Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question, you could use a single photodiode to determine the brightness of your bulbs. Finding that brightest bulb among the many and the background light would be a laborious task, however, unless you expanded your design task to using two photodiodes. The two photodiodes let you find a light's position by monitoring the difference between their output signals.
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