Swing Arm Desk Lamp Reflector Upgrade

The reflector of a swing arm lamp, which is usually painted white, will turn yellow and darken over time due to the paint deteriorating. This will reduce the light emitted by the lamp. By attaching a highly reflective surface to the lamps reflector the light output of the lamp can be increased.

A swing arm desk lamp

Ruler

Fine tip felt marking pen

Heavy duty Aluminum foil

Scissors

Spay on adhesive

I installed a 9 watt (60 watt incandescent equivalent) LED bulb in my lamp a while ago to conserve energy. Recently, I needed more light on a work surface and was going to install a higher wattage bulb when I noticed the reflector was not white any longer. The picture of the reflector’s surface shows the yellowing and darkening.

Before using a higher wattage bulb I tried an experiment to determine if placing a layer aluminum foil on the inside surface of the reflector would increase the light output enough to avoid using a higher wattage bulb.

First, I setup a test to measure the light from the lamp, with the reflector as is, a known distance from the surface. The picture shows the test setup. The reflector is 24 inches from the surface and parallel to the surface. A Foot Candle, Fc, meter is positioned on the surface. The lamp is the only source of light during the measurement and the light meter is moved through the illuminated area to find the maximum light level. Additionally, the bulb is ON for 5 minutes before recording the measurement. This gives the bulb enough time to reach a stable operating temperature and a stable light output. The LED bulb’s circuit temperature will increase by some amount and the light output of a LED bulb decreases with increasing temperature. The light level measurement is made when the light output is stable. The maximum light level was 50.4 Fc.

By using a few small pieces of double backed tape and stripe of heavy duty aluminum foil, I formed the foil to cover the inside of the reflector. The double backed tape held the foil in place long enough to take a light output measurement. The picture shows the foil crudely formed on the inside surface of the reflector. Using the same test setup previously described, the maximum light level for the foil reflector was 85.3 Fc. This is about a 70% increase in the lamps light output.

The following measurements are for the reflector on the swing arm lamp shown. The inside surface of the reflector is is roughly parabolic so a foil lining will have to be applied in stripes to prevent folding of the foil as it is applied. The rim diameter is 6.625 inches and necks down to 2.625 inches in diameter at the bulb’s socket. The circumference of these two diameters are 20.8 inches and 8.25 inches respectively. The distance between the rim and the bulb opening is 4 inches.

I decided to use ten stripes to line the reflector. The length of the reflector is 4 inches. Each strip will be 4 inches long and trapezoidal shaped. The width of the strip at the rim will be 20.8/10 = 2.08 ~ 2.1 inches. The width of the strip at the socket will be 8.25/10 = 0.825 ~ 0.83 inches. The figure shows the dimensioned drawing of the strip.

The pictures for this step show the trapezoidal pattern drawn on the foil and the cut out strips.

A Loctite spray adhesive was used to attach the foil strips to the reflector. The rim and the socket were masked off and a layer of adhesive was spayed on the reflector. The stripes are applied to the reflector's surface , sequentially, with sufficient overlap to provide 100% coverage.

While I calculated the dimensions of the strips for 10 strips, the strips had to be overlapped more then anticipated to get 100% foil coverage and 11 strips had to be used. This was due to the curvature is the reflector.

The reflector with the new foil surface. The light output of the lamp with the modified reflector lamp was 86.1 Fc, measured using the procedure described previously.

A summary of the three light output measurement are:

unmodified reflector 50.4 Fc

crude foil reflector 85.3 Fc

fitted reflector 86.1 Fc

This demonstrates the substantial improvement in the lamp’s efficiency, about 71%, by using a highly reflective surface on the reflector thereby reducing energy consumption by not using a higher wattage bulb.

The lamp, with the upgraded reflector, met the additional light level needed without using a higher wattage bulb.