Getting enough light on the ground without bothering the neighbours is a great start to a good game of evening football. When evaluating what will, or won’t, work there is a number of parameters that will affect the results. Lumens and Lux are 2 of the most common to be looked into so let’s explain a bit more about both terms and then the answer becomes more clear.
Lumens is a measurement of the volume of light being emitted by the lamp. This is controlled by how the manufacturer of the LED chip (Cree, Lumiled etc.) has designed it and the value will vary depending on how much current is put through it. For instance, a 5W chip operating at 5W may produce 90 lumens/watt. That is, for every watt of power put in, it will generate 90 lumens out. However, if you run the same LED at 3W you will get a big increase in the efficiency and generate perhaps 120 lumens/watt. The lamp manufacturer therefore specifies a gross lumens value in two ways – raw lumens and effective lumens. The raw lumens is what the LED manufacturer has put as the value at a given temperature and wattage. This does not take into account the lamp as a system and doesn’t reflect any calculation for losses from optics, temperature etc. The effective lumens is what is calculated once the lamp has been built into a system. This is tested over a period of hours and so allows the lamp time to heat up and is testing the actual light output rather than the theoretical light produced in a laboratory by the LED itself. If the lighting manufacturer doesn’t specify if the values stated are raw or effective, you can safely assume they are raw. Losses from raw to effective may be as much as 30% and is specific to the system i.e. cannot be estimated without knowing more details about the lamp as a whole.
Lux, on the other hand is a practical, field measurement of real light on the ground or at a specified height. This test can be done physically or simulated using software and the IES (digital light pattern) file. Once the lamps have been set up on their poles (or simulated) and the field is illuminated, then you can measure the actual light on the ground. This takes into account the same losses as the effective lumens test but also shows how the light is controlled. This is very important for big-field sports like football, cricket, baseball and rugby. Having a huge mass of bright light that only shines 50m isn’t very helpful on a rugby field. Additionally, the mass of light tends to have a lot of stray light which can tend to annoy spectators and neighbours.
So, whilst you have to have the right lumen values being emitted from the lamp, the way it is controlled is of utmost importance. Just because a light has high power and lots of lumens doesn’t mean the light will go where you want it to go. A proper lighting plan is therefore essential. If you are not sure what lighting levels you need there are a number of sources of this info – a handy quick reference being is the Legacy guide but there are numerous more detailed sources available. Once you know what the standard is, check out your lighting levels with a lux meter. Again there are some helpful app downloads to give you an idea of where your illumination is up to. If this is sufficient, use it as the base for your design. If it isn’t sufficient, work with the lighting guidelines and the designer to come up with a system that works.
It’s a good idea to get a couple of lighting companies so you can compare the lighting plans from different manufacturers. Once done you should be well on the way to making a better, more informed decision, achieving efficiency and low maintenance for many years to come.