How much light are you getting from your LED? The brightness of any object can be a really subjective concept: is that light as bright to me as it is to you? In electronics, we don’t like to make part purchases based on subjectivity; we want empirical values; we want objectivity.
Light source brightness is measured in the SI “candela” unit, which means quite literally, “the brightness of a single candle”. It is similar to lumens. Whereas lumens measure the total brightness emitted by a light source in all directions, candelas measure the brightness by taking into account the steradian at which the light is emitted. A light source that emits 1 candela at a solid angle of 1 steradian is equal to one lumen. A light source that emits 1 candela in ALL directions, a sphere of 4π steradians, is equal to ~12.57 lumens.
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian.
LEDs being very small, and emitting light at very tight angles, have their brightness measured in thousandths of a candela – the millicandela. Lumens, solid angles and steradians aside, the value can be used to gauge the relative brightness of two LEDs against each other. An LED with a rating of 250mcd will be 10x more intense than one with a rating of 25mcd (given the same angle). Taken into account with the LED forward voltage and operating current, you can gather how bright the LED will be for the cost in energy consumed.
Here are some example components…