Shedding light on the technical jargon for the best sun pipe
When looking for the best sun pipe, you’ve probably been scouring the web looking at solar tube systems and will have encountered a wealth of technical jargon that you are expected to understand, such as:
- Lux levels
- Reflectivity and specular reflectivity
- Angle of incident
- Non-specular reflectivity
- Tube efficiency
- U Values for sun pipes
We will attempt to explain what these terms mean and point out misleading information so that you will be able to make an informed decision about your purchase and get the best sun pipe for the job.
Lux is a SI unit of illuminance and luminous emittance. Lux can be easily measured with a hand-held meter but the measurement can easily be manipulated to gain an advantage by less scrupulous manufacturers. The variation of the measurement can be massively altered by moving the light sensor slightly closer or further away from the source to affect the reading.
As with all of these things, unless you’re reading an independent report, treat any figures you read with a large pinch of salt.
Lumen is the SI derived unit of luminous flux. Luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light. Roughly speaking a Lumen is equivalent to a single candle.
Our advice is that these figures are totally unreliable and should be ignored. As with Lux figures, unless you are looking at results from an independent test comparing two units of the same configuration located side-by-side then it’s meaningless as a comparison.
Efficiency, or Global Efficiency (EG), is probably the only true figure that will allow you to be able to make a realistic comparison from one system to the next in terms of the amount of light you will get – and why else would you want a light pipe?
As with any testing, anything published that is not from an independent source should be view with some suspicion. In the UK there is very little independent information available and so you have to ask yourself why. If every manufacturer had to publish an EG figure then you, the consumer, would be able to see at a glance which sun tube would be the brightest for you, in the same way that motor manufactures have to publish MPG information or brewers the alcohol levels on beer.
Like in most things in life, if companies have good news to tell you then they’ll publish it, if they want to keep you in the dark then they’ll hide behind smart marketing and glossy images. I once saw a manufacturer quoting an efficiency figure of 95% for their 350mm solar tube. To our knowledge, and we’re happy to be corrected, the most efficient system of this size is Solarspot at 63%. What they were doing is quoting the reflective value (efficiency) of their tube material which is something completely different – and actually not very good (see below). An exercise in how to make a third-rate product sound like a world-beater… sneaky!
Reflectivity shown as a percentage is the amount of light that reflects back after the light hits it. The most reflective material in the world is 99.7% and is a film used by both Solarspot and Solatube in their system. Of this reflectivity, 100% is what’s called Specular Reflectivity so all of the 99.7% reflected light is captured for the next bounce on its angle of incidence. Most other rigid sun pipe systems use tubing that is between 95-98% reflective (efficient). The main problem with these materials in that a small percentage of the total reflectivity is non-specular so it scatters as it bounces and so, depending on the angle of incidence, up to 5% of the light is lost on every bounce – a massive amount when you compare that to the 0.3% lost by the more reflective pipe.
The diagram below will hopefully explains this a little better
Angle of incidence is the angle at which light bounce off a surface. If the light hits a surface which is smooth at an angle of 40 degrees then it will bounce off the surface at 40 degrees. Systems like the Solarspot use dome lenses to increase the angle of incidence so that the number of light bounces are reduced to increase the efficiency.
With non-specular reflective, every time the light bounces not all of the light carries on traveling at the angle of incidence, an additional percentage is lost as it scatters as it hits the reflective surface. Take a 98% reflective tube as an example; each time the light bounces around 95% will carry on traveling at the angle of incidence whilst 3% will scatter. Some of that scattered light will also carry on down the tube but some is lost back up to the dome.
Tube efficiency is the reflective value of the tube. Most rigid tube systems have a figure of between 99.7% and 95%. But beware, that’s not the whole story. Just because one manufacture has a more reflective tube material doesn’t mean that they will give you more daylight. Solarspot and Solatube both use a 99.7% reflective tube but the solarspot is considerably brighter, by around 70% on a similar sized unit.
Likewise, if you were to compare the Solatube 290 DS (99.7%) with the SunPipe 450mm (98% reflective) you might assume that the smaller solatube would be brighter because of its reflectivity but you’d be wrong. Actually the larger unit is around 50% brighter despite being less reflective.
U Vales are a measurement of thermal value – basically heat loss or heat gain. I’ve seen all sorts of figures quoted and it’s enough to make your brain hurt. Some years ago I was fortunate enough to meet a boffin from the BRE/BBA who was responsible for thermal testing. He explained to me that most of what is quoted online is at best inaccurate with figures varying wildly from manufacturer to manufacturer for systems with the same diameter. When in fact they should technically all be fairly similar.
If you have a number of systems, all roughly the same diameter, all with the same number of lenses (dome, internal lens and diffuser), all with the same thickness of aluminium tubing and all sealed, then you will get roughly the same thermal value. Improving the thermal value to make it potentially more interesting for energy conscious customers is quite easy; simply wrap the tube in layers of insulation and introduce extra lenses into the pipe to create thermal breaks.
Doing this can significantly improve the thermal value, but what is not said is how this affects the light levels. It’s technically possible to reduce the U Value from around 2.0 down to 0.5 but the light will be reduced to such a level that you will need to fit two or three more solar tubes to bring it back up to the desired levels. So you end up with 4 extremely expensive light pipes giving you the same light levels with the same overall thermal values. You’d have been better off sticking with one unit and saving your money. Buyer beware.