Historical studies of the growth in real wages and output depend upon the accurate measure of the price trends of goods and services. Over long periods of time, the consumption bundle has changed profoundly, and most of today's consumption includes items that were not produced, and in some cases not even conceived, at the beginning of the nineteenth century. This paper tackles the issue of the quantitative significance of the qualitative change in consumption by choosing a single service -- lighting -- for which the service characteristic -- illumination -- is invariant. We estimate changes in lighting efficiency and construct a "true" price index back to Babylonian times, with the major emphasis on changes over the last two centuries. A comparison of the true price of light with a traditional light price indicates that traditional price indexes overstate price growth, and therefore understate output growth, by a factor between 900 and 1,600 since the beginning of the nineteenth century. This finding suggests that the "true" growth of real wages and real output may have been significantly understated during the period since the Industrial Revolution.