used for cutting tissue, resurfacing skin, or other medical reasons for ablation or intentional burning. Prolonged usage on a single spot with a high powered laser will easily cause a burn on the tissue, no matter what wavelength it is. You can find many extremely satisfying videos of lasers burning holes through things on YouTube, using mostly Red, Green, or Blue lasers.
Typically in the red and near-infrared wavelengths of 660nm to 850nm we feel apparent skin warming at around 30-40mW/cm^2. At about 100mW/cm^2 the skin temperature may raise quickly. Above 300mW/cm^2 to 500mW/cm^2 prolonged for too much time (like, just a few seconds) would cause rapid heating.  
Some studies with higher powered lasers also use pulsing as a technique to control tissue temperature. This way the scientists are able to use a higher peak intensity with less thermal effects.  Similarly we often see the same pulsing technique used for wavelengths greater than 900nm, to avoid the thermal effect and still get a Photobiomodulation response.
3. LED Device Heating up
LEDs are generally considered more efficient and lower heat generation than their incandescent bulb predecessors. However they still create heat as a byproduct of electricity flowing through them. In fact, it is well known that only 20-30% of the LED power is emitted as light, the rest of the energy is generated as heat.
Using an infrared imaging unit we can see how the heat is being generated by each of the LEDs on this GembaRed Groove panel.
Most LED bulbs simply use natural convection to release heat. This means no fans or active cooling is required for a low powered LED. The LEDs are typically mounted on circuit boards or heat-sinks which help to absorb and dissipate the heat passively.
For example our latest model the GembaRed Beam. It uses lower-powered 1 Watt bulbs with a heat sink design. There are grooves and "fins" on the backside to help dissipate the heat naturally. The fins are a clever way to increase the surface area without taking up extra volume. More surface area increases the passive heat dissipation to the air.
As you increase the power output (higher mW/cm^2), then naturally heat management will play more of a role. Then designers have to move from passive cooling (heatsinks) to more active cooling with fans. So it is very common to find 3 Watt or higher LED bulbs in panels that integrate cooling fans to protect them from overheating.
Likewise if LEDs are wrapped tightly to the skin like in a pad or wrap, then all three components of wavelength, intensity, and device heating come into play. If a device is 50mW/cm^2 or above we must exercise caution with prolonged exposure due to heating when wrapped tightly to the skin. They recommend having the LEDs spaced at less than 0.8 Watts per cm^2 if the device is in contact with the skin. 
So when talking about heat and red light therapy, it is important to consider these three main heat sources of longer wavelengths, intensity, and devices.
The wavelength and intensity will affect the biological or tissue heating effects. So this is most important to manage relevant to the photobiomodulation science. If lasers and LED panels are used in contact to the skin, then it is important also to be mindful of the heat buildup and not allow the device or skin to overheat.
The device heating is important for the manufacturer to design-in appropriate passive or active cooling systems. Obviously as manufacturers increase intensity and the wattage of their bulbs, then they have to incorporate more cooling methods such as fans. So it is important to understand the design and application of each individual PBM product and the best way to balance intensity, heat, and wavelengths.
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