We are often asked to explain the difference between the light irradiated by a laser and the light irradiated by an IPL. It is well known that the one is a single wavelength and the other is a range of wavelengths, but what is the real life implications of this?

The short answer is that you can treat more selectively with a laser than with an IPL. However, you do not always want to be that selective. It really depends on the type of treatment. If we use hair removal as example, you want to be as selective as possible. It is important to have as much as possible of the energy absorbed by the melanin in the hair and as little as possible by other chromophores.
But if you want to do skin rejuvenation you want to target a range of chromophores to stimulate the appearance of a youthful looking skin. This you can best do with an IPL.

There are three mayor differences in characteristics between laser and IPL light. The first is that laser light is coherent, whereas IPL light is incoherent. This means that the laser light waves do not interfere with each other. When similar light waves meet with ups and ups and downs and downs, They merge forming a light wave with a bigger amplitude(bigger ups and bigger downs). This is called constructive interference. When similar waves meet with ups and downs and downs and ups, they cancel each other. Thus, you have very little loss of energy with lasers and it remains stable.

Secondly; because laser light is one wavelength, the light is one colour or monochromatic. Whereas the light from an IPL is polychromatic, or a range of colours that is observed as white light. It is this characteristic that determines which chromophores will react on the light and why you can work more selectively with lasers.

Thirdly; laser light is collimated. It can thus travel over a long distance while it stays in a narrow beam. This is the characteristic that makes the transfer of data over fiber optics possible. This is also clearly visible when you use a laser pointer. Because the light from an IPL is divergent it is necessary to press the handpiece directly on the skin, else the lost of energy is very high and only a small portion of the light produced will reach the skin.