In my job as the technical director of a photometric test lab, it’s not very often that I come face to face with lighting designers in the course of a working day. Generally, these guys are a couple of steps removed from me, as the reports and data our lab creates get passed from my customers on to the designers and they do their thing with the software.
Simulations and reality
At this point I’d like to get a couple of minor niggles about lighting software off my chest. Firstly, very occasionally lighting software plots differ from what I’ve actually measured in my lab. In this case, it’s unlikely to be us that’s got it wrong. A software simulation is just that, a simulation. It’s not a precise rendering of reality. If I’ve measured 500 lux at five metres and your software reports that you should be getting 510 lux, blame the software. This may sound arrogant but all of our equipment is regularly calibrated and we have UKAS traceable procedures for what we do. In other words, trust us, we know what we’re doing!
Second niggle - can people please try and upgrade to the latest versions of the software? I occasionally get customer complaints because when they upload the files we’ve sent them, they get odd error messages coming up.
This is generally because they’re using obsolete versions of Relux and Dialux so they can’t handle the newer versions of IES and LDT files. (This is a subject I may return to in more depth at another time, as it’s worth looking at in detail.) So, if you’re using these programs and you haven’t updated them since before 2010, please do so. It’ll make your job (and mine) a lot easier.
Having said that, I do run into lighting designers from time to time and we get on very well. We might not fully understand what each other does, (because they’re basically architects and I’m an optical physicist) but we both understand the job each other has to do.
So I was intrigued when the Light Corporation contacted me about doing some photometric tests for them. For those of you not familiar with them, they are a lighting design firm that has crossed over into luminaire design, which in my experience is quite a leap. A bit like a road planner actually learning how to design a car.
A downlight, done well
This month I’ll be looking at their adjustable 26-degree LED downlight. In itself this is not an especially innovative product (we’ve all seen downlights before!) but it’s rare to see it done so well. They’ve made a good start by incorporating a Cree LED chip, one to the most reliable on the market and combining it with an excellent optic.
An example: they specified this as a downlight with a beam angle of 26 degrees. Our lab measured a beam angle of 25.8 degrees, as close to a bull’s-eye as makes no odds. It’s good to see such close agreement between the specification and reality. The housing is made from the highest grade of aluminium, which ensures good heat sinking performance and an attractive anodised finish. This gives us an excellent efficacy of 57lm/W. Not the highest I’ve seen, but excellent for a downlight of its size and the associated difficulties with heatsinking.
Take a look at the table below and you can see that all the numbers we usually look for stack pretty well. In addition to what I’ve mentioned above, the colour tolerance is within spec and the power factor is excellent, a fine example of good electrical engineering. I’ve also introduced the R10-R13 values for this product, because they are something this product does very well.
Basically, while CRI (otherwise known as Ra) describes the general colour rendering index of the product, R10-R13 describes the rendering of colours not covered by this metric. Historically, CRI was created to describe the colour rendering of shades best illuminated by fluorescent light sources.
These shades are known as R1 through to R8. R9 and above describe the colour rendering of strongly saturated colours and skin tones. This is something that fluorescent sources don’t do especially well but that LEDs excel in, particularly in the case of this luminaire.
- Total luminous flux 654.8lm
- Input power 11.6W
- Power factor 0.976
- Efficacy 57lm/W
- Beam angle 25.8°
- CCT 2658K
- Colour tolerance 2-step MacAdam ellipse
- CRI 80.5
- R10 (saturated yellow) 72.0
- R11 (saturated green) 76.6
- R12 (saturated blue) 66.4
- R13 (Caucasian skin tone) 79.5
- Dr Gareth John is technical director of Photometric Testing, an independent lighting test laboratory that specialises in the photometric assessment of LEDs, luminaires, lamps and displays