Illuminate with LEDs
By: Tapio Rautonen
Tags: Lighting, Home Automation, Led

We’ve come a long way from incandescent light bulbs and some might even remember the dark years of “energy saving lights”, which were just fluorescent lamps in disguise. Slow to power up, flickering and bad performance. Luckily those days are behind and today everybody should be looking for LED (Light Emitting Diode) lamps.

LED is not a new invention, but only recent years have brought them to general lighting. Mass production and price reduction has made them an affordable choice for indoor lighting. Not to mention the energy savings they can create.

Energy savings and brightness per watt is the easy way to sell LED lamps, but that’s where the easy part ends. LED lamps have many attributes you should consider before buying the cheapest bulb from the local supermarket.

Brightness

Brightness is usually measured by luminous flux, which is the total quantity of human visible light emitted by the source. The SI unit for luminous flux is lumen (lm). Illuminance on the other hand measures how much luminous flux is spread over an area. The SI unit for illuminance is lux (lx) and it is equivalent to 1 lx = 1 lm / m².

All the physics aside. We are interested how much light we need. There are some guidelines how many luxes different surfaces require to work ideally for the purpose. At night when you just need enough light to walk around 1-5 lx is enough. General lighting can go from 20 lx to even 150 lx. Reading, studying and cleaning should be provided from 150 lx to 500 lx. There is quite a lot of variation in the numbers and these are only rough guidelines.

So what do the numbers mean when selecting the lamps. Let’s take an example bedroom with an area of 12 m². The bedroom is used for sleeping, playing and some studying so we should be able provide 250 lx for the whole room. We can calculate the total required luminous flux from the formula x lm = 12 m² * 250 lx. Where x equals 3000 lumens.

Illuminance can be increased by directing a narrow light beam to the desired surface, for example a working desk. Making a proper lighting design should be done with a software that helps to calculate the required light sources, take account of the room heights, areas and the illuminance requirements.

A decent LED can provide from 80 lumens per watt to even 200 lumens per watt. If we take that example bedroom and use a LED light source providing 100 lumens per watt, we need roughly 30 watts of light power to get the brightness we want.

Generally the more lumens per watt a LED lamp can provide the higher quality it is. But remember that brightness is not the only feature to look for.

Color temperature

Light color temperature is measured on Kelvin scale. Incandescent lamps provide the light from thermal radiation and the color temperature for a warm incandescent light bulb is close to 2700K. Other types of light sources generate the light differently and therefore they are capable of creating different color temperatures.

Before LED lighting the typical incandescent lighting generated quite yellowish light and there was not much options for indoor lighting. That was the normal. On the other hand the typical daylight color temperature is around 5500–6000K and on a cloudy day it could go even beyond 6500K. So if you are not into the yellow glow, you can get more clean light from LED lamps.

But beware, everything looks very very different when lighted with 2700K or 5500K light sources. When you are using a light source with color temperature close to daylight the yellowness starts to disappear and everything just gets bright, and clean. Higher color temperature also increases the contrast of colors. Some say this effect reminds of hospitals where everything is white and clean. Daylight might make for example plain wood look pretty weird. It loses the warm and soft feeling and becomes cold and hard looking.

The color temperature is mostly matter of taste and of course affected by the indoor material choices. For our modern house built of concrete and hard surfaces where wood is pretty much inexistent the only real option was to go all in daylight. All main rooms are lighted with four 19x19cm flush mounted LED panels capable of providing 1300 lumens at 5500K. That’s 5200 lumens per room. I can say that we don’t need any daylight floor lamps to brighten the dark autumns of Finland. Just put the ceiling light on full power and get enlightened.

Color rendering index (CRI)

Other attribute that affects how different materials look when lighted is the color rendering index (CRI). It measures how well the colors of various objects are reproduced compared to an ideal light source.

The color rendering index is a scale from 0 to 100 percent indicating how true colors the light source is capable of rendering. Incandescent lights have CRI of 100 by definition, but there’s a lot of variance in LED lamps. CRI over 90 is considered excellent.

Looking an object that is lighted with a light source that has low CRI makes it look different for the human eye than a light source that has CRI close to 100. To make colors look more “real”, aim for higher CRI LED lamps.

Typically low CRI, e.g. 70, is not a problem in places where it doesn’t matter how the colors look. But for a retail store or an art gallery it is important that the objects in there look what they are supposed to look.

Color rendering index can be used as a quality meter too. Usually higher quality LED lamps have better CRI, but that also means higher price. I’d suggest always trying to look for lamps with CRI 90 or more, but sometimes you just don’t have an option.

Casing and form factor

LED lamps come in different forms. There are flush and recess mounted lamps. Panels, strips, spot lights and many other types. You can retrofit light bulbs and even fluorescent tubes with LED alternatives.

Generally LEDs can be fitted in smaller space than incandescent lamps so there are more possibilities for the form factor. LED strips can be only few millimeters thick and flush mounted panels can fit into few centimeters of free height.

Even though LEDs don’t use as much energy as incandescent lamps, they can get hot, really hot. So the cooling must be taken into account. You can fit a flush mounted panel into a really tight space but if there’s not much airspace or the air is not moving, the lamp can get really hot and greatly reduce the operating lifetime of the LEDs or even cause a fire.

Some manufacturers specify that the warranty is void if you don’t mount the light on a heat dissipating surface like extruded aluminium profile. Some LED lamps have built in heat sinks to cool down the LEDs. The cooler the LEDs can run, the longer the operating lifetime is expected.

Powering the smart LEDs

This post was more about general attributes of LED lamps and what to consider when designing LED lighting. I will dig down deeper in future posts how different LED lamps are powered and how all this relates to home automation by making the lamps “smart”.

The sad thing at least in Finland is that even most LED lamp resellers are pretty clueless about the basic stuff about LED lamps. The knowledge is limited to color temperature and that’s it. They sell “some package that provide light when plugged in”.

I will explain in later posts why we need to know more about the lamps when creating automation, lighting scenes, dimming and so on. Stay tuned and follow me to get a deep dive into lighting and home automation.