Candles were one of the few ways people in the Middle Ages could have some lighting during the night. In homes, castles and churches, candles were a cheap but not a very effective way of illuminating the darkness.
In his book Wizards, Aliens and Starships: Physics and Math in Fantasy and Science Fiction, Charles Adler explains why candles would have provided so little light, especially compared to the the modern lighting systems we have today. He notes that “before Edison invented the electric lamp, all means of artificial illumination involved fire – burning something. If you’ve ever had to do your homework during a power outage you know that doing homework by candlelight is not the romantic, nostalgic affair it is painted as in historical novels. Much of the time you are maneuvering your book to catch as much light as possible from the flickering flames. No matter how bright the candle is, it is still pretty dim and very hard to read by.”
The reason for this lies in how sensitive the human eye is towards various light sources. For example, people are not able to see in infrared spectrum and fire emits most of its light in this way. “Only about 1% of the light from the candle is radiated away in visible region of the spectrum,” Adler notes, “compared to about 10% of the light from the bulb.” Add to this that the light bulb also radiates more light away from itself, and you will find that a bulb is going to be about 80 times brighter than a candle.
While candles offered little illumination compared to modern lighting, people in the Middle Ages were able to find ways to compensate. For example, medieval art work often made use of gold-leaf, which would literally glow in candlelight – see more at Golden perception: Simulating perceptual habits of the past