The visible region of the electromagnetic spectrum begins with red at a frequency of around 384 THz (terahertz; 1012 cycles per second) or a wavelength of 780 nm (1 nanometre = 10-9 m) and ends with violet at a frequency of 789 THz or a wavelength of 380 nm. Across the visible region of the spectrum, the frequency ratio is barely more than 1:2 (see Figure 4). In comparison, audible acoustic oscillations (i. e. sound perceptible to humans) ranges from about 50 Hz to approximately 20 kHz, which corresponds to a ratio of 1:400. But the range of visible radiation is centred more or less around the point where the sun’s radiation is at its most intense, which from an evolutionary perspective is probably not just a coincidence. Of course it would be nice if we were able to see infrared radiation at night. But for us to be able to see what an infrared camera can, the visible range would need to extend quite far into the long-wavelength infrared region. After all, the human body radiates with a “colour temperature” of precisely 310 K (see Section 7 for more information about colour temperature).
Another difference between the ways we treat light and sound is that we use a logarithmic scale for sound pressure levels, as our perception of noise levels is logarithmic. While humans also perceive brightness logarithmically, we measure light intensities in the linear units of lumens [lm] or lux [lx]. This logarithmic scale becomes apparent when we consider the fact that office spaces typically require an illuminance of 500 lx [lm/m²] but direct sunshine provides us with no less than 100,000 lx, while at full moon, with only 0.2 lx available, we can still find our way around, and we are still just able to perceive something at an illuminance level of a mere 0.0001 lx. So light from a full moon is only 2 ppm (parts per million) of that on a bright, sunny day. On an overcast winter’s day, which we typically associate with November but which can actually be found throughout most of the winter season, the illuminance level is no more than 1000 lx, which is just 1% of that from direct sunshine. But an overcast November day still doesn’t appear that dark to us. In fact, the range of perceptible brightness covers nine orders of magnitude, which is almost as large as the audible frequency range. Once this has been understood, the struggle to squeeze out another lumen per watt, which is so characteristic of the energy efficiency debate, loses some of its significance. Potentially far greater energy savings could be made by identifying those applications in which half the brightness (strictly, illuminance) would still suffice.