In order to understand why fluorescent lights seem to cause discomfort for the majority of people with Irlen Syndrome, it is necessary to understand about the visible spectrum of light. The human eye can only perceive the colours that we see in the rainbow compared to some insects who can perceive ultraviolet light and/or infrared light. The different colours in the visible spectrum have different amounts of energy as they travel through space from the sun or through the air from artificial lights. Fluorescent lights do not have the same levels of each coloured light as normal sunlight (see the photos). They have peaks of blue, green and yellow light compared to sunlight where the colours are fairly evenly distributed. Some people with Irlen do not like LED lights and find them too bright; the photo shows that there is a lot of blue light in LEDs. Incandescent lights do not have much blue light and lots of light in the red end of the spectrum. Blue light has high energy compared to red light which has lower energy. It is this difference that makes it preferable for people with Irlen to avoid fluorescent lights and to find incandescent lights as the best after good natural light. There are issues for people with Irlen on cloudy days, because the clouds seem to lessen all colours other than blue and so these days are actually more glarey for them than a nice sunny day where the whole spectrum is shining on them. Note that the "energy saving" bulbs (CFS) on the photo, are actually fluorescent lights.