Phototherapy: a guide to the pitfalls of terminology

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Phototherapy: a guide to the pitfalls of terminology

Adrian Warburton, DermaLux Ltd,

Non-medical systems made for beauty and other peripheral uses can suffer from ‘specmanship’ that owes more to the marketing department than to the actuality of the device. This short guide is designed to help you make more sense of the advertising blurb.

Light is that part of electromagnetic radiation that the human eye can see. It lies between 400 and 700 nanometers. All the units for measuring and defining light are based on the Candela, which is the unit defining the luminous intensity from a small source in a particular direction. The Candela was originally based on the light emission from a flame (a tallow candle, hence the name).

The graph below shows the relative response of the eye. The scotopic curve is at night and the photopic is during the day. It is based on a constant power from the light source whatever the wavelength. If we consider the Photopic curve, which has its peak at 555nm (yellow green), the eye sees 510nm (blue green) and 610nm (orange red) as being half as bright as the 555nm reference. So, to achieve the same brightness as 555nm, the 510nm and 610nm would require twice as much actual power.


Similarly, if you had a 660nm source and a 630nm source of identical power, the 630nm would look much brighter than the 660nm. So the perceived brightness of an LED, from one wavelength to another, is not a good indication of its power output.

It is obvious from the above that luminosity-based data, while perfect for a lighting engineer and a marketing man seeking to make their phototherapy unit appear more powerful, are totally unsuitable for defining power for treatment purposes.

The most common source of light for home-use phototherapy units is the Light Emitting Diode (LED). An LED is a semiconductor device encapsulated in a plastic or epoxy module which, on activation by a current passing through it, produces photons (light). The material and structure of the semiconductor determines the wavelength (colour) produced. These devices are long-lasting, very efficient and produce little heat. LEDs come in all shapes and sizes and their output is specified in Candela as they are designed for illumination. The light output is divergent, varying from about 6o to 120o. As you will see later, the types used for phototherapy are generally the narrow angle types.

In passing, I should mention Infrared emitters (usually 800nm upwards) which can be packaged in a similar manner to the LED or in metal cans with windows. These have similar characteristics to LEDs but the output is invisible.

The LASER is a different animal altogether. It produces coherent light, which means that the light is produced in a regular stream rather than the random stream produced by ordinary light. The light comes from what is known as a point ‘source’ so it can be focused or, more usually, collimated. Collimation means producing a parallel beam that can be directed very accurately. Most phototherapy lasers are not collimated and if they are semiconductor versions, they are in fact divergent, although the divergence is accurately quantified. I am yet to discover what a so-called ‘Cold Laser’ is. I have seen units claiming to be ‘Cold Lasers’ which are a very bright light (similar to a projector bulb) passing through a coloured filter to produce the required wavelength and then being administered using a bundled fibre optic.

The only true measurement of power is the Watt - or, in the case of phototherapy, the milliwatt (mW). Phototherapy equipment manufacturers have to measure the power output of LEDs, but the measurement of power alone is meaningless unless we describe the area that the light is covering. For instance, a Light Emitting Diode (LED) shining on the skin will irradiate an area that varies with the distance that it is above the surface. (As mentioned above, all LED emitters are divergent.) The further away, the bigger the area, thus the area treated increases. The LED, however, is giving a constant power, so the power per unit area is decreasing with distance.

It follows that we must not only specify the power, but also the area being irradiated, and so we get to mw per square cm (mW/cm2). This is usually referred to as the Intensity. Ideally, a distance from the surface should be specified for the Intensity. This particularly applies to a single LED as the intensity at the tip when touching the skin is much higher than from 1cm away.

The other important factor in any treatment is dosage. You would not take pills without knowing how many or how often to take them. In light therapy, the accepted measure for dosage is the Joule, which in physics is the quantity of energy.

1 Joule is 1 Watt for 1 Second or 1mW for 1000 Seconds

It is obvious from the above that a Joule on its own does not tell the whole story as 1W for one second will have a different effect on tissue than 1mW for 1000 seconds. So, to get a realistic view of the dosage, it is no good just specifying Joules without specifying the Intensity (mW/cm2) or the length of time for 1 Joule.

The other factor that has an effect on dosage is modulation. Modulation in the digital era is turning the emitting device ON and OFF at various rates. If the light is modulated at 1 second on and 1 second off on a regular basis (1:1 mark space ratio), it is obvious that the Intensity is only on for half the time so the effective intensity is halved and therefore the dose is halved. That is only part of the story because it may be that the light is on for 1 second and off for 10 seconds (1:10 mark space ratio) so the effective intensity is one tenth and so is the dose. It is possible to drive a device harder during modulation as it has the OFF time to rest during each cycle. In either case the average or effective intensity should be quoted.

“But I would see that!” you all say. Yes, you would. But if it was switching on and off at more than 25 times a second (25Hz), you would not see it. Therapy systems can have modulation up to 100,000 times a second (100KHz), particularly laser systems. High-frequency modulation rates in tissue do not have much support, but there is considerable evidence that frequencies around 15Hz stimulate one of the body’s ‘messenger’ routes, the Calcium channel.

The interest in light therapy stems from the work of the late Professor Endre Mester, a Hungarian surgeon at Semmelweis Medical University, Budapest, in the early 70s. Whilst studying the effects of low-level Helium Neon Laser (638nm) light on mice, he observed a biostimulatory effect. Encouraged by the idea of immune system biostimulation via light stimuli, Mester and his co-workers studied the light effect on various biological systems.

Over the next 30 years, work with light therapy continued. It has been demonstrated that light therapy (mainly Red) stimulates fibroblast proliferation and collagen production by fibroblasts due to the induction of growth factor release from macrophages and other cells following light adsorption. Yes, it can improve the quality of your skin!

It is obvious from the above that its main thrust has been in the field of tissue repair. Other areas have been researched, particularly the treatment of Acne Vulgaris. In the 70s, some scientists trying to find a way of counting the number of ‘bugs’ on surface tissue discovered that the bugs fluoresced when irradiated with 420nm light. The reason for this reaction was the bug’s use of a porphyrin as part of its respiratory chain. The bug does not like too much oxygen and the fluorescing was the porphyrin producing oxygen, so the bugs died.

Various other wavelengths have this same effect on the porphyrin, but to greater and lesser degrees. 420nm is probably the most effective on the surface, but unfortunately the adsorption of light into the skin is not linear. The adsorption of light at 420 nm (close to UV) is much less than the longer wavelengths like the red end of the spectrum. This is easily demonstrated in life. The reason that we get sunburnt is that all the energy at the UV end of the spectrum is adsorbed in the first fractions of millimeters of the skin so all the energy dissipating in that very thin layer burns the skin. At the infrared end the light penetrates much further and gets adsorbed by the water and warms the tissue. To get the best of both worlds, the combination of red and blue seems logical.

So the argument is: if 660nm also kills the bug and penetrates much further, reaching the bugs trapped in the sebaceous ducts, which is the most effective? The red also has an anti-inflammatory property, calming down the infected area. I think that the argument as to the difference between 620nm – 680nm is invalid because the body is fairly easy to stimulate and, on the healing side, it is probably the same mechanisms that are stimulated by other electromedical products like ultrasound, galvanic currents or whatever is the flavour of the month.

A word about Rosacea

As far as I am aware, there have been no controlled clinical trials using red light with rosacea. However, several people have been using all-red versions of the Dermalux with some success and at least two are using wound-healing red LED clusters on the condition. There are various schools of thought on the causes of Rosacea, one of which is that it is the same bug (p. acnes) that causes Acne Vulgaris but working in a different way. If this is the case, then red light should have a beneficial effect because it can kill the bug. If it is not the case, then the anti-inflammatory properties of red light should ‘calm down’ the enlarged capillaries, whatever the original cause.