REVIEWED Harvard Engineering RetroSON6 gear tray

We see quite a few streetlight heads here in the lab. The expansion of LED technology and the need for councils to meet their carbon reduction targets has meant that we see a profusion of different luminaire designs as lighting companies get to grips with the challenge of creating the next generation of streetlights.

We’ve seen all sorts here, from a LED-based Windsor street lantern so big and heavy it took an entire bag of a hundred cable ties to keep it on the gonio, to a brand new design that had special modifications to stop pigeons sitting on it. Thankfully they tested the pigeon proofing aspect of the housing after it had been sent to our lab for the photometric tests.

Not all customers are so considerate. I was once asked to conduct gonioradiometric measurements on a set of flytraps (the client wanted to know the UV output as a function of angle). They sent them to me used and full of dead flies. Not fun.

Most the new lighting schemes installed by councils are planned on the assumption of one-for-one replacement, with LED-based streetlight heads replacing high and low-pressure sodium heads. The reasons are fairly obvious: councils want save money by not replacing lighting columns as well as the luminaires. So for existing lighting schemes it’s best to offer a retrofit option.

Harvard has managed to keep the colour temperature to a not-too-cool 3851K

Fit and forget
Which brings me to this month’s product, Harvard Engineering’s RetroSON6 gear trays. This is a fit-and-forget solution and can be clipped into a lantern in place of a traditional high-pressure sodium (SON) lamp. The advantages of this are obvious: it lets councils use the existing stock of lanterns and lighting columns and doesn’t alter the look of the street.

The cost considerations are crucial here because councils justify the cost of installing the new schemes by offsetting the initial capital outlay against the expected savings of the efficient replacement luminaires. If the lanterns stay the same, the initial cost outlay is less and the risk for the initial investment is lower. This has a clear plus in the payback period, with Harvard expecting significant improvements in this metric. I can vouch for this because, for example, the power consumption of the 2,600 lm package is 19W, only 27 per cent of the rating of the 70W SON it’s designed to replace.

Great selling point
The ease of replacement of the gear tray makes for a great selling point too. I’ve swapped around these gear trays in the lab so I can attest to the ease of removal, it’s just a matter of a twist of an Allen key and a couple of screws. I’ve more than once skinned my knuckles while trying to swap lamps in a luminaire so the speed and simplicity of this system is welcome.

Recently we tested three of the RetroSON gear trays, all with different lumen packages. This month, we’ll look at the 2,600 lm option, designed to replace 70W SON lamps.

The spectral distribution of the light source - with the blue peak that's characteristifc of LED, but plenty of warm tones too

The table shows the results of our measurements. The first thing that struck me was how most of the measured values exceeded the manufacturer’s expectations. The luminous flux is slightly above Harvard’s stated value, leading to a slight improvement in the efficacy. Harvard was also expecting the CRI to be 70 per cent, whereas in fact it’s reached 75 per cent. Sometimes I get into discussions with manufacturers because the results are below expectations, so it’s nice to report for once that a manufacturer’s estimates of performance line up very well with reality. They’ve also managed to tune the trade-off between CCT and S/P ratio. In this case the CCT stays well below 4500K – a tolerable, not-too-blue light – while keeping the ratio high enough to pay dividends. Of course, given Harvard’s experience with driver design, the power factor is excellent.

Finally, the manufacturers says the product will operate at 90 per cent of peak light output at a temperature of 65°C for at least 50,000 hours. It pleases me that Harvard has stated a life for the luminaire at an estimated operating temperature. Too many manufacturers arrive at their lifetime estimates by simply taking the life of the LED from the datasheet, which is based on 25°C data.

  • Total luminous flux 2,687
  • Input power 19.23
  • Power factor 0.949
  • Efficacy 139.7
  • CCT 3851
  • CRI 75.31
  • S/P ratio 1.5
  • Harvard has done a great job on thiseasy-to-install, high-performance retrofit product


  • 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