Test 3
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Test 3: Staggered and Circular Arrays

The shape of ‘lamps’ are often round (as in downlights) or rectangular (as in fluorescent tubes). To simulate these geometries, two new arrays were created, shown in Figure 16 below. The picture is slightly out of focus since the the camera’s focusing method has a hard time with these active light sources. The power to the LEDs was trimmed to less than 10% just to make the shot.

The rectangular array contains 30 red, 18 green and 18 blue LEDs while the circular array uses 24 white, 18 green and 19 red LEDs. For comparison, the circular array has the same diameter as an ordinary 60W light bulb. The rectangular array could be daisy-chained since all power leads on the circuit board run from end to end. An example of such an extended light strip would be for cove lighting.

Note that, in one case, we strive to obtain white light form red-green-blue mixing, while the circular arrangement is intended to achieve the same result by ‘correcting’ the blueish white LEDs by adding green and red.

The desired light was obtained in both cases when viewed on a target several feet away with some aberrations due to the misconvergence of the colors.

Plans: Obtain translucent Plexiglas sheets for mixing the colors better and to hide the direct view onto the LEDs.

 

 

April 2004: The Plexiglas arrived. It is 1/8th inch thick and passes about 50% of the light. The latter feature is the price one has to pay for the diffusion. This type of translucent material is often used to “hide” fluorescent tubes.

The diffuser does an excellent job in mixing the colors. When one views the Plexiglas surface, one can still see different colors. However, the illumination of a target just a few inches away is very uniform without any color hot spots.

When the RGB controller was adjusted for a warm white light, the pulse width ratios showed

    66% for red - 100% for green - 49% for blue.

This suggests a ratio of 20 : 18 : 9 for red, green and blue LEDs. Since a chain length of 10 for red and lengths of 6 for green and blue are convenient, a suitable distribution would consist of 40 red, 36 green and 19 blue LEDs.

Additional tests are needed to clear up some of these issues.

 

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