LED colour quality, CRI and binning.
One main criticism of LEDs used for room and space lighting is that the colour or hue of the light appears cold and harsh. Some LEDs appear as tiny, brilliant lights that are too bright to look at, but seem to give very little illumination. There is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source called Colour rendering index, or CRI. We are used to a much warmer light from conventional incandescent lights because filament bulbs have a broader spectrum of light that is biased towards the warm red frequency.
White light is made up of all colours, which is why we see colours of objects evenly when lit by a balanced white light. If we reduce the output of any colour in a white light source, we reduce the reflective strength of that colour in objects. For example if we look at a ripe tomato under a blue light, we don’t see its red colour, because there is no red light available to reflect off it and onto our eyes. The ability to render colours fully and accurately is one major drawback with LED lighting that has not been desiged correctly.
The warmer the Light, the lower the lumens/watt
Unfortunately, the warmer light output of phosphor based LEDs is considerably lower in lumens than the uncorrected versions and this is an area of development still requiring more research. This means Warm white LEDs produce lower lumens (light brightness) than daylight or natural white LEDs
Mixing colours to make the right white
Binning and labelling
For the DIY LED enthusiast, there is another definitive guide that helps greatly in deciding the colour quality of the LED. Quality manufacturers such as SMD, Cree, Seol, Philips and Nichia provide detailed specification data sheet for their LED emitters. Along with technical details of forward voltage and current requirements, these manufacturers will also show a codes relating to a particular LED range and its colour output based on the CRI or colour rendering index, as discussed earlier. Put simply, this labelling follows a prescribed measurement of what sort of white the output is, taking into account, the dominant wavelength and scope of the total output. This means you can buy a LED emitter and choose a binning code that gives you cold white, daylight white, natural white, warm white, or some other description. What is important for the user is to understand the actual colour temperature in Kelvin according to the table below. When a white LED is described in this way what it means is that it will have a dominant tint or hue to the light ranging from red to the blue end of the spectrum. This gives the subjective feeling of warmth or coolness.
Putting it together