The amount of visible light that the human eye can see in all directions is known as luminous flux. Put simply, this quantifies the overall perceived brightness of a product. It is measured in lumens (lm), a unit which is particularly useful for products such as work lamps, where the overall light output of the lamp is important. The higher the lumen rating the brighter the light will be, and the lower the lumen rating the dimmer the light will be.
It is worth noting that lumens can be written using raw lumens or effective lumens, and the values of each can differ by a huge amount. The difference is that raw lumens are calculated by multiplying the lumen rating per LED by the number of LEDs in the lamp. This provides a theoretical rating, but doesn’t take factors such as product housing into consideration. As a result, the reading is only true for the LEDs themselves, and not an assembled lamp.
Effective lumens are the most accurate reflection of the luminous flux. The rating takes into consideration any energy lost from heat, product housing and the stabilisation of light, for example.
In some cases, it’s beneficial to understand the light output of a product at a specific angle. This is where luminous intensity is useful, as it describes how much light is emitted from a light source in a particular direction (as opposed to all directions, which is luminous flux).
Luminous intensity is measured using candelas (cd), and it is a particularly important measurement for testing road lighting for legal requirements. The candela rating will be the same at all distances from the light source, at a specific point or angle. Did you know one candela is approximately equivalent to the light emitted by one regular candle. This was the original method of measuring light intensity.
The amount of visible light (luminous flux) on a particular surface area, is described as illuminance, and is measured using lux. This is helpful in determining the intensity of light in a particular area. One lux is equivalent to one lumen per square meter of a surface area. Unlike candelas, the lux value changes depending on the distance of the light source from a surface. This makes it useful for ensuring work areas are sufficiently illuminated, or for interior lighting designs where a consistent light level is desired.
You may also see footcandles used instead of lux. This is simply an imperial measurement, in which illuminance has been measured by square feet instead of square meters, according to this ratio: 1 footcandle = 10.764 lux.
Each type of measurement is important as they all give us different insights into how useful a lamp will be for a particular purpose. Whilst lumens can provide a general overview of the brightness of a product, candelas and lux should also be considered where relevant to ensure the light spread suits a particular application.