In order to find the electrical conditions for operating high-pressure sodium lamps with a constant correlated color temperature to get high color rendering from lamps of the same type having different correlated color temperatures, we conducted a set of experiments. We operated 150W lamps under various electrical conditions to determine the relationship between the correlated color temperature and the electric properties. Based on the results, we derived the following formula:
T
cp=a·W
1a+b· V
1a+c,
where T
cp is the correlated color temperature, W
1a is the lamp power, Via is the lamp voltage, and a, b, and c are constants. This formula gives the electrical conditions of W
1a and V
1a for a specific value of T
cp. The same formula is also applicable to lamps of other wattages by changing the values of the constants a, b, and c. If the correlated color temperature of the lamp is limited to±150K by controlling these electrical conditions, the deviation of the lamp from the black body locus is preferable for vision. In the same manner, we derived
T
cp=d· V
1a+e· I
1a+f,
where T
cp is the correlated color temperature, V
1a is the lamp voltage, I
1a is the lamp current, and d, e, and f are constants. The sensing circuits for a specific value of T
cp can be the simple sensing circuits of V
1a and I
1a.
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