Published 11 November 2012

using the Superlux SD125F fitting

When a kitchen incandescent downlight fitting recently failed, it was time to investigate whether a CFL replacement could be equivalent in light output. The Superlux SD125F ceiling downlight fitting, which is complete with 15 watt spiral CFL was claimed to be equivalent to a 100 watt incandescent reflector bulb. Link to Superlux website: SD125F page


We were replacing an existing mains fitting in a gib ceiling. The old hole was 105mm diameter and I needed to increase that to 125mm, which was simple enough. You just have to take the precautions that the AC cable is un-powered and out of the way. Batts insulation also needs to be pushed aside for the task and a mask is useful to avoid inhaling a meal of gib dust. Ensure the hole has at least 20mm clear space all round, plus a height of at least 160mm above.Old downlight New downlight

Fitting the SD125F was straightforward although it took me a few minutes to pass the intelligence test of working out just how the retaining clips are installed. You also need three hands for a short time while you are reconnecting the wires. I guess it took me an hour at a gentlemanly pace; the next ones will be quicker.

How bright is the CFL
compared to the incandescent?

I had taken some readings of the incandescent with the lux meter before replacing the fitting. The 100 watt downlight produced 380 lux directly underneath at my measurement plane 1.71 metres below. It was a narrow beam; the light reducing to half that figure at a distance only 55cm away. By comparison the 15 watt CFL that came with the SD125F fitting produced (after warmup) only some 133 lux directly underneath but with a much wider half power circle of 105cm radius. So, although the intensity of the CFL is lower than the 100 watt downlight, the actual lumens contained in a half-power circle is higher for the CFL and fitting. The incandescent bulb results in 255 average lumens in a half power circle, while the CFL gives 326 lumens in its half power circle area. The diagram below illustrates this comparison:

lux comparison diagram

Of course, the light output doesnt stop at the edge of the half-illumination circle, but the circle will contain over 70% of the total lumens available. (This is slightly simplified since I assumed a exponential decay of light from the centre to the edge of the circle having measured only two points)

Trying some alternative lamps

Better aesthetics
with the floating glass accessory

I don't like the look of naked CFL's in downlight fittings. Superlux will supply a 'floating glass' accessory (for about NZ$25) which replaces the original flange and provides a partly frosted glass cover spaced off from the flange by about 30mm. This looks very good in place. However, it does reduce the light output by about 23%. The original 15W CFL and fitting without glass gave 133 lux underneath, but with the glass, that drops to 103 lux. The average lumens in a half-brightness circle also drops by the same amount. Offsetting that to some degree is that some light is reflected up to the ceiling from the glass cover resulting in a pleasing annulus, or halo around the fitting. This is seen in the photo below.

SD125F and glass cover

For and against

To the eye, the combination of SD125F fitting, 15W CFL and glass cover light up the space at least as well, if not better than the original laser-beam-like reflector downlight, although the intensity directly below is not as great. This makes them less suitable as task or reading lights. To achieve uniform area coverage, fittings should be spaced between 1.5 metres and 2 metres apart.

The SD125F is relatively cheap (about NZ$43 for fitting and lamp),and it is easy to replace an existing downlight. The fitting closes the room space off from the ceiling, and fibreglass ceiling insulation may be abutted.

One thing against all CFL's, is their lengthy warm-up time. Some can take 3 minutes to achieve full output, however the one supplied with the Superlux fitting appears reasonably quick.

I am pleasantly surprised by the effectiveness of this fitting and are likely to proceed to replace all the old incandescent downlights in the lounge/kitchen area.

Economics and payback time

If replacing 100 watt downlights, you will use 85 watts less power per fitting replaced. As an example, we have six fittings in the area. Replacing all these will mean reducing from 600 watts to 90 watts consumption. In winter we would have these lights on for about 6 hours each evening, but only say 3 hours in summer. Taking an average of 4.5 hours per night, we would save 510 x 4.5 = 2.295kWh per day. In a year that is 837.7kWh. Our rate here is 28 cents per kWh. So, we would save $234.55 of electricity every year. Given that six new downlight fittings plus glass covers will come to $408, we would pay the cost of the new fittings back in about the first 21 months, after which we would be saving money. If you needed to pay an electrician to instal them, then payback time would be longer; perhaps double the time. The cost of replacement lamps should be factored in, but if these lamps do in fact achieve the lifetime quoted, then replacements do not significantly affect the pay-back period.

Axino-Tech Consulting and Services
November 2012


Using the diffuser causes overheating
Frits Schouten

Those downlight are indeed very nice when used with the floating glass.
But, and this is a very big but, if a SD-DIF-SBF IPX5 (Diffuser For SD125F) is fitted then the 15 watt cfl will blow in about 30 minutes and deforming the reflector while dying from overheating. I fitted 22 of them in wet areas and on the various decks around the house. All 22 CFL's blew on the first day of installation. (All lights were turned on for testing purposes). They basically cooked themselfs because, with the defuser, there is absolutely no ventilation in the lamp holder. I contacted SuperLux about this and they basically told me that there is nothing wrong with the lightbulbs and they must have been installed wrongly. Well, all 22 of them? The 15watt CFL's are supplied with the SD125F by SuperLux. The only thing I was expecting from them was an acknowledgement of this issue and perhaps a down rating to 8 watt CFL's or so. But in their view, it was my fault that the lamps failed. I did fit 8 watt CFL's all 22 of the lights that have a diffuser but, because they are smaller, they don't line up too well with the reflector reducing the light output even more. They do last though...

That's my 5cts worth on the SD125F.

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