Light variability from batch to batch with LED lamps ‐ a case study. At NZMS we like to keep abreast of modern technologies and how to best use them in the digitisation sphere. LEDs in particular have been a product we've been keen to try for digitisation of heritage documents ‐ not only for their lower power demands and lack of heat, but also due to the absolute lack of ultra violet light meaning any potential impact of bright light on delicate materials is minimised even further. We've been trialling LED lights in our Southern Regional office for some months now with a degree of success for documents where colour accuracy wasn't necessarily as paramount as some work. They've been working well with our Canon DSLR rig. However, we had a failure in one of our light banks and received a replacement lamp under warranty. To our surprise when we set the new light bank up we noticed it was a subtly different colour to the other bank ‐ it seemed as if it was bluer. This led us to thinking ‐ isn't white light white? How can a light that's sold as daylight white (5600K) actually be different from batch to batch? First, it's important to understand how LEDs work, and why they're different. Traditionally, lighting has used incandescent light bulbs, that output light in a very even way ‐ and very closely matched daylight. But incandescent lights are being used less often now ‐ not only because they're expensive, but they also don't last very long, they output lots of ultraviolet light which is bad for delicate materials, and they also put out quite a lot of heat. LED and fluorescent light is very different to incandescent light bulbs. An incandescent light bulb is a simple device with a tungsten filament that consistently produces the same type of light with the same characteristics ‐ until it dies. However, LED and fluorescent lights are far more complicated. In an incandescent bulbs the piece that makes the light produces a light that is made up of all the colours in the spectrum. In contrast, in an LED or fluorescent light that piece that creates the light produces light of a single wavelength ‐ in other words, a single colour. If you remember science experiments at school, shining light at a prism to split up the light you'll recall that white light is a mixture of colours. So, if an LED or fluorescent light only outputs light in one colour how come it looks white? The answer is "it isn't." The original light is actually a high‐intensity blue. In order to make it look white, they add a phosphor to the coating on the light. A phosphor is an element which emits light. In this instance, it emits a light of a different colour when you shine light on it. The blue light excites the phosphor, causing it to glow yellow. And the mixture of the blue and yellow results in white. By varying the mixture of blue and yellow, as well as using phosphors of varying shades, the resulting colour can be controlled. Because the light is made up of basically just two colours, the spectrum isn't complete. Even though the overall light colour may be warm, there are some colours that aren't well represented, so some colours can look dull compared with other sources of light. Light variability from batch to batch with LED lamps April 2014 If you look at the figure following you will see that if you compare the spectral profiles of daylight compared to a High CRI fluorescent lamp, daylight is comprised of an even spread of colours from Ultra violet through to Infra red. You can see this effect when you use a prism to split white light into the different wavelengths, which we see as colours. Notice that the colours close to the Ultra violet and Infra red aren't as defined in fluorescent light, and that there is a "spike" in colour in the blue and green. This is caused by the original colours coming through ‐ the original blue, and the combination of the blue and the yellow phosphors causing a green ‐ the "fluorescence" effect gives you all the other colours. The end result is that colours are not as contrasty and strong when lit by fluorescent light as when they are lit by daylight ‐ and we usually refer to the digitised images as being slightly "flat" or dull. The effect is even stronger when you look at the LED lights. Because the original light is so narrowly focussed on one particular wavelength, the fluorescent effect is less pronounced. This means that the light still "looks white" to our eyes, but to a camera device it really only sees a strong light blue and a yellow, with a small amount of greens and reds. This is more clearly shown in the figure below. To further complicate things, as the blue light is concentrated in a particular wavelength, the effect of the phosphors is very strong ‐ a small amount of difference can create a quite different looking light colour to our eye (and the camera's sensor). Light variability from batch to batch with LED lamps April 2014 The coating process that manufacturers of LEDs use creates significant inherent variations that have an impact on the lumens (how strong the light is), the colour temperature (how far towards the blue or red part of the spectrum the light tends to appear), and the voltage of the LEDs. Huge amounts of dollars have been spent to minimise this variation in production but we are not yet capable of producing highly consistent and tightly controlled production of LEDs. So manufacturers use a production process called binning. In an effort to maximise their yields and recognising that the lighting industry has a wide range of needs, they've introduced a method of sorting whereby LEDs are categorised by lumen, colour temperature and voltage. Each bin allows a tolerance or variation amount, and the more tighter the tolerance is, the higher the price, as these LEDs are made in smaller quantities (and you have to throw a lot of them away). And this small amount of tolerance in the binning is what's causing this issue ‐ because over a large LED panel with, say 500 LEDs, that variance can make a real difference. Enough to make one light bank not look like another light bank. This isn't so much an issue when you have two light banks of the same batch in use, as a white balance calibration can take into account the difference from batch to batch, but if you have two light banks that are subtly different, it can cause one side of the image to be a slightly different colour to the other. This is why, if one of your LED light banks fail, you'll need to replace both of the banks. You need to weigh the benefit of the more expensive fluorescent lighting banks and the more consistent lighting they produce against the cheaper LEDs. But remember to include in your analysis the cost of replacing both lighting banks in the event of a single failure. Light variability from batch to batch with LED lamps April 2014