Candlepower (or candles): I've spent several weeks debating with myself as to whether I should mention this.
Candela (the modern term for candlepower) is a base unit of luminous intensity in the International System of Units (SI). (source: Wikipedia Candela article) Lumen is a SI derived unit of luminous flux. There is a direct relationship between candela and lumen: one lumen equals one candela times one steradian. (Steradian is a measurement of a conical section of a sphere.)
Note: 'luminous intensity' versus 'luminous flux.' While candela and lumen both measure visible light, each measures it in distinctly different ways.
Yes, understanding the two units requires a better than average understanding of physics and three-dimensional geometry. Don't let that stop you from exploring the subject on Wikipedia. You're never too old to learn and the best way to learn is to wade into topics, see where things lead, and try to understand.
I am fortunate in having a good understanding of physics and geometry. However, I cannot (in any way, shape, or form) be considered a physicist or mathematician. (I'm a generalist -- sometimes called a 'polymath' -- rather than a specialist.) On the whole, the Wikipedia articles on visible light are well written and lean towards 'plain English' description rather than dense science and mathematics. (The same cannot be said for related articles elsewhere on the Internet.)
I found it interesting to know the history behind the evolution of both units of measurement, which is covered in the Wikipedia. (I believe it's important to know how we got where we are today.)
That being said, lumens are a good means for the average person to make reasonably accurate comparisons about light emitted by light bulbs and fixtures.
