L70 is the amount of time it takes for a light to degrade to 70% of initial lumen output
CRI is Colour Rendering Index. It is a measurement of the quality of light. Whereas lumens is a measurement of the quantity of light, CRI is the measurement of quality. It is a scale between 0 and 100. 0 is bad, 100 is great. LED's typically are in the 70-95 range. The higher the CRI, the more expensive the LED chip, and therefore the more expensive the product itself.
Colour temperature for LED’s is measured on the Kelvin scale. LED’s come in a variety of colours, but the most common are in a range from 2700K to 7500K. The unfortunate thing is that the only consistent measurement is the Kelvin scale, many manufacturers like to use terms like Day White, Natural White, Warm White and Cool White, but there is not a standard that defines a Kelvin temperature to associate with those terms. The general rule is WARM (2700K) to COOL (6500K+). The warmer the light, the more yellow tones, the cooler the light, there are more blue tones.
- Here are the typical range of colour temperatures
- 2700-3200 - WW - Warm White
- 4000-4500 - NW - Natural White
- 5000-5500 - DW - Day White
- 6500-7500 - CW - Cool White
In the study of colour vision, a MacAdam ellipse is a region on a chromaticity diagram which contains all colours which are indistinguishable, to the average human eye, from the colour at the center of the ellipse. The contour of the ellipse therefore represents the just noticeable differences of chromaticity. So when an LED supplier proudly claims to offer you LEDs binned to a 4-step MacAdam ellipse tolerance (or 4xSDCM), keep in mind that this is better than LEDs that are binned to 5-steps but you will still see a colour difference over the range of LEDs supplied to that specification. Try to use 3xSDCM to avoid visible colour shift
It is the measurement of the fixtures light output angle until the output has fallen off to 50%. This means that you disregard the light out of that 50% and above zone for this measurement, which makes it helpful in knowing how much “usable” light the fixture puts out in a fairly even field.
Always buy based on lumens, watts is a number that is used to determine how much energy it takes to produce those lumens. It is important to pay attention to lumens per watt, or lm/w. The higher the number, the less energy you will need to produce the light. You will notice that some lights take less energy to produce the same amount of light, and this is something you should watch for. Typically, the higher the ratio, the better.