Today's Lesson

Colour Fringing & Shadowing

LED lighting can create some effects that you may not be used to. They aren't necessarily bad, just different, and you should be aware that depending on what LED products you are using (and how), they may be unavoidable.

So what are these effects?

I briefly touched on colour fringing or shadowing when discussing white light. The effect is essentially where you have a different colour light around the edge of your light beam. Now this can have a few possible causes.

From White LEDs

As already discussed, white LEDs are generally created as a blue LED covered with a yellow phosphor. Cheaper white LEDs may have some blue light sneaking around the edges of this phosphor. This fine blue ring can then be made worse by lensing. If buying LED fittings from Digilin, you can rest assured that not only do we use high quality LEDs, we test all our LED and lens combinations to minimise if not completely avoid colour fringing with white LEDs.

From RGB LEDs

Figure 1. While straight on both these LED strips are the same
colour, from this side on angle they appear quite different. This
is caused by which of the individual colours in the LED is closest
to the viewer.

Colour fringing and blotchiness from RGB LED fittings is caused by the fact that they are 3 separate light sources. Even in LEDs that contain the three primary colours in a single package, there is still a small gap between the individual LED dies. You can actually see the effect of this spacing if you were to look side on at a Harlequin RGB LED strip. This strip uses a full colour RGB LED. If you look straight down the strip from one end, then change to look from the other end you should see red being dominant colour from one end, and blue from the other. This is because at one end the red die is closest to you, whereas at the other the blue and green dies are closest. If you look at Figure 1, you can clearly see that the right hand strip appears to be much bluer and the one on the left much closer to white. This is despite them being connected and running the exact same colour (I promise, looking at them from the normal angle they are the same colour).

As shown in the earlier light lesson with the prism, the different frequencies of light bend slightly differently through glass and plastic mediums, such as lenses. This means a lens on an RGB light source may actually slightly separate your red, green and blue light. This is the reason why Digilin do not tend to use narrow beam angle lenses on our RGB LED fittings. This effect can be minimised by making sure that the surface that the light is shining on isn't too close, thus giving enough distance for the 3 colours of light to fully mix.

Shadowing

LED light fittings can create some interesting shadow effects due to the fact that normally, LED light fittings use multiple light sources (ie multiple LEDs) and thus can create multiple shadows. This is really only noticeable when the object casting the shadow is close to the light source. Further back and the light from adjacent sources can wash out the shadows.

To be honest, in white light, it's not so much of a problem, as it doesn't really create anything that the end viewers aren't used to (that's not to say you shouldn't be aware of the phenomenon). But with an RGB source, an object close to the light source could actually create 3 different coloured shadows (one without red, one without green and one without blue). This can look quite odd, but again, is simple enough to avoid.